The OGC Web Coverage Processing Service (WCPS) Standard
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The OGC Web Coverage Processing Service (WCPS) Standard Peter Baumann Jacobs University Bremen 28759 Bremen Germany [email protected] ABSTRACT 1. MOTIVATION Imagery is more and more becoming integral part of geo More and more, imagery is becoming integral part of geo services. More generally, an increasing variety of sensors is services. In a more general perspective, a variety of remote generating massive amounts of data whose quantized nature and in situ sensors generate massive amounts of data where frequently leads to rasterized data structures. Examples in- the quantized nature of the measurements frequently leads clude 1-D time series, 2-D imagery, 3-D image time series to a rasterized data structure. Examples include 1-D time and x=y=z spatial cubes, and 4-D x=y=z=t spatio-temporal series, 2-D imagery, 3-D image time series and x=y=z spatial cubes. The massive proliferation of such raster data through cubes, and 4-D x=y=z=t spatiotemporal cubes. In parallel a rapidly growing number of services make open, standard- to the massive proliferation of such raster data through a ized service interfaces increasingly important. rapidly growing number of services the demand arises for not just simple data extraction and download services, but Geo service standardization is undertaken by the Open Geo- more flexible retrieval functionality. Spatial Consortium (OGC). The core raster service stan- dard is the Web Coverage Service (WCS) which specifies This foreseeably will soon go beyond just offering subsetting retrieval based on subsetting, scaling, and reprojection. In from some server-side imagery. Actually, we face the tran- 2008, OGC has issued a companion standard which adds sition from mere data services to value-adding information flexible, open-ended coverage processing capabilities. This services, including adequate data processing capabilities { in Web Coverage Processing Service (WCPS) specifies a cover- other words, we face a paradigm shift from data stewardship age processing language allowing clients to send requests of to service stewardship. arbitrary complexity for evaluation by the server. In this contribution we adopt a standards-based perspective, This contribution reports on the WCPS standard by giv- introducing a recently published standard for a high-level ing an introduction to its coverage model and processing language allowing complex server-side processing of multi- language. Further, design rationales are discussed, as well dimensional raster data. Standardization of open, inter- as background and relation to other OGC standards. 1-D operable geo services is performed by the Open GeoSpa- to 4-D use case scenarios illustrate intended use and bene- tial Consortium (OGC, www.opengeospatial.org) in collab- fits for different communities. Although the paper focuses oration with ISO, OASIS Open, W3C, and other relevant on conceptual issues, the WCPS reference implementation, bodies. One of the historically first and still most promi- PetaScope, is briefly addressed. The author is co-chair of nent standards is the Web Map Service (WMS) Implementa- the coverage-related working groups in OGC. tion Standard [9] which defines map image generation based on client-side parameter specification controlling server-side Categories and Subject Descriptors rendering ("portrayal"). While the results of such protrayal E.1 [DATA STRUCTURES]: Arrays; H.2.8 [DATABASE are ready for, e.g., immediate display in a Web browser, MANAGEMENT]: Database Applications|Image data- such rendered images usually are not suitable for further bases, Scientific databases, Spatial databases and GIS processing, e.g., in some analysis tool. For this purpose, the Web Coverage Service (WCS) Standard has been devel- oped [39]: "Unlike the WMS, which portrays spatial data to return static maps (rendered as pictures by the server), the Web Coverage Service provides available data together with their detailed descriptions; defines a rich syntax for re- quests against these data; and returns data with its original semantics (instead of pictures) which may be interpreted, extrapolated, etc. - and not just portrayed." WCS basically allows retrieval based on subsetting, scaling, and reprojec- tion. As such, WCS is the central OGC standard for simple, easy-to-use coverage services. The term "coverages" is used by ISO and OGC to denote a "space-varying phenomenon", which in practice today boils down to raster data. A more detailed discussion of the term will be given in Section 5.1. Effective December 2008, OGC has issued an extension to WCS which adds flexible, open-ended coverage processing capabilities to WCS. This Web Coverage Processing Service (WCPS) Interface Standard [2] specifies a processing