Terrain Analysis (Intermediate)
Overview of information products derivable from DTM's
Information products such as elevation, slope, aspect, profile curvature and plan curvature can quite easily be derived from digital terrain models (DTM) (see basic lesson Geländeanalyse). Sometimes these basic terrain parameters are not sufficient and more complex information is required in order to make well-founded decisions. Imagine somebody wants to model permafrost in the Alps. He/she would need information about slope and aspect, which is quite easy to derive from a DTM. Topographic shadows are possibly a little more demanding but having received this information it can be used for modelling potential solar radiation. This again can be employed in an even more sophisticated permafrost computer model. The following illustration from Gruber et al. (2001) shows the result of a permafrost model:
Permafrost model in the Matterhorn region (Gruber et al. 2001)This illustration is based on a multiple regression with the potential direct shortwave radiation in summer and the sea level is the independent variable; the BTS (basis temperature of snow-cover in late winter) is the dependent variable. Ca. 450 measurements have been used as a basis for the model. Violet means possible permafrost and blue means likely permafrost. To better demonstrate the context, the authors fused satellite images with the permafrost classes. Computergraphics by Stefan Biegger. Data source: Swisstopo (1991). Satellite images: © ESA/Eurimage, CNES/Spotimage, NPOC.
So in this lesson we are proceeding from primary information products like slope, aspect and curvature towards more realistic applications in the realm of hydrology (unit Applicatons in hydrology) and visibility analysis (unit Visibility analysis). Table 1 lists some information products derivable from DTM’s. The table is subdivided into primary and secondary information. Primary information can be derived quite directly from DTM’s and is used universally. Secondary information only results after several processing steps and its application is more specific. Always keep in mind that the type of terrain model utilised (raster/vector, high/low resolution, photogrammetric/radar…) has to suit the respective application. An engineer planning a road through a mountainous area might use a very precise TIN whereas a low-resolution grid might be adequate for a vegetation model in a flat area.
| Comment | Possible fields of application | |
|---|---|---|
| Primary topographic information | ||
| Elevation | Universal | |
| Slope | Angle of steepest descent [0-90°] | Universal |
| Aspect | Angle between north and direction of steepest descent [0-360°] | Universal |
| Curvature (plan, profile) | [Convex-concave] | Universal |
| Secondary topographic information | ||
| Local drain direction net (ldd net) | Topology of water drainage in a grid. Eight directions for each cell. | Hydrology |
| Upstream elements, catchment area, flow accumulation | Number of cells upstream of a given cell. | Hydrology |
| Catchment length | Distance from highest point to outlet | Hydrology |
| Wetness index | Measure of wetness | Hydrology |
| Stream power index | Measure of the erosive power of overland flow | Hydrology |
| Sediment transport index | Characterisation of erosion and deposition processes (compare universal soil loss equation) | Hydrology |
| Stream length | Length of longest path along ldd upstream of a given cell | Hydrology |
| Stream channel | Cells with a certain minimum number of upstream elements | Hydrology |
| Ridge | Cells with no upstream elements | Hydrology |
| Intervisibility, viewshed | Visible/invisible area | Positioning of radio antennas |
| Cast shadow | Shadows cast by nearby terrain | Radiation modelling |
| Horizon line | Line between farthest points on the terrain still visible from a given point | Engineering, heuristic for visibility analysis |
| Potential direct solar radiation | Solar radiation received at a location in the terrain depending on the incoming solar radiation, the aspect and slope of the terrain and shadows cast by terrain nearby | Engineering, vegetation modelling, permafrost modelling |
Learning Objectives
- You understand how digital terrain models can be applied in hydrology to derive flow paths, hydrologic indices, and geomorphologic features related to hydrology.
- You know how DTM’s can be used in order to derive horizon lines, viewsheds, topographic shadows and maps of potential direct radiation.
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