The use of terrain maps for practical purposes by persons who are not Geoscientists usually requires the preparation of interpretive or derivative maps. A derivative map presents terrain data in a form in which it can be easily understood and utilized by planners, resource managers and others (Ryder and MacLean, 1980). Usually, each derivative map will have a specific theme, such as geological hazards or aggregate resources. Some derivative maps are based on terrain information plus data from another type of thematic map, such as vegetation, or hydrologic features, or bedrock geology.
10.2 Examples
Derivative maps are made by selecting a subset of data from the base data (terrain) map. This subset may be one part of the terrain symbol, for example, hazard maps have been prepared by selecting the geological process symbol. Thus a terrain polygon mapped as "Cv/Mv-A" for example, would translate into an area of snow avalanche ("-A") hazard, as illustrated in Figure 12. More commonly, however, preparation of a derivative map is done by applying criteria that are based on more than one terrain component. For example, the hazard interpretation system indicated in Figure 13 is based on all components of the terrain symbol. The method of interpretation illustrated in Figure 12 is appropriate for small scale (e.g., 1:250,000) reconnaissance maps, whereas the method illustrated in Figure 13 is appropriate for larger scale maps.
In both methods of hazard interpretation described above, the resulting map gives a generous impression of the extent of hazardous terrain. A symbol such as "Cv/Rs-Rd" for example, would be used by a mapper to indicate that debris flows ("-Rd") occur within the terrain polygon. The debris flows would probably be restricted to specific "tracks" however, and large parts of the polygon would be unaffected. Thus the derivative map indicates the presence of a hazard, and more detailed mapping -- of specific debris flow tracks -- would be required to delimit specific hazard zones. It is also worth noting that derivative maps of hazards do not provide information about the frequency or magnitude of the hazardous processes: determination of these parameters is a step that would normally follow terrain mapping and preparation of the hazards map.
Derivative maps showing slope stability and erosion potential are commonly prepared from terrain maps for use in the planning of forestry activities. Usually, terrain mapping at moderately large scales (1:20,000 or larger) is carried out specifically for the preparation of stability maps. Soil drainage conditions and slope steepness are also mapped and used for the interpretation of slope stability. A combined terrain and slope stability map is illustrated in Figure 14, and the criteria that were applied to derive those stability classes are shown in the legend to that figure. Guidelines for the preparation of slope stability maps for forestry purposes are available from the Ministry of Forests, (e.g., Schwab, 1993; Mapping and Assessing Terrain Stability Guidebook, 1995). Derivative maps showing sediment sources are being used increasingly for management of forestry and other land uses on steep slopes. Sediment sources that are commonly mapped include sites where sediment is presently being released, such as recent debris slides, and undercut stream banks. Potential sediment sources, ie., steep slopes where road construction or clear cutting could result in initiation of slope failures, are also shown on sediment source maps. Thus preparation of the derivative map involves transfer of on-site symbols from the base map (e.g., symbols for debris slide scars and tracks, undercut stream banks, etc.) as well as identification of potentially unstable areas (i.e., terrain polygons) by application of criteria based on all parts of the terrain letter symbol.
Terrain maps are commonly translated into maps of aggregate resources by selecting areas underlain by gravely and sandy materials. Thus fluvial and glaciofluvial polygons are selected, and colluvium may also be indicated as a potential source of fill. To some extent, the quality of the aggregate source can be estimated from the geomorphological nature of the deposit. For example, ice-contact materials (e.g., gFGh) are likely to have more adverse characteristics, such as a high degree of variability,
lenses of silt, or boulder beds, than outwash gravels (e.g., gF*t).
Derivative maps showing "terrain constraints" to development of various facilities, such as waste disposal sites, roads, and residential subdivisions are prepared from terrain maps. Constraints are conditions that hinder construction or development but that can be overcome by appropriate engineering techniques. They include steep slopes, poor drainage, high water table, rocky ground, compressible soils, and the presence of permafrost. Such a map would be used for preliminary planning purposes, such as the identification of several alternative sites for a subdivision within a wider area. Maps of terrain constraints are prepared at various scales, as appropriate. For example, a scale of 1:50,000 is appropriate for a map showing terrain constraints to logging roads, where information is to be used for preliminary planning of road access in an undeveloped area. Terrain constraints to urban development are normally portrayed at large scales, such as 1: 10,000.
In recent years, derivative terrain maps have been used increasingly as an aid to both the planning and the interpretation of geochemical drift exploration surveys. These surveys are used to identify anomalously high concentrations of economic minerals in till, and then the source of the sediment is sought by examining underlying bedrock or by following glacier (or stream) flow paths upstream. These techniques are effective only where sediment (surficial material) was derived from underlying bedrock or where its source upstrearn can be identified. Where surficial materials are thick, and where the surface materials rest upon older sediments of a different kind, till sampling is less likely to yield information about underlying bedrock. Thus information about types of surficial materials and their thickness, glacier flow directions, and meltwater flow directions can be used for planning the location and lay-out of the geochemical survey grids (see Proudfoot et al., 1995). This information can be read directly from terrain maps, or, for use by people unfamiliar with the terrain legend, it can be shown on a derivative map (Fig. 15).
Derivative maps are derived from a terrain (or other) data map by the application of predetermined criteria that are based on the definitions of the classes that are to be portrayed on the derivative map, e.g., legend to Figure 12. As noted above, these criteria may refer to only one part of each terrain symbol, such as texture, or they may refer to two or more parts, or they may refer to parts of the terrain symbol plus additional mapped parameters, such as slope steepness. Criteria may also be based on data derived from more than one type of map. Where a digital (or other) data base contains additional information that is keyed to the polygons on the terrain map, then this too can be incorporated into the derivative criteria.
Interpretations can be done "by hand" by the terrain specialist, or by a GIS program. The advantage of the former method is that a person who is familiar with the mapped area can assess the significance of features observed in the field or on the air photos, but not shown on the terrain map, with regard to the interpretations. For example, slope profiles (irregular vs. smooth) and exposure to wind (and hence tree blow-down potential) may influence interpretations for slope stability in some polygons. However, this information can be entered into a GIS program to carry out various analyses, e.g., highlight all potentially unstable slopes, etc.
Terrain mapping and interpretations can be portrayed on a single map. In the simplest case, a terrain map can be colour coded according to the derivative criteria. Alternatively, interpretative symbols can be added to the terrain map (e.g., Fig. 14). This method of presentation has the advantage of allowing the map user to easily review the basic terrain data; it has the disadvantage of appearing cluttered; especially if terrain polygons appear small at the map scale.
If simple symbols are required, such as when maps will be used for public presentation, then a separate derivative map can be prepared. In this case, a copy of the line work (polygon boundary lines), to be used later for the derivative map, should be made when the terrain map is being drafted. The derivative map can be prepared on the same base map as the terrain map, or it can be presented as an overlay to the terrain map, or it can be presented on an orthophoto.
Minimum Requirements for Derivative Maps:
|