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):
- Name of the DEM of the study site
- Name of the output grid [actually two grids are generated: <outfile>
and <outfile>s]
- Number of pixels for smallest search radius
- Number of pixels for largest search radius
- Number of pixels to increment search radius
- Whether to keep the exposure grids of
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
