Slope aspect map

The slope aspect map presents the classification of the surface according to orientation towards the points of the compass. The number of the categories has to be determined first and this depends primarily on the resolution of the base map (elevation model):

• 4: only the main points of the compass (N, S, E, W), where the width of the categories is 90°. It is generally applied in the case of small resolution maps where small landforms cannot be presented therefore more detailed categorization is pointless.

• 8: main and auxiliary points of the compass (N, NE, E, SE, S, SW, W, NW), where the width of the categories is 45°. It is applied in the case of detailed maps. This is the most frequently used categorization.

• 16: main, auxiliary and subsidiary points of the compass (N, NNE, NE, ENE, E, ESE, SE, SSE, S, SSW, SW, WSW, W, WNW, NW, NNW), where the width of the categories is 22.5°. It can be used in the case of detailed maps where micro-forms can be seen as well. It is used rarely as the high number of categories makes the reading of the map more difficult.

Please, note that the first category extends into both directions from the starting orientation (which is generally north). For example in the case of 4 categories North extends from 315° till 360° and from 0° till 45°. Therefore not only 4 but 5 orientation categories have to be defined in the course of re-categorization (0°-45°, 45°-135°, 135°-225°, 225°-315°, 315°-360°) where the first and last categories mean the same and they have to be displayed similarly as well! ArcView considers this when its default settings are used therefore it has to be regarded only at re-categorizing.

Always make sure that one more categories are required than the number of the points of the compass: because plain areas have to be excluded from the categorization as slope aspect has no meaning in their case. (For example the least dissected landscape of Hungary is the Hortobágy where the land rises 50 cm by 1 km on average resulting in a slope angle of 0.05% - in theory the western aspect of the eastern part of the landscape could be determined /as it ascends towards the east) but it has no sense in an area practically flat like a table.) Land-use greatly determines what is regarded to be a flat area: mostly 1°, 2°, or 5° are regarded to be the upper limits.

For creating slope aspect maps select the Surface → Derive Aspect… command that opens the Output Grid Specification window (Figure 9.5) where the following characteristics can be determined:

• Output GRID Extent: the size of the deduced map can be determined compared to any of the existing layers.

Options are the following:

• Same As View: size similar to that of the view (map).

• Same As Display: size similar to that of the actual display.

• Same As … (TIN modell): size similar to that of the TIN model on which the slope category map was based – this is the practical option.

• Same As … shp (name of Theme): size similar to that of the theme (shp)

• Output Grid Cell Size: on the deduced map the pixels size can be determined.

• As Specified Below: : it can be determined with the following three characteristics (Cell Size, Number of Rows, Number of Columns). This is to be selected if we have no deduced map yet the parameters of which we would like to copy (the analysis is completely new) or we wish not to use the already existing parameters.

• Same as… (the layer of the deduced map): copying the parameters of an already existing map that is based on the TIN model. It can be selected only if one is existing (e.g. slope aspect, hillshade – for details see later).

• Cell Size: the output (result) cell size. Reducing the pixel size increases resolution and increases the size of the database quadratic.

• Number of Rows: number of rows in the model.

• Number of Columns: number of rows in the model.

Figure 9.5: Setting the parameters of the slope aspect map

Figure 9.6: Slope aspect map with 8 categories

Following the determination and acceptance of the parameters the calculation process may take - depending on the selected resolution - a long time. In the meantime the belt beside the Interpolating grid script at the bottom of the screen shows the actual stage. The slope category map appears as a new theme in the top of the theme list at the end of the procedure (i.e. contrary to the creation or import of a new theme it does not have to be added to the project). Figure 9.6 presents the displayed slope aspect map with default settings (categories, colour palette).

Constructing the categories is mostly the same as discussed in the case of TIN model construction and re-categorization (chapter 8.1) - with the difference that the number of categories is known here right from the beginning (6 in our example) - therefore it is not discussed here. Easy overview shall be achieved when the filling pattern (colours) is defined stick to the golden mean: using the shades of one colour is not practical as they would be difficult to read (only if greyscale is required in a publication), however, too many colours also make the reading of the map difficult. Most frequently four colours are used for the four main points of the compass and in the case of 8 or 16 categories the shades of these four colours are applied. Colour selection is determined by both tradition and subjective feelings as well:

• Northerly slopes are blue as they are known (and felt) to be colder than the rest.

• Southerly slopes are red as they are the hottest.

• Easterly slopes are in-between the previous two receiving a neutral colour: generally yellow.

• Westerly slopes are also transitional but warmer than the easterly ones therefore they are generally green.