12.0 Considerations Regarding the Reliability of Terrain Maps
The "reliability" of a terrain map refers to the extent to which it correctly shows the real characteristics of the terrain. Reliability can be considered in terms of the three types of information shown on the map: polygon boundary lines, letter symbols, and on-site symbols.
The reliability of polygon boundary lines can be assessed with regard to:
Accuracy: How well do boundary lines either delimit (or separate) the basic elements (Section 4.1 ) of the landscape (simple terrain polygons) or group them into composite terrain units? In other words, the spatial pattern of the boundary lines can be assessed. This will include characteristics such as the size and number of polygons.
Precision: This is represented by how far the boundary lines deviate from the true boundaries of the basic elements. The mapper indicates her/his impression of the precision of a boundary by the use of solid, dashed or dotted lines.
Both of these aspects of reliability are determined primarily by air photo interpretation: those parts of a polygon boundary lines that are actually checked in the field constitute only a very small proportion of their total length. Accuracy will depend partly upon the nature of the terrain that is being mapped -- how dissimilar are adjacent basic units and, therefore, how apparent are their boundaries. It will also depend upon the skill and experience of the air photo interpreter, especially where there are only subtle differences between adjacent basic units. Precision will depend upon the clarity of the boundaries, air photo scale, the points already mentioned above, and whether or not boundaries are partly obscured by forest.
The degree of correctness of terrain letter symbols depends upon how well the mapper is able to determine the type of material, its texture, and associated landforms within each polygon. This correctness will depend upon the skill and experience of the air photo interpreter, upon the effectiveness and amount of field checking carried out, and upon the variability of the materials, and so on, within each polygon.
The use and placement of on-site symbols also depends on mapper experience and air photo interpretation, because, except for mapping at TSIL A, few locations of on-site symbols are visited in the field.
Those aspects of a mapping project that influence the reliability of a terrain map can then be listed: