A formal model for the geologic time scale and global stratotype section and point, compatible with geospatial information transfer standards
Simon J.D. Cox* CSIRO Exploration and Mining, P.O. Box 1130, Bentley, 6102 WA, Australia Stephen M. Richard* Arizona Geological Survey, 416 W. Congress St., #100, Tucson, Arizona 85701, USA
ABSTRACT A formal notation is used so as to provide a Structure of the Paper rigorous description of the various elements The geologic time scale is a complex data required to describe the structure and cali- This paper is structured as follows. An in- structure composed of abstract elements bration of the time scale in a manner that troduction and summary of key aspects of that represent time intervals and instants allows the logical consistency of the model stratigraphic methodology is provided in the and their relationships with speci®c con- to be evaluated. This is important, since al- ®rst section. Next, we brie¯y introduce infor- crete representations in the geologic record though stratigraphic methodology is one of mation standardization activities that provide as well as the observations made of those the most rigorously studied aspects of geo- the modeling framework and notation used in concrete representations. The International logical practice, it has evolved throughout this study. We then describe a general frame- Union of Geological Sciences' International the era of historical geology. There have been work for temporal reference systems and de- Commission on Stratigraphy guidelines signi®cant changes in best practice, in partic- velop a formal model for the geologic time recommends a very precise usage of the re- scale and its calibration within that frame- lationships between these components in ular in the shift from characterizing units to de®ning the boundaries between them. Nev- work. This conceptual model is used as the order to establish a standard time scale for basis for an XML implementation of the mod- use in global correlations. However, this ertheless, the time scale itself remains based on named units and eras. Other residues of el, presented using example documents de- has been primarily described in text. Here, scribing the International Union of Geological earlier practice remain visible in the descrip- we present a formal representation of the Sciences (IUGS)'s International Commission tion of the time scale, particularly where model using the Uni®ed Modeling Lan- on Stratigraphy (ICS) time scale with global agreement on the application of current prac- guage (UML). The model builds on existing stratotype section and point (GSSP) referenc- tices is incomplete. In this context, a rigorous components from standardization of geo- es. Some theoretical issues arising from the characterization of the relationships between spatial information systems. The use of a models are discussed. A summary of UML formal notation enforces precise de®nition the elements of the time scale, the evidence notation is given in Appendix 1. of the relationships between the compo- in the geologic record, and the application of nents. The UML platform also supports a speci®c procedures to effect numeric calibra- THE GEOLOGIC TIME SCALE direct mapping to an eXtensible Markup tion of the scale is essential. Language (XML)±based ®le format, which A secondary goal is to develop a machine- Units, Boundaries, and Stratotypes may be used for the exchange of strati- processable format for the exchange of in- graphic information using Web-service formation related to the time scale. The mod- The conventional geologic time scale is a interfaces. eling notation selected (Uni®ed Modeling reference system de®ned by a contiguous se- Language [UML]) is a commonly used soft- Keywords: time scale, stratigraphy, infor- quence of time intervals, each identi®ed with ware engineering standard, so the model can mation model, XML, UML. a name. These are recursively subdivided, re- be implemented easily on various standard sulting in a hierarchy composed of intervals INTRODUCTION platforms. In particular, an eXtensible Mark- of various ranks. The units in the scale are up Language (XML) document schema can ordered, so the relative temporal positions of Goals be derived directly from the model. The geologic objects and events may be recorded XML implementation supports lossless data or asserted, denoted by the names of units The goal of this paper is to provide an in- transfer using standard Web protocols, and it from the scale. formation model for the geologic time scale. provides a basis for formal encoding and As originally conceived, units of the geo- *E-mails: [email protected]; steve.richard@ computer processing of information that is logic time scale identify intervals, each cor- azgs.az.gov. the basis for de®ning geologic time scales. responding to the time during which a partic-
Geosphere; December 2005; v. 1; no. 3; p. 119±137; doi: 10.1130/GES00022.1; 6 ®gures; 1 table.
For permission to copy, contact [email protected] ᭧ 2005 Geological Society of America 119
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ular sequence of rocks (stratigraphic unit) was plete time scale. However, a complete com- called global standard stratigraphic ages deposited. Historically, the units were chosen pilation is possible only based on sections that (GSSA). because, within the region where they were explicitly include boundaries. Implicitly, the Ages recorded using named units from the de®ned, they could be recognized through uni- shift from unit stratotypes to boundary stra- geologic time scale allow ordering without re- form lithological and biostratigraphic charac- totypes recognizes that, although the time quiring numeric values. However, while the teristics, which correlated with a relatively scale is based on time intervals covering the assignment of numeric values is not necessary consistent geological environment in a single domain, its logical consistency is contingent for use of the time scale, it is convenient to period. The representative object or prototype on a globally recognizable, unambiguously or- calibrate the time scale against a time line. For in a stratigraphy de®ned in this way is a type dered sequence of events. those boundaries de®ned by a stratotype, it section for the geologic unit of interest, for- provides a locality from which specimens may mally called a unit stratotype. This approach Governance and Calibration be collected, the age of which may then be supports stratigraphic practice in which as- measured using quantitative techniques. When signment of strata to a unit is based on the the material in the stratotype is unsuitable or The guidelines of the ICS formalize this presence of characteristics that match the stra- insuf®cient for estimating the numerical age, practice (Remane et al., 1996). These are used ta to the type section for the unit. The basis then specimens from locations that can be cor- for the matching, or correlation, may be lith- by the GSSP project. The GSSP provides a related with the stratotype may be used in- ologic, paleontologic, or geochronologic, de- forum for the speci®cation of boundary stra- stead. The experimentally determined dates ®ning lithostratigraphic, biostratigraphic, and totypes used for correlation on a global scale. from such specimens provide an estimate of chronostratigraphic units. Local stratigraphic de®nitions will still be the chronologic position (numeric coordinate Construction of a consistent time scale us- used for local stratigraphic correlations, but on a time line) of the boundary. ing the unit stratotype approach depends crit- global stratotypes provide the ultimate refer- ically on stratigraphic completeness and the ence for inter-regional correlation. Local Formalization ability to geochronologically correlate bodies stratigraphic schemes must be related to the of rock globally. However, the corollary of a GSSP stratotypes through a coherent chain of This overview of the construction and cal- model based on continuity within units is that correlations in order to be connected to the ibration of the geologic time scale refers to a the boundaries between units correspond with global time scale. Alternatively, geochrono- variety of concepts, instances of which are re- changes in the geological environment and metric methods may be used for objects with- lated to each other in various ways. Effective discontinuities in the stratigraphic record. in those parts of the time scale where chro- use of the time scale for stratigraphic corre- Such discontinuities, and the incongruity of nometric values for the boundaries have been lation requires that these concepts and their lithostratigraphic and biostratigraphic units accepted. interrelationships be precisely understood. with chronostratigraphic units, result in incon- Physical changes in the rock record ob- There is a signi®cant body of literature that sistent and ambiguous correlation in the vicin- served at the boundary horizon are inferred to describes the system, an introduction to which ity of chronostratigraphic unit boundaries. result from a geologic event or events. Ideally, is provided by Walsh et al. (2004) and refer- For these reasons, the complementary ap- these are manifested globally by similar or re- ences therein, in the ICS guidelines (Remane proach to speci®cation of the time scale is lated physical changes in sediment deposited et al., 1996), and in the overview to the In- now preferred, focusing on the boundaries at the same time. Correlation of the boundary ternational Stratigraphic Chart (International rather than the intervals. Within this model, horizons by correlation of physical changes in Commission on Stratigraphy, 2004). the focus is on a representative point within other stratigraphic sections is the basis for es- However, the model for the construction the geologic horizon corresponding to the tablishing the relative age of strata throughout and calibration of the geologic time scale has boundary of interest, formally called a bound- Earth. been described primarily in prose. This leaves ary stratotype and often known as the ``golden A GSSP is a particular sequence of strati- open the possibility of ambiguity and omis- spike.'' Observations made at the point pro- ®ed rocks de®ned to contain a particular, sion. The most formal representation of the vide a basis for characterizing the boundary. physically located stratigraphic point that conceptual model is provided by the schemata For example, estimates of the precise age of serves as the reference object to de®ne the (column headings) of various tabulations of the boundary may be made using geochrono- boundary between units in the time scale. In the time scale and associated components. In logic techniques on specimens sampled from chronologic terms, each point corresponds to this paper, we attempt to improve on this by the stratotype point. These observations sup- a time instant at the boundary between time providing a description of the system in a port progressive chronometric calibration of intervals that compose the time scale, while in standard framework used for modeling tech- the time scale one boundary at a time. stratigraphic terms, the stratigraphic point de- nical information, using a standard symbolic Boundary stratotypes are de®ned, wherever ®nes the lower boundary of rocks formed dur- notation. The framework is object-oriented possible, within sections where the geologic ing a named interval. (OO) modeling and analysis, and the notation record is continuous across the boundary. The In the context of the GSSP project, bound- that we use is the class diagram from the UML material above and below the point provide ary stratotypes provide the ultimate de®nition (Object Management Group, 2004). context for the boundary, showing evidence of elements of the time scale from the begin- We use the UML notation for three reasons: relating to the geological history adjacent to ning of the Ediacaran up to but (probably) not (1) it provides a rich description of concepts the boundary. With enough stratotype points, including the beginning of the Holocene. For and their interrelationships that allows us to each representing boundaries of units of suf- the earlier parts of the time scale, boundaries capture most of the nuances required, while ®ciently ®ne rank, the partial units represented between intervals are de®ned chronometrical- its graphical nature allows these to be com- in the sections containing the points overlap ly by assigning a numerical age to the bound- municated to readers of various levels of ex- to provide a basis for characterizing the com- ary. These numerically de®ned boundaries are pertise; (2) it is commonly used in software
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engineering, so models de®ned in UML can ``feature model'' that serves as the basic will be of interest to the broader geoscience be easily converted into software representa- framework for classifying the items of in- community. tions, including XML (Yergeau et al., 2004), terest in the application domain; GeoSciML is building on earlier work, in Java, Cϩϩ, C#, etc.; and (3) it is the notation ISO TS 19103, Conceptual Schema Language, particular, that of the XMML and NADM used by the leading organizations involved in lays out the UML pro®le used as the nota- projects. The geologic time scale encoding de- standardization of geospatial information sys- tion across the ISO 19100 series; scribed in this report is likely to be incorpo- tems. This allows us to construct specialized ISO 19108, Temporal Schema, describes a rated in GeoSciML. software components for the geosciences that consistent model for temporal objects and leverage existing technology. reference systems; and STANDARDS COMPONENTS USED IN A particular advantage of the OO approach ISO DIS 19136, Geography Markup Lan- THE TIME SCALE MODEL is that the key information can be partitioned guage (GML), is an XML schema for com- into different objects, and relationships be- ponents for geographic information. Base Classes tween objects can be described independent of GML (Cox et al., 2004) was developed by the level of detail known about each of the OGC, and includes an implementation of the objects. While it is out of scope in this paper temporal schema from ISO 19108. In this re- Most of the classes shown in the following to provide an introduction to OO modeling port, we lean heavily on the theoretical frame- sections and diagrams inherit some standard and analysis (there are many introductory work provided by ISO 19108 and its XML properties from common base classes. The books), a brief summary of UML as used in implementation in GML. The models shown base classes are taken from ISO 19108 and this report is provided in Appendix 1. in this report utilize the UML notation de®ned from the UML representation of GML 3, in ISO 19103. GeoSciML, and O&M. Important base classes STANDARDS CONTEXT A model and encoding for observations and referred to in this report include: measurements (O&M) has also been pub- De®nition carries a mandatory ``name'' prop- General Geospatial Information lished through OGC (Cox, 2003). This pro- erty, plus an unlimited set of aliases, and vides a basis for describing specialized date has an optional ``description,'' which car- Most investigations in geology have a measurements used for calibration of the time ries the text of the de®nition or a link to a strong geospatial ¯avor, and database and geo- scale. source. graphic information system (GIS) technology Observation is an event, producing a ``result'' are commonly used for the management and Geoscience Information describing the value of some property of its display of geologic data. It is therefore both target (e.g., a specimen). The result may convenient and ef®cient to develop models In addition to these activities that standard- take many forms: a Measurement is a spe- and encodings for geologic information that ize generic geospatial information, there are cial case that results in a numeric value with leverage developments in geospatial stan- several related projects directly addressing the unit-of-measure. Every Observation has a dards. The standardization framework that we geosciences. These include: featureOfInterest and uses a Procedure. use also addresses temporal issues in a manner eXploration and Mining Markup Language Procedure is a description of a process, such that allows us to combine the concerns in a (XMML) (http://xmml.arrc.csiro.au)ÐGML- as an instrument, sensor, sample prepara- single model and encoding. based XML encoding for mineral exploration tion, calculation, simulation, etc. The methodology used in this report fol- data. This includes components related to ob- Section, Station are identi®able locations with lows procedures and notation used in speci®- servations and measurements that are used in a shape corresponding to a curve and point, cations issued by the International Organiza- this report. respectively, usually associated with mak- tion for Standardization Technical Committee The North American Data Model (NADM) ing observations and retrieving specimens. 211 (ISO/TC 211) and by the Open Geospatial (http://nadm-geo.org)Ðconceptual models for Specimen is material retrieved by sampling at Consortium (OGC). These organizations have information associated with geologic maps, a location or on another feature, typically been active since the mid-1990s, working to presented using UML. This includes models used as the subject of an observation or standardize models, representations, and pro- for earth materials, geologic units, genesis, measurement. cessing services for geographic information. geologic structure, etc., which de®ne a basic ISO/TC 211 is responsible for around 40 framework for representing geoscience infor- Temporal Reference Systems international standards and reports in the ISO mation in computer information systems. 19100 series. These speci®cations primarily NADM was developed by a group of geolo- Figure 1 shows components concerning ge- describe conceptual or abstract models and ap- gists and information specialists from state neric temporal objects and reference systems. proaches. OGC plays a complementary role in and federal geological surveys in Canada and The classes pre®xed ``TM'' are taken directly developing and testing speci®cations for geo- the United States. from ISO 19108, while the other classes rep- spatial data access and processing services, GeoSciMLÐa project under the auspices of resent the extensions required for the model and implementation encodings for some of the the IUGS Commission for Geoscience Infor- in this paper. information models developed through ISO. mation, to develop a GML-based XML en- All temporal reference systems derive from Of interest to us in the context of this report coding for geosciences. The principal stake- a common TMReferenceSystem. This has a are the following: holders are statutory data custodians, and the mandatory property, domainOfValidity, which ISO 19109, Rules for Application Schema, focus is on the information required to support describes the spatiotemporal scope of the ref- formally recognizes the existence of com- the maintenance of geologic maps. However, erence system (e.g., common era, global or munities with needs for thematically spe- this is being done through a high-level con- Cambrian, Australia). Four concrete speciali- ci®c information models and provides a ceptual model that has many components that zations are de®ned as follows:
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Figure 1. Model for temporal reference systems, adapted from ISO 19108:2003. TimeOrdinalEraBoundary class is a modi®cation re- quired for stratigraphy and archaeology. Note: a parent class not shown in the diagram is indicated by its name in italics above the name of the derived class.
TMCalendar is a reference system based on the term ``era'' is used generically, and is central since it is the temporal concept that is years, months, and days. used for component intervals of all ranks. associated with the boundary stratotype. TMClock is based on hours, minutes, and In the de®nition of TOE, we have found it Finally, the TMPosition data type carries a seconds in a particular day. necessary to introduce a variation to the ISO ``frame'' property, which indicates which ref- TMCoordinateSystem provides the basis for de®nition. In the standard model (ISO 19108: erence frame is used, usually an instance of numerically describing temporal position. 2003), the limits of TMOrdinalEra are de- one of the temporal reference systems. This uses two properties to de®ne a time ®ned precisely by attributes of type DateTime. line: an origin, which ties the scale to an However, in historic, archaeological contexts, MODEL FOR GEOLOGIC TIME external temporal reference position, and and certainly in the geologic time scale, while SCALE interval, which provides the basic unit or the order of eras within a TORS is known, the precision, such as seconds or millions of In the portrayal of the model in this report, positions of the boundaries are often not pre- years. the information is partitioned between several cisely known and can only be estimated. We TimeOrdinalReferenceSystem (TORS) pro- diagrams for convenience (Figs. 2±5). The suggest that standard practice is better repre- vides the system required for a time scale union of these describes a general model for based on named intervals. A TORS is com- sented by a model using an explicit Time- time scales and the relationships with evi- posed of an ordered sequence of one or OrdinalEraBoundary (TOEB) element to carry dence in the geologic record. It is intended to more component TimeOrdinalEra (TOE) el- information concerning the transition between include all components necessary to describe ements. A TOE may be recursively decom- two TOEs. The temporal position of the era the practice speci®ed in the ICS guidelines posed into ordered member TOE elements, boundary is given by an associated Time- (Remane et al., 1996), but it also includes el- thus allowing a hierarchical system to be Instant, but the TOEB exists in its own right ements and relationships that relate it to other constructed. Each era is characterized by its even if its position is not known. In the con- methodologies, in particular, for de®ning local name (inherited from De®nition). Note that text of the geological time scale, the TOEB is or regional time scales. In order to illustrate
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Figure 2. Basic geologic time scale (many attributes suppressed for clarity).
the application of the model to the GSSP, a aries composing the reference system as ®rst- NumericEraBoundary is provided for those summary diagram is provided in which rela- order elements. Geochronologic boundaries de®ned chronometrically. It has no tionships that are not required by the ICS specializations of each are provided. additional associations, but a ``status'' attri- guidelines are omitted (Fig. 5). GeochronologicEra is a kind of Time- bute is provided. OrdinalEra with boundaries de®ned by geo- The Geologic Time Scale logic evidence. It specializes TimeOrdinalEra Geologic Evidence and the Time Scale by adding a ``rank'' attribute, whose value is Figure 2 shows the way the generic com- one of the standard terms, such as eon, era, Figure 3 shows relationships of the concep- ponents are extended for geochronologic period, etc. Some additional properties are dis- tual objects with certain physically realizable purposes. cussed in a following section. geologic feature types, including units and GeologicTimescale is a kind of TORS that Two specializations of TOEB are intro- sections, samples of which may play roles as is composed of one or more ordered TOEs duced. GeochronologicBoundary represents stratotypes. Both eras and boundaries are rep- together with two or more TOEBs. an era boundary de®ned with reference to resented in the geologic record. GeologicTimescale is, thus, a temporal com- some geologic evidence. Its properties are dis- A ChronostratigraphicUnit is the (notional) plex that includes both the eras and the bound- cussed in detail in the following section. feature composed of all the rocks formed dur-
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Figure 3. Relationship of geological features to elements of the time scale (many attributes suppressed for clarity).
ing the associated GeochronologicEra. Both geological unit or boundary may be made, a the process of estimating the numeric position ChronostratigraphicUnit and the commonly single instance must provide the reference lo- of a boundary. used LithostratigraphicUnit are kinds of cality or stratotype for the associated era or DateMeasurement is a kind of measurement GeologicUnit. Similarly, a Chronostrati- era-boundary, respectively. whose result is a (numeric) value with refer- graphicBoundary is the (notional) compound A further important concept is the ence to a TimeCoordinateSystem (Fig. 1). In surface marking the upper or lower bound of StratigraphicEvent. In general, events are as- common with all observations, it relates to a a unit. Both ChronostratigraphicBoundary and sociated with time primitives of either zero or physical target or featureOfInterest, usually ei- LithostratigraphicBoundary are kinds of a ®nite extent (i.e., time instants or time pe- ther (1) a specimen, or (2) a sampling site StratigraphicBoundary. In practice, the com- riods). However, in the context of the geolog- such as a StratigraphicPoint in its context plete shape of any ChronostratigraphicUnit ical time scale, a useful event has negligible within its host StratigraphicSection. The mea- and ChronostratigraphicBoundary instance duration, and is associated with a boundary surement uses a DatingProcedure, preferably will not be precisely known, so while the ex- and characterized by a StratigraphicPoint. a precise numeric method such as one of the istence of a unit and its boundaries is a fact, radiometric methods or based on astronomical they will never be fully described. The Calibration of the Time Scale cycles. If these are not suitable for the phys- ChronostratigraphicUnit is the complete body ical evidence, then less precise methods are of rock formed during (formedDuring) a used. GeochronologicEra, and the Chronostrati- The time scale is calibrated by estimating The GeochronSpecimen is some material graphicBoundary correlates with (correlates- the position or time-coordinate of boundaries that samples a site. The site will strictly be a With) a GeochronologicBoundary. A Chrono- within it (Gradstein and Ogg, 2004). Deter- small interval bracketing a point of interest stratigraphicUnit carries a ``rank'' attribute, mination of geologic age fundamentally relies (i.e., a short section), but may often be treated whose value is one of the standard terms such on isotopic dating of mineral phases that can as a point at the scale of interest. If the ma- as system, stage, zone, etc. be related to the age of the enclosing rock (see terial at the stratotype itself is unsuitable for Conventional samples of both units and Faure, 1977), or to the correlation of calculat- date determination, then the featureOfInterest boundaries may be de®ned, with a spatial di- ed changes in Earth's orbital parameters as a related to the actual measurement may sample mensionality two orders less than the parent. function of time to patterns of physical prop- a different locality that is correlated with the For a solid unit, this sample is a section erty variations related to those parameters in stratotype, or with another known relationship (whose shape is a curve), while for a surface stratigraphic sequences (Laskar, 1999; Shack- with the stratotype. the sample is a point. These are shown in the leton et al., 1999). Thus, estimation of the po- Finally, StratigraphicDateEstimate repre- model as StratigraphicSection and Strati- sition of a boundary is based on observations sents an identi®ed interpretation of temporal graphicPoint, respectively. A Stratigraphic- made on specimens collected from stratigraph- position and is substitutable for TimeInstant. Point is always contained within a ic sections that contain, or may be correlated The observationalBasis of the Stratigraphic- StratigraphicSection, which is its hostSection. with, a boundary stratotype, or observations DateEstimate may be one or more observa- In principle, a StratigraphicSection may host made concerning the position of a boundary tions (e.g., DateMeasurements). Thus, the as- any number of StratigraphicPoints. While an stratotype within patterns displayed in its host sociation labeled ``Geometry'' between unlimited number of samples of the concrete section. Figure 4 introduces classes supporting TimeInstant and TimeOrdinalEraBoundary
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Figure 4. Calibration of the time scale through date determinations on specimens or points.
usually refers to a StratigraphicDateEstimate jects in the system, but which are not required recursively nested and assigned a rank within when the GeochronologicBoundary descen- or are deprecated in modern stratigraphic a standard hierarchy. GeochronologicEra and dent is involved practice, as de®ned in the guidelines of the GeochronologicBoundary are specializations Note the various cardinalities on the associa- ICS (Remane et al., 1996). of the standard eras and boundaries. tions. A measurement is associated with a single The diagram in Figure 5 shows a complete One StratigraphicPoint plays the role of procedure and a single target object. Specimens model, constructed by combining elements in- stratotype for a GeochronologicBoundary, may be associated with a point or interval. Mea- troduced in Figures 1±4, but suppressing clas- which records a GeochronologicEvent. The surements may be made on a target. Some of ses and associations that either con¯ict with GeochronologicBoundary corresponds with the associations are only traversable in one di- or are not used by the practice described in the initiation of rock formation during the rection: a procedure does not know about all the the guidelines (Remane et al., 1996). Further- GeochronologicEra for which the Strati- measurements made using it; a Date- more, in this diagram most of the required graphicPoint is the lower boundary-stratotype. Measurement does not know if it is used as the class attributes are shown. For example, a Under ICS guidelines there is no correspond- basis for a StratigraphicDateEstimate. number of attributes describe details of points ing association of a unique stratigraphic sec- and sections, some of which are inherited tion with a GeochronologicEra. Unit strato- Representation of the ICS Model from parent classes as indicated by the types may be used for regional and local annotation. purposes, but their use is deprecated for spec- The model shown in Figures 1±4 provides We may summarize the story told in this i®cation of the global time scale. a description of a relatively comprehensive set model as follows. DateMeasurements are made on either (1) a of relationships between objects involved in GeologicTimescale is a specialized TORS StratigraphicPoint in its context (e.g., for de- the de®nition and calibration of the geologic and is composed of an ordered sequence of terminations based on astronomical cycles), or time scale. This includes a number of associ- TOE elements, along with the TOEB elements (2) on a GeochronSpecimen (e.g., for radio- ations that re¯ect relationships between ob- that act as reference points. TOE elements are metric date determinations). Specimens may
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TABLE 1. MAPPING BETWEEN REQUIREMENTS OF ICS GUIDELINES AND ELEMENTS IN THE MODEL PRESENTED IN THIS PAPER
ICS requirement Corresponding model element Name of the boundary GeochronologicBoundary::name GSSP de®nition StratigraphicSection::description StratigraphicPoint::boundary:GeochronologicBoundary::nextEra Stratigraphic rank and status of the boundary :GeochronologicEra::rank StratigraphicPoint::status StratigraphicPoint::boundary:GeochronologicBoundary::nextEra Stratigraphic position of the de®ned unit :GeochronologicEra::name Or StratigraphicSection::era:GeochronologicEra::name Type locality of the GSSP StratigraphicSection::begin Geologic setting and geographic location StratigraphicSection::geologicSetting StratigraphicSection::boundedBy Lithology/sedimentology/paleobathymetry StratigraphicSection::geologicDescription Map Accessibility, both logistically and politically StratigraphicSection::accessibility Conservation StratigraphiSection::conservation Identi®cation in the ®eld StratigraphicPoint::description Stratigraphic completeness of the section StratigraphicSection::completeness Global correlation using, where applicable, StratigraphicPoint::primaryGuidingCriterion biostratigraphy, magnetostratigraphy, stable StratigraphicPoint::additionalCorrelationProperty[0..*] isotope stratigraph, and other stratigraphic tools and methods Best estimate of age in millions of years StratigraphicPoint::boundary:GeochronologicBoundary::position :StratigraphicDateEstimate::timePosition StratigraphicPoint::reference References to background studies StratigraphicPoint::boundary:GeochronologicBoundary::position :StratigraphicDateEstimate::reference StratigraphicSection::reference
be sampled in the stratotype section, or anoth- itself (the golden spike) that provides the ul- IMPLEMENTATION er StratigraphicSection that is correlated with timate reference for the boundary, so its po- the stratotype. A StratigraphicDateEstimate sition will remain unchanged even if new ev- XML Document Format provides the preferred value of the position of idence modi®es the interpretation of the the GeochronologicBoundary. The estimate is stratigraphic event (Walsh et al., 2004). UML is a convenient means to represent an usually based on one or more Date- StratigraphicEvent inherits from the event information model. To make use of the model, Measurements, but may be derived from some class (not shown) an eventTime association we require an implementation that allows in- other basis. with a notional time object. However, as used stances that conform to the model to be ex- A StratigraphicDateEstimate has a quality here, it is assumed that the position of a pressed. Some software development environ- associated with it, which allows the estimated StratigraphicEvent is not available directly, ments support automatic con®guration and error to be recorded. StratigraphicDate- code generation of data structures and repre- but may be recovered by tracing the associa- Estimate along with both StratigraphicPoint sentations based on a UML model. The in- tion with a boundary or prototype point, for and StratigraphicSection have status attributes stances may take various forms, including ta- which estimates of the position are available. that can be used to record whether these are bles and messages. The ICS guidelines (Remane et al., 1996) rati®ed through GSSP. In this work we focus on a message format A suitable StratigraphicPoint has an asso- provide a set of information that must be sup- using XML (Yergeau et al., 2004). XML is a ciation with one or more StratigraphicEvents, plied for a proposed GSSP. Table 1 shows how text-based method for serialization of structured which are associated with observable evidence these are implemented in the UML model pre- and semi-structured data primarily developed in the section that de®nes the point, such as sented here. All the required information has for transfer using Web protocols, but may also the appearance or disappearance of particular suitable slots in the model, so this means that be used to de®ne ®le-formats for persistent fossil taxa, or the beginning or end of some the record of a submission to ICS could take storage. A particular advantage of the plain-text climatic phenomenon. Note, however, that in the form of a document structured according encoding is that it allows inspection and mod- the ICS approach, it is the StratigraphicPoint to this model. i®cation using basic text-processing tools to
Figure 5. The elements of the time scale model used by the Global Stratotype Section and Point (GSSP) project. This is based on the model shown in Figures 1±4, but only the relationships that are formally used in the GSSP project are shown. The colors provide an informal high-level classi®cation: those in yellow are generic time scale components, green are generic sampling and observation elements, light brown are related to concrete objects in the ®eld, and blue are abstract components associated with the geologic time scale. N
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supplement processing using specialized soft- This depends on the model using the UML nested element pattern embodies a set of re- ware. However, it is important to note that pro®le de®ned in ISO 19103 and GML 3, lationships that have a tree form. XML is not a presentation format, and the in- mentioned above. In summary, the serializa- Rules 2 and 3 mean that the resulting in- formation in an XML document should be tion method implements the metamodel in the stance document is ``striped,'' with nested el- transformed and formatted for human con- following ways: ements alternating between class and property sumption. This might include transformations 1. The structure of the XML document cor- names, and where the appearance of a class as of values from mathematical formats into con- responds to a view of the UML model as a a descendent of another class is always me- ventional presentation forms. For example, lat- tree rooted at the class of interest; diated by a container element corresponding itude and longitude appear as signed decimal 2. Both classes and properties (UML attri- to a property. degrees in a GML document, but cartographic butes and associations) appear as XML ele- The method uses an intermediate World practice prefers a sexadecimal (degrees-minutes- ments in the XML instance document; Wide Web Consortium (W3C) XML Schema seconds-hemisphere) representation; geologi- 3. The XML element name matches the (Fallside et al., 2004) that implements the cal dates are negative numbers relative to the class or property name (i.e., UML attribute model and supports schema-validation of in- standard origin, but are usually viewed as pos- names, or in the case of associations, the role- stance documents. itive numbers corresponding to age (see fur- name at the target class); Examples ther discussion below). 4. Where the value of a property has a As discussed above, XML representation of complex structure (i.e., shown as a class rather Following the model and encoding rules de- data using GML is at the core of various Web- than a data type in the UML model) it may service interfaces de®ned for access to geo- scribed above, a representation of (parts of) be given either as a structure of sub-elements spatial data by the Open Geospatial Consor- the geologic time scale and its calibration fol- nested within the property element (``inline''), tium, such as Web Feature Service (Vretanos, lowing the ICS guidelines is shown in Listings or via a reference to a value elsewhere using 2005). GML is being rati®ed through ISO/TC 1±5. an ``xlink:href'' attribute on the property ele- 211 and currently has the status of Draft In- Listing 1 represents the complete geologic ment; and ternational Standard 19136. time scale, though only the three eras of rank 5. Generalization is implemented as substi- Eon are shown, along with descriptions of the Serialization Rules tution group af®liation in the XML schema. two intermediate boundaries. An illustration Rule 1 is concerned with accommodating of the ®ner decomposition of parts of the A pattern for XML serialization is provided the fact that, in general, a UML model is a Phanerozoic and Late Permian is shown in by ISO 19118 and GML 3 (Cox et al., 2004). graph of links between classes, while XML's Listing 2.
