4.1 toposcale.aml

This is a simple AML that allows to identify topographic exposure (ridge, slope, toe slope, etc) at various spatial scales, and to hierarchically integrate these features into a single grid. Topographic position, necessary e.g. to classify soil properties over large areas, can be calculated using a hierarchically nested approach. In this AML, circular moving-windows with increasing radii are applied to a DEM, and the difference between the average elevation of the window and the center cell of the window is calculated and written to temporary output grids. The user is promted for the number search radii, for which the temporary grids are generated. The resulting (temporary) maps are interpreted as relative topographic exposure at different spatial scales. The exposure can be interpreted as a ridge or peak if the center cell in the mowing window has a higher elevation than the average elevation of the cells in the window. Contrarily if the center cell is of lower elevation than the average elevation of the window, then the center pixel can be interpreted as "toe slope" or "valley bottom".

A hierarchical integration into a single map is achieved by starting with the standardized exposure values of the largest window, then adding standardized values from smaller windows where the (absolute) values of the smaller (search-) scale grids exceed the values of the larger scale map.

It is important to notice that the resulting maps are somewhat smaller in size than the original DEM. This is due to the fact that when analyzing the DEM in a circular window the output is reduced by the size of the search radius because no analysis can be performed accurately at the outermost cells. Thus, the resulting integrated map is reduced in its lower left and upper right corner by the radius (in cells) of the largest search window!

The resulting map can then be classified into 4 principal topographic classes: ridge, slope, toe slope and bottom, using the topoclass.aml. When running toposcale.aml, the user is prompted for (see example):

Such topographic characteristics can be used, together with an ecological classification of the surface geology, to define coarse fragment content, soil depth, soil texture, and specific soil moisture holding capacity (see Roberts et al., 1993, and soilprop.aml and bucket.aml). There is no distinction between plains and homogenous slopes in toposcale.aml, since both topographic positions reveal no concave or convex deviation from the surrounding terrain. However, it is possible to distinguish plains from slopes based on the average slope angle (as calculated in the same search window). The user has to consider, however, whether plains and slope need to be distinguished for the application of interest. In many instances, slopes and plains can be considered as similar (similar ratio of erosion/accumulation, etc.).

When starting the AML, the user is prompted for the name (&path, if different from the directory the AML was started) of the DEM the topographic position has to be derived from. Further, the minimum and maximum radius of the search window has to be entered, as well as the increment to enlarge the search radius stepwise. The AML does increment the search radius (and repeat the exposure calculation for each radius) until the incremented radius is bigger than the max. radius. It then starts integrating hierarchically the generated temporary grids. Finally. Two versions of relative topographic exposure are available as a grid, one containing the “raw” values, and a second containing a smoothed version of the topographic exposure. The smoothed version has an "s" added to the output file name.

General specifications of the AML:

Command: &r  toposcale     (at GRID prompt)
Required input: DEM
Output units: Standardized topographic exposure (in z-units of DEM)
Speed of calculations: Relatively slow due to intensive calculations
Flexibility of the routine: High; user can adjust AML at interface
User interface: Limited, but simple and easy
Known errors:  -
Programmer N.E. Zimmermann
Download: toposcale.aml      (use: "save link as")
Contact: niklaus.zimmermann @ wsl.ch

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Last Updated: 10/27/00
By Niklaus E. Zimmermann