lan- guage as client/server interface which can be characterized as "SQL for coverages". WCPS allows for manifold ad-hoc processing of coverage data, such as deriving the vegetation index, determining statistical evaluations, and generating Figure 1: Some of the most basic OGC services different kinds of plots, like classification, histograms, etc. The purpose of this contribution is to present the WCPS stones to which OGC standards need to adhere. For exam- standard to the geo and GIS community. To this end, the ple, OWS Common mandates a GetCapabilities request type remainder is organized as follows. Section 2 puts WCPS for every service which allows clients to retrieve information into the context of OGC's work. The main considerations about data sets offered and service capabilities provided by and requirements that have guided WCPS development are a server implementing the standard. Further, common sets discussed in Section 3, followed by a review of the state of of metadata as well as canonical structures for the request the art in Section 4. Section 5, the core part, presents WCPS XML schemas are laid down there. model and language concepts. For a practical assessment, a cross-dimensional set of use case scenarios is discussed in Figure 1 sketches the WFS, WCS, and CS-W standards Section 6. Section 7 gives a summary and outlook. suites, plus WMS. WFS-T, WCS-T, and CS-T represent so-called transactional services which allow for updating of 2. OGC data sets on a server. Filter Encoding (FE in Figure 1) and OGC issues a family of modular geo service standards which OWS Common Query Language (CQL) serve for predicated- are accessible free of cost. While the number of individual based retrieval on vector and meta data, respectively, a role specifications sometimes is perceived as a disadvantage, the which WCPS takes on for coverages. The core difference complexity of the matter actually mandates such modular- between WMS and the other interfaces mentioned is that ization. Of course, maintaining harmonization between the WMS delivers portrayed feature and coverage data which specifications represents a continuous challenge which re- are suitable for human viewing, but not for further process- mains on the agenda of the specification writing working ing; WFS, WCS, and CS-W, on the other hand, deliver data groups. without semantic loss so that the output can be fed into fur- ther processing tools or pipelines (such as GISs). A possible grouping of OGC specifications runs as follows: In this contribution we exclusively focus on service interface standards for coverages, which center around WCS. These • core services: for the classical triad of geo data { vec- are dealt with by the WCS Standards Working Group, in tor, raster, and metadata {, the Web Feature Service short: WCS.SWG, and the WCPS Working Group. In ad- (WFS) [29], WCS [39], and Catalog Service (CS-W) dition, the Coverages Discussion Working Group (Cover- [25] are provided. ages.DWG) acts as a platform for the exchange and dis- • value-adding services: on top of the core services, ad- cussion of coverage-related topics, such as features and their formulations in forthcoming specification versions. The au- ditional service specifications allow for browser-based 1 navigation (such as WMS), processing (such as the thor co-chairs these working groups . One very actively Web Processing Service (WPS) [34]), and additional contributing interest group is GALEON (Geo-Interface to service features (such as Digital Rights Management, Atmosphere, Land, Earth, Ocean, NetCDF) which regu- GeoRM [36]), to name but a few. larly reports about implementation experience as well as their view on requirements for future WCS versions; see • topical specification families, such as the Sensor Web www.ogcnetwork.net/galeon. Enablement (SWE) [7]. Historically, the WCS 1.0 specification has been the first attempt to standardize raster services. Some shortcomings All specifications are based on a common architecture in perceived with respect to clarity and conciseness have been support of OGC's vision of geospatial technology and data remedied in version 1.1, however at the cost of an overall interoperability. The Abstract Specification document series harder to understand specification, as it turned out. The provides the conceptual foundation for OGC specification current version is WCS 1.1 Corrigendum 2, in short: WCS development activities. Open interfaces and protocols are 1.1.2 [39]. Additionally, beyond the original scope of mainly built and referenced against the Abstract Specification, thus remote sensing imagery meantime a lot more communities enabling interoperability between different brands and dif- and their particular data types need to be considered, such ferent kinds of spatial processing systems. Several of the as 1-D time series