Listing 1. The geologic time scale decomposed to eon level. Ͻ?xml versionϭ``1.0'' encodingϭ``UTF-8''?Ͼ Ͻgt:GeologicTimeScale gml:idϭ``ICSTimeScaleEonsOnly''Ͼ Ͻgml:descriptionϾThe geologic timescale, as de®ned by ICSÐdecomposed to eons onlyϽ/gml:descriptionϾ Ͻgml:nameϾICS Geologic TimescaleÐeons onlyϽ/gml:nameϾ Ͻgml:domainOfValidityϾEarthϽ/gml:domainOfValidityϾ Ͻ!± ϭϭϭϭϭϭϭϭϭϭϭϭϭϭÐϾ Ͻgt:componentϾ Ͻgt:GeochronologicEra gml:idϭ``AR''Ͼ Ͻgml:nameϾArcheanϽ/gml:nameϾ Ͻgt:start xlink:hrefϭ``urn:x-seegrid:items:exceptions:unde®ned''/Ͼ Ͻgt:end xlink:hrefϭ``#ARPR''/Ͼ Ͻgt:rankϾEonϽ/gt:rankϾ Ͻ/gt:GeochronologicEraϾ Ͻ/gt:componentϾ Ͻgt:componentϾ Ͻgt:GeochronologicEra gml:idϭ``PR''Ͼ Ͻgml:nameϾProterozoicϽ/gml:nameϾ Ͻgt:start xlink:hrefϭ``#ARPR''/Ͼ Ͻgt:end xlink:hrefϭ``#PRPH''/Ͼ Ͻgt:rankϾEonϽ/gt:rankϾ Ͻ/gt:GeochronologicEraϾ Ͻ/gt:componentϾ Ͻgt:componentϾ Ͻgt:GeochronologicEra gml:idϭ``PH''Ͼ Ͻgml:nameϾPhanerozoicϽ/gml:nameϾ Ͻgt:start xlink:hrefϭ``#PRPH''/Ͼ Ͻgt:end xlink:hrefϭ``#present''/Ͼ Ͻgt:rankϾEonϽ/gt:rankϾ
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Ͻ/gt:GeochronologicEraϾ Ͻ/gt:componentϾ Ͻ!± ϭϭϭϭϭϭϭϭϭϭϭϭϭϭϭϭϭϭϭϭϭϭϭϭÐϾ Ͻ!ÐReference timesÐϾ Ͻ!ÐPositions of Global Stratotype Points as given in http://www.stratigraphy.org/gssp.htm 2004±04±25ÐϾ Ͻ!± ϭϭϭϭϭϭϭϭϭϭϭϭϭϭϭϭϭϭϭϭϭϭϭϭÐϾ Ͻgt:referencePointϾ Ͻgt:NumericEraBoundary gml:idϭ``ARPR''Ͼ Ͻgml:description xlink:hrefϭ``citations.xml#Ev14p1391991''/Ͼ Ͻgml:nameϾBase ProterozoicϽ/gml:nameϾ Ͻgml:nameϾBase PaleoproterozoicϽ/gml:nameϾ Ͻgml:nameϾBase SiderianϽ/gml:nameϾ Ͻgt:previousEra xlink:hrefϭ``#AR''/Ͼ Ͻgt:nextEra xlink:hrefϭ``#PR''/Ͼ Ͻgt:positionϾ Ͻgml:TimeInstant gml:idϭ``PROrigin''Ͼ Ͻgml:timePosition frameϭ``tcs.xml#geologyMA''ϾϪ2500.Ͻ/gml:timePositionϾ Ͻ/gml:TimeInstantϾ Ͻ/gt:positionϾ Ͻgt:statusϾGSSAϽ/gt:statusϾ Ͻ/gt:NumericEraBoundaryϾ Ͻ/gt:referencePointϾ Ͻgt:referencePointϾ Ͻgt:GeochronologicBoundary gml:idϭ``PRPH''Ͼ Ͻgml:descriptionϾThree different estimates of the position of this boundary are included.Ͻ/gml:descriptionϾ Ͻgml:nameϾBase PhanerozoicϽ/gml:nameϾ Ͻgml:nameϾBase PaleozoicϽ/gml:nameϾ Ͻgml:nameϾBase CambrianϽ/gml:nameϾ Ͻgml:nameϾBase Lower CambrianϽ/gml:nameϾ Ͻgt:previousEra xlink:hrefϭ``#PR''/Ͼ Ͻgt:nextEra xlink:hrefϭ``#PH''/Ͼ Ͻgt:positionϾ Ͻgt:StratigraphicDateEstimate gml:idϭ``PHOrigin''Ͼ Ͻgml:timePosition frameϭ``tcs.xml#geologyMa''Ͼ-540.0Ͻ/gml:timePositionϾ Ͻgt:qualityϾ Ͻmeta:QuantitativeAssessmentϾ Ͻmeta:explanationϾErrorϽ/meta:explanationϾ Ͻmeta:values uomϭ``Ma''Ͼ5Ͻ/meta:valuesϾ Ͻ/meta:QuantitativeAssessmentϾ Ͻ/gt:qualityϾ Ͻgt:statusϾInformalϽ/gt:statusϾ Ͻgt:observationalBasis xlink:hrefϭ``dates.xml#PHOrigin1''/Ͼ Ͻgt:observationalBasis xlink:hrefϭ``dates.xml#PHOrigin2''/Ͼ Ͻgt:observationalBasis xlink:hrefϭ``dates.xml#PHOrigin3''/Ͼ Ͻgt:metadataϾϽgsml:ObjectMetadataϾϽgsml:sourceReference xlink:hrefϭ``citations.xml#chronos''/ϾϽ/gsml:ObjectMetadataϾ Ͻ/gt:metadataϾ Ͻ/gt:StratigraphicDateEstimateϾ Ͻ/gt:positionϾ Ͻgt:event xlink:hrefϭ``urn:x-seegrid:items:exceptions:unknown''/Ͼ Ͻgt:stratotype xlink:hrefϭ``gssp.xml#PHOriginPoint1''/Ͼ Ͻ/gt:GeochronologicBoundaryϾ Ͻ/gt:referencePointϾ Ͻ/gt:GeologicTimeScaleϾ
Note that in this and the subsequent ex- namespace for which the ``gt'' pre®x is used. ements from the ``meta'' namespace, and amples, the XML document is composed of Components from other parts of GeoSciML components relating to observations and mea- elements from several different namespaces use the pre®x ``gsml.'' General components surements and sampling, are from the ``om'' (Bray et al., 1999). The components that are inherited from GML are indicated by the and ``sa'' namespaces. speci®c to the geologic time scale are in a ``gml'' namespace pre®x, some supporting el- The time scale is contained within a
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GeologicTimeScale element, and is composed and as the values of start or end properties of ponent. Although it is not necessary for the of a set of GeochronologicEra, Numeric- the relevant eras. Note that these have multiple handle to have any semantic signi®cance, here Boundary, and GeochronologicBoundary names, which are equivalent. The Archean- we use the standard symbols for the handle elements. Proterozoic boundary is a NumericBoundary for eras as a mnemonic device (except that G The role of each GeochronologicEra in the whose position is given as a TimeInstant. On instead of is used for the Cambrian Period. GeologicTimeScale is indicated by its contain- the other hand, the Proterozoic-Phanerozoic This allows encoding using the reduced char- er component element. GeochronologicEra boundary is a GeochronologicBoundary, for acter set available on most standard key- carries a name, start, end, and rank properties. which the position is a StratigraphicDate- boards), and for boundaries we concatenate Ordering of the GeochronologicEras is encod- Estimate, based on several (links to) Date- the symbols for the two adjoining eras with ed in the sequence of elements in the XML Measurements. For a more complete illustra- an underscore. tion of StratigraphicDateEstimate and Date- document. Following a standard GML pattern Listing 2 shows an expansion of the Phan- (Cox et al., 2004), the values of the start and Measurement, see Listing 3 below. The erozoic eon. This has three member end properties are given via references that boundary is associated with an event, and has GeochronologicEra elements, describing the link to de®nitions of boundaries available a stratotype, whose value is a link to a de- Paleozoic, Mesozoic, and Cenozoic eras. The elsewhere. The value of each link is a URI- scription of a StratigraphicPoint. For a more Reference (Berners-Lee et al., 1998), pointing complete illustration of StratigraphicEvent, conventional decomposition of the Paleozoic to a fragment in a document identi®ed by a and StratigraphicPoint, see the discussion of is shown by ®ve member elements carrying URI. In the examples shown here, many of the Listing 4 below. links (to descriptions of eras not shown here) references are internal to the same document, Note that each of the elements representing and a ®nal member element which contains a so the short-form of pointer is used, compris- distinct identi®able objects (i.e., those that in- GeochronologicEra element describing the ing a pound symbol followed by the handle of stantiate classes shown in Fig. 5) carries Permian period. The latter is decomposed fur- the target element. ``gml:id'' attribute. The value of this is unique ther into GeochronologicEra elements repre- Both the NumericBoundary and Geochron- within the document, and provides a handle senting (a subset of) the relevant epochs and ologicBoundary play the same role, as for the document element and its contents, ages. Elements describing subage and chron referencePoints in the GeologicTimeScale, which supports cross-references to this com- ranks are not shown.
Listing 2. The Phanerozoic era, decomposed to age level (Late Permian only). Ͻgt:GeochronologicEra gml:idϭ``PH''Ͼ Ͻgml:nameϾPhanerozoicϽ/gml:nameϾ Ͻgt:start xlink:hrefϭ``#PRPH''/Ͼ Ͻgt:end xlink:hrefϭ``#present''/Ͼ Ͻgt:memberϾ Ͻgt:GeochronologicEra gml:idϭ``PZ''Ͼ Ͻgml:descriptionϾ Paleozoic Era. Note that this era de®nition contains references to some eras that are not yet described here: viz. G, O, S, D, C.Ͻ/gml:descriptionϾ Ͻgml:nameϾPaleozoicϽ/gml:nameϾ Ͻgt:start xlink:hrefϭ``#PRPH''/Ͼ Ͻgt:end xlink:hrefϭ``#PZMZ''/Ͼ Ͻgt:member xlink:titleϭ``Cambrian'' xlink:hrefϭ``#G''/Ͼ Ͻgt:member xlink:titleϭ``Ordovician'' xlink:hrefϭ``#O''/Ͼ Ͻgt:member xlink:titleϭ``Silurian'' xlink:hrefϭ``#S''/Ͼ Ͻgt:member xlink:titleϭ``Devonian'' xlink:hrefϭ``#D''/Ͼ Ͻgt:member xlink:titleϭ``Carboniferous'' xlink:hrefϭ``#C''/Ͼ Ͻgt:memberϾ Ͻgt:GeochronologicEra gml:idϭ``P''Ͼ Ͻgml:descriptionϾ Permian-Carboniferous time scale is derived from calibrating a master composite section to selected radio- metric agesϽ/gml:descriptionϾ Ͻgml:nameϾPermianϽ/gml:nameϾ Ͻgt:start xlink:hrefϭ``#CP''/Ͼ Ͻgt:end xlink:hrefϭ``#PZMZ''/Ͼ Ͻgt:memberϾ Ͻgt:GeochronologicEra gml:idϭ``P1''Ͼ Ͻgml:nameϾCisuralianϽ/gml:nameϾ Ͻgt:start xlink:hrefϭ``#CP''/Ͼ Ͻgt:end xlink:hrefϭ``#P1P2''/Ͼ Ͻgt:group xlink:hrefϭ``#P''/Ͼ Ͻgt:rankϾEpochϽ/gt:rankϾ Ͻ/gt:GeochronologicEraϾ
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Ͻ/gt:memberϾ Ͻgt:memberϾ Ͻgt:GeochronologicEra gml:idϭ``P2''Ͼ Ͻgml:nameϾGuadalupianϽ/gml:nameϾ Ͻgt:start xlink:hrefϭ``#P1P2''/Ͼ Ͻgt:end xlink:hrefϭ``#P2P3''/Ͼ Ͻgt:group xlink:hrefϭ``#P''/Ͼ Ͻgt:rankϾEpochϽ/gt:rankϾ Ͻ/gt:GeochronologicEraϾ Ͻ/gt:memberϾ Ͻgt:memberϾ Ͻgt:GeochronologicEra gml:idϭ``P3''Ͼ Ͻgml:nameϾLopingianϽ/gml:nameϾ Ͻgt:start xlink:hrefϭ``#P2P3''/Ͼ Ͻgt:end xlink:hrefϭ``#PZMZ''/Ͼ Ͻgt:memberϾ Ͻgt:GeochronologicEra gml:idϭ``p8''Ͼ Ͻgml:nameϾWuchiapingianϽ/gml:nameϾ Ͻgt:start xlink:hrefϭ``#P2P3''/Ͼ Ͻgt:end xlink:hrefϭ``#p8p9''/Ͼ Ͻgt:group xlink:hrefϭ``#P3''/Ͼ Ͻgt:rankϾAgeϽ/gt:rankϾ Ͻ/gt:GeochronologicEraϾ Ͻ/gt:memberϾ Ͻgt:memberϾ Ͻgt:GeochronologicEra gml:idϭ``p9''Ͼ Ͻgml:nameϾChanghsingianϽ/gml:nameϾ Ͻgt:start xlink:hrefϭ``#p8p9''/Ͼ Ͻgt:end xlink:hrefϭ``#PZMZ''/Ͼ Ͻgt:group xlink:hrefϭ``#P3''/Ͼ Ͻgt:rankϾAgeϽ/gt:rankϾ Ͻ/gt:GeochronologicEraϾ Ͻ/gt:memberϾ Ͻgt:group xlink:hrefϭ``#P''/Ͼ Ͻgt:rankϾEpochϽ/gt:rankϾ Ͻ/gt:GeochronologicEraϾ Ͻ/gt:memberϾ Ͻgt:group xlink:hrefϭ``#PZ''/Ͼ Ͻgt:rankϾPeriodϽ/gt:rankϾ Ͻ/gt:GeochronologicEraϾ Ͻ/gt:memberϾ Ͻgt:group xlink:hrefϭ``#PH''/Ͼ Ͻgt:rankϾEraϽ/gt:rankϾ Ͻ/gt:GeochronologicEraϾ Ͻ/gt:memberϾ Ͻgt:memberϾ Ͻgt:GeochronologicEra gml:idϭ``MZ''Ͼ Ͻgml:descriptionϾ Mesozoic Era Note that this era de®nition contains references to some eras that are not yet described here: viz. T, J K.Ͻ/gml:descriptionϾ Ͻgml:nameϾMesozoicϽ/gml:nameϾ Ͻgt:start xlink:hrefϭ``#PZMZ''/Ͼ Ͻgt:end xlink:hrefϭ``#MZCZ''/Ͼ Ͻgt:member xlink:titleϭ``Triassic'' xlink:hrefϭ``#T''/Ͼ Ͻgt:member xlink:titleϭ``Jurassic'' xlink:hrefϭ``#J''/Ͼ Ͻgt:member xlink:titleϭ``Cretaceous'' xlink:hrefϭ``#K''/Ͼ Ͻgt:group xlink:hrefϭ``#PH''/Ͼ Ͻgt:rankϾEraϽ/gt:rankϾ Ͻ/gt:GeochronologicEraϾ Ͻ/gt:memberϾ
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Ͻgt:memberϾ Ͻgt:GeochronologicEra gml:idϭ``CZ''Ͼ Ͻgml:descriptionϾ Cenozoic Era Note that this era de®nition contains references to some eras that are not yet described here: viz. Pg, Ng.Ͻ/gml:descriptionϾ Ͻgml:nameϾCenozoicϽ/gml:nameϾ Ͻgt:start xlink:hrefϭ``#MZCZ''/Ͼ Ͻgt:end xlink:hrefϭ``#present''/Ͼ Ͻgt:member xlink:titleϭ``Paleogene'' xlink:hrefϭ``#Pg''/Ͼ Ͻgt:member xlink:titleϭ``Neogene'' xlink:hrefϭ``#Ng''/Ͼ Ͻgt:group xlink:hrefϭ``#PH''/Ͼ Ͻgt:rankϾEraϽ/gt:rankϾ Ͻ/gt:GeochronologicEraϾ Ͻ/gt:memberϾ Ͻgt:rankϾEonϽ/gt:rankϾ Ͻ/gt:GeochronologicEraϾ
Listing 2 primarily illustrates how the component GeochronologicEra elements are nested, following the structure of TimeOrdinal- ReferenceSystem given by ISO 19108. Listing 3 shows the details of two GeochronologicBoundary elements, which delimit the Changhsingian age shown in Listing 2. The estimate of the time position of each is carried by a StratigraphicDateEstimate element, each of which in turn points to their observationalBasis in the form of DateMeasurements shown in Listing 4. The structure of DateMeasurement follows the Observations and Measurements model (Cox, 2003) to capture various metadata about the details of the measurement. In the cases shown here, the target of all DateMeasurements is indicated simply as the stratotype, but in general a feature such as a GeochronSpecimen may be indicated, supporting a full record of the details of the experimental process. Each boundary has event and stratotype elements which carry links to a StratigraphicEvent and StratigraphicPoint, respectively.
Listing 3. Two Geochronologic Boundry descriptions and associated StratigraphicDataEstimate and the DateMeasurement elements relating to one of the eras shown in Listing 2. Ͻgt:referencePointϾ Ͻgt:GeochronologicBoundary gml:idϭ``p8p9''Ͼ Ͻgml:nameϾBase of ChanghsingianϽ/gml:nameϾ Ͻgt:previousEra xlink:hrefϭ``#p8''/Ͼ Ͻgt:nextEra xlink:hrefϭ``#p9''/Ͼ Ͻgt:positionϾ Ͻgt:StratigraphicDateEstimate gml:idϭ``p9Origin''Ͼ Ͻgml:timePosition frameϭ``tcs.xml#geologyMA''Ͼ-253.8Ͻ/gml:timePositionϾ Ͻgt:qualityϾ Ͻmeta:QuantitativeAssessmentϾ Ͻmeta:explanationϾErrorϽ/meta:explanationϾ Ͻmeta:values uomϭ``Ma''Ͼ0.7Ͻ/meta:valuesϾ Ͻ/meta:QuantitativeAssessmentϾ Ͻ/gt:qualityϾ Ͻgt:statusϾInformalϽ/gt:statusϾ Ͻgt:observationalBasis xlink:hrefϭ``dates.xml#p9Origin1''/Ͼ Ͻgt:metadataϾϽgsml:ObjectMetadataϾϽgsml:sourceReference xlink:hrefϭ``citations.xml#Ogg1''/ϾϽ/gsml:ObjectMetadataϾ Ͻ/gt:metadataϾ Ͻ/gt:StratigraphicDateEstimateϾ Ͻ/gt:positionϾ Ͻgt:event xlink:hrefϭ``gssp.xml#p8p91''/Ͼ Ͻgt:stratotype xlink:hrefϭ``gssp.xml#gsspbasechanghsingian''/Ͼ Ͻ/gt:GeochronologicBoundaryϾ Ͻ/gt:referencePointϾ Ͻgt:referencePointϾ Ͻgt:GeochronologicBoundary gml:idϭ``PZMZ''Ͼ Ͻgml:nameϾBase of MesozoicϽ/gml:nameϾ Ͻgml:nameϾBase of TriassicϽ/gml:nameϾ Ͻgml:nameϾBase of Lower TriassicϽ/gml:nameϾ
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Ͻgml:nameϾBase of InduanϽ/gml:nameϾ Ͻgt:previousEra xlink:hrefϭ``#p9''/Ͼ Ͻgt:previousEra xlink:hrefϭ``#P''/Ͼ Ͻgt:previousEra xlink:hrefϭ``#PZ''/Ͼ Ͻgt:nextEra xlink:hrefϭ``#MZ''/Ͼ Ͻgt:nextEra xlink:hrefϭ``#T''/Ͼ Ͻgt:nextEra xlink:hrefϭ``#t1''/Ͼ Ͻgt:positionϾ Ͻgt:StratigraphicDateEstimate gml:idϭ``MZOrigin''Ͼ Ͻgml:timePosition frameϭ``tcs.xml#geologyMA''Ͼ-251.0Ͻ/gml:timePositionϾ Ͻgt:qualityϾ Ͻmeta:QuantitativeAssessmentϾ Ͻmeta:explanationϾErrorϽ/meta:explanationϾ Ͻmeta:values uomϭ``Ma''Ͼ0.4Ͻ/meta:valuesϾ Ͻ/meta:QuantitativeAssessmentϾ Ͻ/gt:qualityϾ Ͻgt:statusϾGSSP Rati®ed 2001Ͻ/gt:statusϾ Ͻgt:observationalBasis xlink:hrefϭ``dates.xml#MZOrigin1''/Ͼ Ͻgt:metadataϾϽgsml:ObjectMetadataϾϽgsml:sourceReference xlink:hrefϭ``citations.xml#E24p1022001''/ϾϽ/gsml:Object- MetadataϾϽ/gt:metadataϾ Ͻ/gt:StratigraphicDateEstimateϾ Ͻ/gt:positionϾ Ͻgt:event xlink:hrefϭ``gssp.xml#PZMZ1''/Ͼ Ͻgt:stratotype xlink:hrefϭ``gssp.xml#gsspbasetriassic''/Ͼ Ͻ/gt:GeochronologicBoundaryϾ Ͻ/gt:referencePointϾ
Listing 4 contains the details of DateMeasurements that are the basis for StratigraphicDateEstimates. This listing is referred to as dates.xml in Listing 3.
Listing 4. Details of DateMeasurements that are the basis for StratigraphicDataEstimates. This listing is referred to as dates.xml in Listing 3. Ͻgml:featureMemberϾ Ͻgt:DateMeasurement gml:idϭ``p9Origin1''Ͼ Ͻgml:descriptionϾCalibration of a master composite section to selected radiometric agesϽ/gml:descriptionϾ Ͻom:timeϾ Ͻgml:TimeInstant gml:idϭ``p9OriginMeasured''Ͼ Ͻgml:timePositionϾ2001Ͻ/gml:timePositionϾ Ͻ/gml:TimeInstantϾ Ͻ/om:timeϾ Ͻom:location xlink:hrefϭ``urn:x-ogc:def:nil:OGC:unknown''/Ͼ Ͻom:procedure xlink:hrefϭ``http://www.stratigraphy.org/procedures/geochronology/compositeSectionCalibration''/Ͼ Ͻom:observedProperty xlink:hrefϭ``urn:x-ogc:def:phenomenon:OGC:Age''/Ͼ Ͻom:qualityϾ Ͻmeta:QuantitativeAssessmentϾ Ͻmeta:explanationϾErrorϽ/meta:explanationϾ Ͻmeta:values uomϭ``Ma''Ͼ0.7Ͻ/meta:valuesϾ Ͻ/meta:QuantitativeAssessmentϾ Ͻ/om:qualityϾ Ͻom:featureOfInterest xlink:hrefϭ``gssp.xml#gsspbasechanghsingian''/Ͼ Ͻom:result uomϭ``Ma''Ͼ253.8Ͻ/om:resultϾ Ͻ/gt:DateMeasurementϾ Ͻ/gml:featureMemberϾ Ͻgml:featureMemberϾ Ͻgt:DateMeasurement gml:idϭ``MZOrigin1''Ͼ Ͻgml:descriptionϾU-Pb ages bracket GSSPϽ/gml:descriptionϾ Ͻom:timeϾ Ͻgml:TimeInstant gml:idϭ``MZOriginMeasured''Ͼ
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Ͻgml:timePositionϾ1998Ͻ/gml:timePositionϾ Ͻ/gml:TimeInstantϾ Ͻ/om:timeϾ Ͻom:location xlink:hrefϭ``urn:x-ogc:def:nil:OGC:unknown''/Ͼ Ͻom:procedure xlink:hrefϭ``http://www.stratigraphy.org/procedures/geochronology/UPb''/Ͼ Ͻom:observedProperty xlink:hrefϭ``urn:x-ogc:def:phenomenon:OGC:Age''/Ͼ Ͻom:qualityϾ Ͻmeta:QuantitativeAssessmentϾ Ͻmeta:explanationϾErrorϽ/meta:explanationϾ Ͻmeta:values uomϭ``Ma''Ͼ0.4Ͻ/meta:valuesϾ Ͻ/meta:QuantitativeAssessmentϾ Ͻ/om:qualityϾ Ͻom:featureOfInterest xlink:hrefϭ``gssp.xml#gsspbasetriassic''/Ͼ Ͻ/om:qualityϾ Ͻom:featureOfInterest xlink:hrefϭ``gssp.xml#gsspbasetriassic''/Ͼ Ͻom:result uomϭ``Ma''Ͼ251.0Ͻ/om:resultϾ Ͻgt:metadataϾ Ͻgsml:ObjectMetadataϾ Ͻgsml:sourceReferenceϾ Ͻmeta:SimpleCitation gml:idϭ``B1998''Ͼ Ͻgml:nameϾBowring et al., 1998Ͻ/gml:nameϾ Ͻ/meta:SimpleCitationϾ Ͻ/gsml:sourceReferenceϾ Ͻ/gsml:ObjectMetadataϾ Ͻ/gt:metadataϾ Ͻ/gt:DateMeasurementϾ Ͻ/gml:featureMemberϾ
Finally, Listing 5 illustrates the structure of the descriptions of StratigraphicEvent and StratigraphicPoint elements referred to by the event and stratotype properties of the GeochronologicBoundary elements in Listing 3. Note that since a StratigraphicPoint element potentially describes a golden spike in the calibration of the time scale, this has a status property to indicate if it has been rati®ed through the GSSP program.
