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Single Raster Analysis Tools III Using FILTER |
A-Z | Single Raster Analysis Tools V QUADRAT, CENTER - Point Pattern Analysis |

- Computing Slope and Aspect with
**SURFACE**The

**SURFACE**module can derive three surficial characteristics:*Slope**Aspect**Analytical Hillshading*(= shaded relief)

In terms of IDRISI the values in a

**slope image**reflect the maximum slope around each pixel among four directions - N, W, E, S. Only these 4 neighbours are taken into account.**Be critical**of what you get 'at your fingertips', calculations of slope aren't that clear-cut they may seem to be (background reading e.g. BERRY 1993, p. 147ff.^{ *}). Be prepared to receive considerably - in terms of reliability - different results throughout GIS software products treating one and the same dataset. Disturbing, isn't it?Slopes may either be expressed as decimal degrees (0 - 90°) - - or percent (45° equals to 100%, whereas 90° slopes approach to infinity) - - in the SURFACE module. Slope images may be used to derive constraint areas, e.g. for planning factories, roads, ... they are part of predicting soilerosions, or danger of avalanches and so on. Keep in mind that slope computing is by no means strictly restricted to DEM datasets.

Let us now proceed to the**aspect images**. IDRISIs SURFACE module calculates them as 'the direction in which the maximum slope faces'. The values are decimal degrees of type real that range between 0° and 360° (N). The direction is clockwise (90° means east, 180° south, ...). Values of -1 are assigned to completely flat areas (where slopes are 0).Last but not least we may simulate the state of illumination for a DEM at given sun azimuth (260 - 290°) resp. elevation angle (0 - 90°). As with the ORTHO-module before we could apply TRANSPOS to the DEM, rotate it, run the

**analytical hillshading**and rotate the resulting image back to the origin, thus simulating light sources from any direction. The algorithmic basis of the analytical hillshading is a simple combination of trigonometrical calculations with slope, aspect and the zenith angle. Hillshade images are intended only for visualizations (e.g. enhancing textural and directional elements). Values range from 0 to 1 so a STRETCHing to 0 - 255 may be expedient. Notice, that cast shadow is*not*taken into account!

The following graphic is the 3D-representation of a hillshade draped over the raw DEM. Rivers, lakes and glaciers have been added subsequently in a prevoius step. The blue sky was a little bit tricky but has been realized completely with IDRISI:

Joseph K. BERRY, 1993: Beyond Mapping: concepts, algorithms, and issues in GIS. GIS World Books, Fort Collins

Index Single Raster Analysis Tools III

Using FILTERA-Z Single Raster Analysis Tools V

QUADRAT, CENTER - Point Pattern Analysis