Landform Characterization with Geographic Information Systems Jacek S. BI Abstract landforms therefore often include the way they were formed, The ability to analyze and quantify morphology of the sur- their composition, and the environment in which they were face of the Earth in terms of landform characteristics is es- formed. sential for understanding of the physical, chemical, and The ability to map landforms is an important aspect of biological processes that occur within the landscape. How- any environmental or resource analysis and modeling effort. ever, because of the complexity of taxonomic schema for Traditionally, mapping of the aspects of the environment has landforms which include their provenance, composition, and been accomplished through in situ surveys. The advent of function, these features are difficult to map and quantify us- aerial photography and satellite remote sensing have made ing automated methods. The author suggests geographic in- surveys of large areas easier to accomplish, although this technology still requires in situ verification and ground-tru- formation systems (GIS) based methods for mapping and classification of the landscape suqface into what can be un- thing. While remote sensing technology can provide tremen- derstood as fourth-order-of-relief features and include convex dous amounts of information about the surface of the Earth, it areas and their crests, concave areas and their troughs, open is incapable of providing all of the data needed. The most concavities and enclosed basins, and horizontal and sloping complete approach to mapping the distribution of various en- flats. The features can then be analyzed statistically, aggre- vironmental parameters requires an integrated approach that gated into higher-order-of-relief forms, and correlated with relies on remote sensing and geographically referenced field other aspects of the environment to aid fuller classification survey data, whether in cartographic or tabular format. By of landforms. combining mapped data hom various sources, it is often pos- sible to make informed guesses about those characteristics of the environment that remain hidden to satellite sensors or aer- Introduction ial cameras. For example, through correlation of mapped in- The ability to analyze and quantify morphology of the sur- formation on vegetation types with data on local climate, face of the Earth is essential for understanding the physical, topography, hydrology, geology, and general distribution of chemical, and biological processes that occur within the soils and through application of the knowledge of surface pro- landscape. The shape of terrain influences flow of surface cesses that relate to soil formation, detachment, and transport, water, transport of sediment and pollutants, climate both on it is possible to make relatively accurate predictions regarding local and regional scales, nature and distribution of habitats distribution of different types of soils, something not currently for plant and animal species, and migration patterns of many possible using remote sensing data alone. animal species. It is also an expression of geologic and Traditionally, these types of studies have been performed weathering processes that have contributed to its formation. using a manual overlay process that relies on maps hand-drawn Knowledge of terrain morphology also is essential for any on transparent velum. Currently, geographic information sys- engineering or land-management endeavors that affect or dis- tems (GIS) permit integration of geographic data using comput- turb the surface of the land. ers. Surveyed information is entered into a GIs and presenred in The primary science that deals with understanding, de- digital format, where it can be combined with remote sensing scription, and mapping of the shape of terrain is geomor- images to generate new maps using various automated spatial phology, defined in the Random House Webster's Dictionary data processing algorithms. The knowledge-based process that as the study of the characteristics, origin, and development combines existing geographic data to generate new information of the form or surface features of the Earth, i.e., landforms. is generally known as cartographic modeling. The component Landforms are defined as specific geomorphic features on the maps that show the geographic distribution of a single environ- surface of the Earth, ranging from large-scale features such as mental parameter or a single category of parameters are known plains and mountain ranges to minor features such as indi- as geographic themes. vidual hills and valleys. Geomorphology encompasses a When a theme represents a category of information that spectrum of approaches to the study of landforms within two consists of many data elements, it is possible to generate major interrelated conceptual frameworks: functional and new maps from a single geographic theme by selectively dis- historical. The functional approach tries to explain the exis- playing the elements. For example, a single soil mapping tence of a landform in terms of the circumstances which sur- unit within a soils theme can have numerous attributes asso- round it and allow it to be produced, sustained, or trans- ciated with it, such as data on the organic matter content, pH formed such that the landform functions in a manner which factor, salt content, percentages of silt and clay, erodibility, reflects these circumstances, while the historical approach structure, and other information. Using a GIS, each kind of tries to explain the existing landform assemblage as a mix- - ture of effects resulting from the vicissitudes through which Photogrammetric Engineering & Remote Sensing, it has passed (Chorley et al., 1985). Taxonomic schemes for Vol. 63, No. 2, February 1997, pp. 183-191. National Applied Resource Sciences Center, Bureau of Land 0099-1112/97/6302-183$3.00/0 Management, Denver Federal Center, Bldg. 50, P.O. Box O 1997 American Society for Photogrammetry 25047, Lakewood, CO 80225-0047. and Remote Sensing PE&RS February 1997 information can be selectively identified, displayed, and (1959). According to Belcher, each landform presents sepa- saved as an independent data layer for use within a carto- rate and distinct soil characteristics, topography, rock materi- graphic model. als, and groundwater conditions. He adds that the recurrence In the above example, a GIS functions mainly as a dis- of the landform, regardless of the location, implies a recur- play or selection device for various soil attributes already rence of the basic characteristics of that landform (Belcher, known. However, a GIS also permits automated extraction of 1948). Lueder, on the other hand, describes a unit landform completely new information from an existing theme. This is as a terrain feature or terrain habit, usually of the third or- particularly the case with topographic data traditionally der, created by natural processes in such a way that it may available as elevation contour maps. From the contour map be described and recognized in terms of typical features various types of information, such as watershed boundaries, wherever it may occur, and which, when identified, provides steepest flow paths, slope gradients, slope aspects, and more, dependable information concerning its own structure and ei- can be derived by a specialist through painstaking manual ther composition and texture or uniformity (Lueder, 1959). In analysis of the shape of the contours and the distances be- identifying landforms as the third-order-of-relief features, tween contour lines. Lueder narrows the definition down by placing landforms In digital format, elevation data are generally available more in the context of shape of the land, and within the tra- as grids, where each grid cell represents a particular eleva- ditional orders-of-relief framework. In that framework, the tion above a certain vertical reference, such as the mean sea first-order-of-relief is represented by continents and ocean level. Geographic information systems include various algo- basins; the second order by mountain ranges, plains, conti- rithms that mathematically analyze the digital elevation data nental shelf, continental rise, and the abyssal plains; while to automatically derive information that otherwise would the third order by the landscape features such as individual take tremendous effort and amount of time to obtain. Typical hills, mountains, and valleys. Of course, for the purposes of terrain analysis algorithms in a GIs include methods for gen- precise mapping, the question still remains, where to place erating slope gradients, slope aspects, watershed boundaries, the boundaries of features. and flow paths. The added bonus of using these automated Lueder's definition serves as a departure point for analy- means is that new information is always calculated in pre- sis of landform characteristics based purely on elevation cisely the same way, eliminating subjective judgement on the data. While it is unclear whether identification of the third- part of the analyst. order-of-relief features will provide dependable information Automated extraction of new information from digital el- concerning the landform's own structure, composition, or evation models (DEMS) has many levels of complexity. Gener- texture, elevation data implicitly contains information on the ation of slope and aspect maps is a relatively simple process, shape, vertical order, and magnitude of relief features. How- while delineation of watershed boundaries