Listing 5. The StratigraphicPoint and StratigraphicEvent elements associated with the boundries shown in Listing 3. Ͻgml:featureMemberϾ Ͻgt:StratigraphicPoint gml:idϭ``gsspbasechanghsingian''Ͼ Ͻgml:descriptionϾLeading candidates are in ChinaϽ/gml:descriptionϾ Ͻmeta:reference xlink:hrefϭ``citations.xml#Ogg1''/Ͼ Ͻsa:surveyDetails xlink:hrefϭ``urn:x-ogc:def:nil:OGC:unknown''/Ͼ Ͻsa:relatedObservation xlink:hrefϭ``dates.xml#p9Origin1''/Ͼ Ͻsa:position xlink:hrefϭ``urn:x-ogc:def:nil:OGC:unknown''/Ͼ Ͻgt:boundary xlink:hrefϭ``ICStimescale.xml#p8p9''/Ͼ Ͻgt:hostSection xlink:titleϭ``China''/Ͼ Ͻgt:offset uomϭ``m'' xsi:nilϭ``true''/Ͼ Ͻgt:primaryGuidingCriterionϾConodont biostratigraphyϽ/gt:primaryGuidingCriterionϾ Ͻgt:event xlink:hrefϭ``#p8p91''/Ͼ Ͻgt:statusϾInformalϽ/gt:statusϾ Ͻ/gt:StratigraphicPointϾ Ͻ/gml:featureMemberϾ Ͻgml:featureMemberϾ Ͻgt:StratigraphicPoint gml:idϭ``gsspbasetriassic''Ͼ Ͻgml:descriptionϾBase of Bed 27c, Meishan, Zhejiang, ChinaϽ/gml:descriptionϾ Ͻmeta:reference xlink:hrefϭ``citations.xml#E24p1022001''/Ͼ Ͻsa:surveyDetails xlink:hrefϭ``urn:x-ogc:def:nil:OGC:unknown''/Ͼ Ͻsa:relatedObservation xlink:hrefϭ``dates.xml#MZOrigin1''/Ͼ Ͻsa:position xlink:hrefϭ``urn:x-ogc:def:nil:OGC:unknown''/Ͼ Ͻgt:boundary xlink:hrefϭ``ICStimescale.xml#PZMZ''/Ͼ Ͻgt:hostSection xlink:titleϭ``Bed 27c, Meishan, Zhejiang, China''/Ͼ Ͻgt:offset uomϭ``m''Ͼ0.0Ͻ/gt:offsetϾ
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Ͻgt:primaryGuidingCriterionϾConodont biostratigraphyϽ/gt:primaryGuidingCriterionϾ Ͻgt:event xlink:hrefϭ``#PZMZ1''/Ͼ Ͻgt:additionalCorrelationPropertyϾTermination of major negative carbon-isotope excursionϽ/gt:additionalCorrelationPropertyϾ Ͻgt:additionalCorrelationPropertyϾAbout 1 myr after peak of Late Permian extinctions.Ͻ/gt:additionalCorrelationPropertyϾ Ͻgt:statusϾGSSP Rati®ed 2001Ͻ/gt:statusϾ Ͻ/gt:StratigraphicPointϾ Ͻ/gml:featureMemberϾ Ͻgml:featureMemberϾ Ͻgt:StratigraphicEvent gml:idϭ``p8p91''Ͼ Ͻgml:descriptionϾNear lowest occurrence of conodont Clarkina wangiϽ/gml:descriptionϾ Ͻom:time xlink:hrefϭ``ICStimescale.xml#p8p9/gt:position''/Ͼ Ͻom:locationϾ Ͻom:LocationCodeϾ Ͻom:geographicDescriptionϾGlobalϽ/om:geographicDescriptionϾ Ͻ/om:LocationCodeϾ Ͻ/om:locationϾ Ͻ/gt:StratigraphicEventϾ Ͻ/gml:featureMemberϾ Ͻgml:featureMemberϾ Ͻgt:StratigraphicEvent gml:idϭ``PZMZ1''Ͼ Ͻgml:descriptionϾConodont, lowest occurrence of Hindeodus parvusϽ/gml:descriptionϾ Ͻom:time xlink:hrefϭ``ICStimescale.xml#P1P2/gt:position''/Ͼ Ͻom:locationϾ Ͻom:LocationCodeϾ Ͻom:geographicDescriptionϾGlobalϽ/om:geographicDescriptionϾ Ͻ/om:LocationCodeϾ Ͻ/om:locationϾ Ͻ/gt:StratigraphicEventϾ Ͻ/gml:featureMemberϾ
The examples shown in Listings 1±5 show Interval scale or coordinate systemÐa val- coordinate system or ratio scale. It is impor- a representative subset of a time scale. A com- ue on an interval scale describes position rel- tant to note that while an ordinal system de- plete time scale would reuse the patterns ative to a datum or origin. The distance from pends on the ordering of the events that de®ne shown here for the full set of eras of all ranks the datum is given as an amount scaled by a the boundaries between units in the system; and their associated boundaries. unit of measure, in arbitrary precision. The the positions of these boundary events is not position of the origin of an interval scale is necessarily known. Walsh et al. (2004) refer SOME THEORETICAL IMPLICATIONS arbitrary, so positions on both sides of a da- to ordinal units as classi®catory pigeonholes. tum are possible, hence the value must be The differences between the types of scale Reference Systems and Time Scales signed. The value of a potential must be ex- are also shown by the operations that are valid pressed using an interval scale. In a temporal on values using them and their results. The There has been some discussion of the re- context, a position or date may be expressed relative quantities of two measures may be de- lationship between the geologic time scale and as a numeric value relative to a time coordi- termined by subtraction or division, with the other measurement systems (Walsh et al., nate system. In geochronology, the conven- result being a measure or a ratio, respectively. 2004). The model for temporal reference sys- tional origin for numeric scales is 1950, The relative separation of two positions on an tems summarized above provides a useful though this is only distinguishable from ``the interval scale may only be determined by sub- framework for this. Broadly, there are three present'' for very high precision dating meth- traction, with the result being an amount on a kinds of reference system or scales involved ods dealing with the relatively recent past. ratio scale. here: Ordinal reference system or ordinal scaleÐ The common practice of giving geologic Ratio or absolute scaleÐa value on a ratio values given as an ordinal unit or classi®er, age as an unsigned number is consistent with scale describes the ``amount of'' something. denoted by a symbol such as a word or code. considering age to describe the ``amount of The amount, or measure, is given as an un- Relative sizes or positions may be described years'' in an object. Age and temporal signed number that is scaled by some unit of using an ordinal reference system, with a ®xed position are often used interchangeably in measure. This may be expressed in arbitrary precision determined by the extent of the or- geochronology, with little confusion, because precision (though not necessarily accurate or dinal unit, which may vary across the scale. of the practice of setting the datum as the pres- meaningful). Mass, length, and concentration Ordinals may be used for classi®cation of ab- ent. In the context of the encoding shown are measured on ratio scales. In a temporal solute values (e.g., the well-known grain-size here, the position of a boundary is given as a context, the length of a time interval or the classi®cations in sedimentology) as well as signed number on an interval scale, while the age of an object may be given as a number of position (the geologic time scale), so the or- result of an age determination should be a seconds, years, etc. dinal scale may be calibrated against either a measure on a ratio scale. However, in order to
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utilize the standard structures provided by ISO 19108:2003 and GML, the Stratigraphic- DateEstimate inherits the position property from TMInstant, and thus gives the value as a (signed) position on an interval scale.
Ordinal Reference System versus Constrained Topology
The GeologicalTimeScale described here is structured as a TimeComplex, composed of eras and boundaries corresponding to the time Figure 6. Schematic topological complex, illustrating the constraints required for this to edges and time nodes of a temporal topology serve as a reference system. complex (ISO 19108:2003). There are two is- sues with expressing the time scale as a to- pology complex, however. numerical positions of the end points of some of UML that allows us to generate an XML The ®rst is that this would require multiple- or all of the eras are not known, or not known encoding compatible with geospatial standards inheritance, with the TORS class deriving precisely. from ISO and OGC. The latter means that in- from both TMReferenceSystem and The parts of the graph colored blue contain formation related to the time scale may be TMTopologyComplex. While useful in prin- an alternative primary decomposition of era B, transferred using standard Web-service inter- ciple, multiple inheritance is notoriously prob- labeled b1, b2, etc, where the elements of the faces, such as Web Feature Service. lematic in practice, and alternatives such as decomposition have the same rank as the el- The UML model and XML schema, and ex- interfaces are commonly used. Thus, in ISO ements in the existing decomposition. Note ample instances described in this report, are 19108 the concepts of ordinal reference sys- that, unless the positions of the nodes are pre- available online from https://www.seegrid. tem and topology complex are kept separate. cisely calibrated on a numeric scale, it is not csiro.au/subversion/xmml/trunk/GeoSciML/ This re¯ects a preference for single-inheritance possible to determine the relative temporal po- draft/model/, https://www.seegrid.csiro.au/ in the model, with the ordinal reference sys- sitions of features whose ages are b3 and B24. subversion/xmml/trunk/GeoSciML/draft/ tem grouped with reference systems rather The order of components is ambiguous, so the schema/, and https://www.seegrid.csiro.au/ than topology complexes. complex including both does not qualify as a subversion/xmml/trunk/GeoSciML/draft/ The second issue concerns the constraints valid reference system. instances/geoTime/. that must be imposed so that the complex can The blue subset may, however, comprise a ful®ll the requirements of a reference system. different reference system for era B, for ex- APPENDIX 1. INTRODUCTION TO UML These are as follows. The ordinal eras and or- ample, having a different (spatial) domain of CLASS DIAGRAMS dinal era boundaries must form a connected, validity. Note that the temporal relationship The UML (Object Management Group, 2001) is covering network or complex for the domain between objects characterized using different a well-known notation, and is described in many of the reference system. Furthermore, the reference systems is in general indeterminate. introductory and advanced books (e.g., Fowler and complex must be constrained such that each This describes the common situation in stra- Scott, 2000). It may be used to model various tech- era may only be subdivided once by a set of tigraphy where the relative age of objects from nical, social, and natural systems, and is commonly eras of a lower rank. In terms of the topology different regions may not be possible if local used for analysis of business processes and in soft- ware design, particularly of interfaces. complex, the set of edges that either starts or time scales are in use. Correlation projects at- The UML includes several diagram types. In this ends at any node must include exactly one of tempt to resolve this by discovering, or as- report we use only class diagrams (see Figs. 1±5). each rank between the highest and lowest rank serting, relationships between elements of These are super®cially similar to the entity-relation- represented. The single hierarchy that results time systems de®ned originally for different ship (E-R) notation used in data modeling for re- lational database design. However, the UML in- ensures there is no ambiguity in the relative domains of validity. If successful, this may re- cludes re®nements to support the description of positions of eras. We might term this an sult in a merging