A terrain map shows numerous irregularly-shaped areas that are known as "terrain polygons". Ideally, polygons are delimited in such a way that terrain variability within each polygon is minimized. This means that the terrain within each polygon is relatively uniform with regard to the criteria upon which the terrain classification system is based: type and texture of surficial material, type of landform and/or material thickness, and present-day geomorphological processes (see Howes and Kenk, 1988). In practice, the variability of terrain is such that many polygons cannot be internally homogeneous; they include two or even three types of materials and landforms that are sufficiently intermixed that they cannot be separated at the scale of the mapping. In such cases, however, polygons are delimited so that the variability of terrain characteristics within a polygon is less than that between adjacent polygons.
The physical landscape is a mosaic of different kinds of materials and landforms (Mitchell, 1991). The simplest bit of the landscape, which can be referred to as a "basic element", is one kind of material formed by a single process and forming one kind of landform. An alluvial fan and a hillside covered with till of fairly uniform thickness are two examples of basic elements. A series of river terraces and a dune field are also basic elements. If map scale permits, a polygon should consist of a single element, ie., it should be internally homogeneous; this is a "simple terrain polygon". Alternatively, if single elements are too small to delimit, a polygon may include a repeated pattern of two or at most three basic elements; this is a "composite terrain polygon". Composite terrain polygons of bedrock and colluvium are common on steep mountain sides. Composite polygons of floodplain, river terraces and alluvial fans are common on valley floors. Wherever possible, composite terrain polygons should be delimited so that they include surficial elements of related genesis or surficial elements linked by common processes. For example, alluvial fans should be grouped with an adjacent floodplain, rather than with adjacent steep slopes; talus slopes and the rock bluffs from which the talus was derived are appropriately grouped together.
In other words, a polygon is a homogeneous part of the earth's surface. Internal variability within a polygon must be less than the difference between adjacent polygons. A single polygon can encompass a repetitive pattern of landforms and materials -- it can be homogeneous by virtue of its variability.
Basic elements vary considerably in area, for example a narrow floodplain may cut across an extensive till plain, and so adjacent terrain polygons may be of quite dissimilar size. The minimum size of a terrain polygon that is normally practical to delimit on a map is about 1 cm2. The average size of terrain polygons may vary from one landscape or physiographic region to another. Obviously, terrain polygons on a large scale map will encompass smaller true areas (i.e., less hectares) than polygons on a small scale map, as shown on Table 4.
Considerations outlined in Section 4.1 lead to definition of the following minimum requirements for mapping of terrain polygons.
Minimum Requirements:
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