of different systems to form systems according to object-oriented principles. In UnambiguousTimeTopologyComplex. a single system (hierarchy) with a domain of particular, the relationships between concepts are For example, in the temporal topological validity that is the union of the domains of the classi®ed in various ways, indicated on the diagram complex shown in Figure 6, we show edges contributing systems. by different line and arrow styles with annotations. In addition to attributes, other kinds of properties representing eras as arrows, between nodes may be speci®ed for each concept. representing boundaries shown as ®lled cir- SUMMARY Furthermore, we use the capabilities of class di- cles. The eras have various ranks implied by agrams in a constrained way, broadly corresponding the thickness of the line, and are labeled B, We have presented an integrated model for to the pro®le described in ISO 19103 and in Annex E of the GML speci®cation (Cox et al., 2003). The the geologic time scale, its formal de®nition C1, etc. Some of the nodes are labeled B C, key elements used are summarized in the following B4 B5, etc. using type localities according to ICS guide- paragraphs. The parts of the graph colored green rep- lines, and the measurements involved in cali- Each concept of interest is represented as a class, resent a valid ordinal reference system. For brating it against a numeric scale. The model and shown on the class diagram as a multi-com- example, a feature assigned the age B22 is is represented using a formal notation, the partment box. The top compartment holds the name of the classi®er, optionally preceded by the name of unambiguously earlier than a feature of age UML Class Diagram, which is widely used in the package it belongs to. An instance of a class is B4, and is during the life of a feature of age software engineering and business-process called an object, with an ``is a'' relationship with B. These relationships are clear even if the analysis. Furthermore, we have used a pro®le the classi®er (e.g., Abby is a person). In the case of
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abstract classes, which exist to support a coherent gram, describing a different subset of relationships Faure, G., 1977, Principals of isotope geology: New York, class hierarchy but will never supply instances, the with other classes in each diagram. These provide John Wiley and Sons, 464 p. name is shown in italics. Attributes of the class are the joining points between the subsets of the model Fowler, M., and Scott, K., 2000, UML Distilled (second edition): Reading, Massachusetts, Addison-Wesley, listed in the second compartment, each by an entry shown in different diagrams. The complete set of 185 p. of the form ``name:Type,'' with optional cardinality. properties of a particular class is the union of prop- Gradstein, F.M., and Ogg, J.G., 2004, Geologic time scale Operations, responsibilities, constraints, tags, etc., erties shown where it appears in the various dia- 2004ÐWhy, how, and where next!: International are shown in additional compartments. We are pri- grams, together with other information that may not Commission on Stratigraphy, http://www.stratigraphy.org/ marily interested in class attributes. be shown on any diagram. GTS04.pdf (January 2005). Relationships between classes are indicated in the Thus, while UML diagrams may be constructed International Commission on Stratigraphy, 2004, Interna- diagram by lines of various styles. In this study, we with generic drawing tools (including paper and tional Stratigraphic Chart (Overview): International use two types of relationship: generalization and pencil), professional UML tools maintain an ab- Commission on Stratigraphy, http://www.stratigraphy.org/ over.htm (October 2005). association. stract representation of the model, and use that to International Organization for Standardization, 2002, ISO Association is denoted by a line that may be or- ensure consistency between different views. 19108:2002; Geographic informationÐTemporal namented with various arrowheads and labels at ei- Following the usage prescribed by ISO 19109 schema: Geneva, International Organization for Stan- ther or both ends. These indicate ``has a'' relation- and used in GML, class attributes and associations dardization, 56 p. ships between instances of the classes (e.g., Abby are referred to collectively as properties, with the International Organization for Standardization, 2003, ISO (a person) owns Iko (a cat)). Almost all relation- attribute name or association rolename providing 19103:2003; Geographic informationÐConceptual ships shown on an E-R diagram are comparable to the name of the property. Rolenames are required schema language, draft technical speci®cation: Gene- va, International Organization for Standardization, 71 p. UML associations. However, in the UML, these re- on the traversable ends of associations. Further- International Organization for Standardization, 2003, ISO lationships may be named, and each end of the as- more, following a lexical rule prescribed in GML 19107:2003; Geographic informationÐSpatial sche- sociation may also carry a rolename. Cardinality 3, classnames are in UpperCamelCase, while attri- ma: Geneva, International Organization for Standard- may be expressed as an integer or a range, where, bute and rolenames use lowerCamelCase as far as ization, 186 p. for example, ``2..*'' implies that at least two in- possible. International Organization for Standardization, 2004, ISO stances of the association are required but an unlim- 19109:2004; Geographic informationÐRules for ap- ited number may be provided. No cardinality con- plication schema: Geneva, International Organization ACKNOWLEDGMENTS for Standardization, 84 p. straint implies exactly one. The association may be International Organization for Standardization, 2004, ISO directed, shown by a stick arrowhead (→), such that This study was initiated as a contribution to the 19118:2004; Geographic informationÐEncoding: Ge- an instance of the class at only one end knows about neva, International Organization for Standardization, instances of the class at the other end. Filled and Chronos project. The work has been improved as a 117 p. open diamond-arrowheads may be used to indicate result of discussions with Cinzia Cervato, Morishi- Laskar, J., 1999, The limits of Earth orbital calculation for tight and loose association (known as composition ge Ota, Ilene Rex and Charles Roswell, and com- geological time scale use: Royal Society of London and aggregation), but are mostly not used here. ments by reviewer Peter Sadler. Cox's contributions Philosophical Transactions, ser. A, v. 357, were supported by the XMML consortium, CSIRO, p. 1735±1759. Specialization and/or generalization is denoted by Object Management Group, 2004, Uni®ed modeling lan- a line with an open arrowhead (᭞) adjacent to the and the Predictive Mineral Discovery Cooperative Research Center. guage (UML), version 2.0: Needham, Massachusetts, generalized class. This indicates a relationship at the Object Management Group, http://www.uml.org/ (Oc- model level, where the child class bears an ``is a tober 2005) type of'' relationship to a parent (e.g., a cat is a Remane, J., Bassett, M.G., Cowie, J.W., Gohrbandt, K.H., type of animal). Specialization usually adds attri- REFERENCES CITED Lane, H.R., Michelson, O., and Naiwen, W., 1996, Revised guidelines for the establishment of global butes and relationships to those inherited from the chronostratigraphic standards by the International parent class, but may involve other constraints. As Berners-Lee, T., Fielding, R., and Masinter, L., 1998, Uni- Commission on Stratigraphy (ICS): Episodes, v. 19, well as inheritance of properties, generalization usu- form resource identi®ers (URI): Generic syntax, re- p. 77±81. ally also implies polymorphism, such that instances quest for comments: Internet Engineering Task Force Shackleton, N.J., Crowhurst, S.J., Weedon, G., and Laskar, of the child class are considered to be instances of Report 2396, http://www.ietf.org/rfc/rfc2396.txt (Oc- L., 1999, Astronomical calibration of Oligocene- tober 2005) the parent. Thus, an association with a class implies Miocene time: Royal Society of London Philosophical Bray, T., Hollander, D., and Layman, A., eds., 1999, Na- Transactions, ser. A, v. 357, p. 1909±1927. a potential association with any of its descendents mespaces in XML: World Wide Web Consortium, Vretanos, P.A., ed., 2005, Web Feature Service 1.1; (e.g., if a person owns an animal, this might be a http://www.w3.org/TR/REC-xml-names/ (October OpenGIS Implementation Speci®cation: Open Geo- dog, cat, ®sh, or hamster, etc.). This last feature is 2005) spatial Consortium Document 04-094, https:// particularly important and is used extensively in the Cox, S.J.D., ed., 2003, Observations and measurements, portal.opengeospatial.org/®les/?artifactidϭ8339 (Oc- model here. OpenGIS Recommendation Paper: Open Geospatial tober 2005). It is important to understand that the diagram is Consortium Document 03-022r3, 129 p., http:// Walsh, S.L., Gradstein, F.M., and Ogg, J.C., 2004, History, portal.opengeospatial.org/®les/?artifactidϭ1324 (Oc- philosophy, and application of the Global Stratotype merely a representation of an underlying model. tober 2005). Section and Point (GSSP): Lethaia, v. 37, p. 201±218, Furthermore, one diagram will usually not show the Cox, S.J.D., Daisey, P.W., Lake, R., Portele, C., and White- doi: 10.1080/00241160410006500. entire model, but rather just a view of a selection side, A., eds., 2004, Geography Markup Language Yergeau, F., Bray, T., Paoli, J., Sperberg-McQueen, C.M., of related classes, perhaps with only certain prop- (GML) 3.1.1, OpenGIS Recommendation Paper: and Maler, E., eds., 2004, Extensible markup language erties displayed. This is convenient, since it means Open Geospatial Consortium Document 03-105r1, (XML) 1.0 (third edition): World Wide Web Consor- that unnecessary detail can be suppressed in order 580 p., http://portal.opengeospatial.org/®les/?artifact tium, http://www.w3.org/TR/REC-xml/ (October ϭ 2005). to allow a diagram to illustrate particular points. But id 4700 (October 2005). Fallside, D.C., Walmsley, P., Thompson, H.S., Beech, D., MANUSCRIPT RECEIVED BY THE SOCIETY 31 MAY 2005 in order to understand the entire model, it is nec- Maloney, M., Mendelsohn, N., Biron, P.V., and Mal- REVISED MANUSCRIPT RECEIVED 26 OCTOBER 2005 essary to combine the information from several hotra, A., eds., 2004, XML schema (second edition): MANUSCRIPT ACCEPTED 26 OCTOBER 2005 diagrams. World Wide Web Consortium, http://www.w3.org/TR/ Some classes will appear in more than one dia- xmlschema-0/ (October 2005) Printed in the USA
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