Slope Computations--Edge Effects


Used the following geomorphometry programs

  • GDAL: GDAL 3.8.5, released 2024/04/02
  • GRASS: version 8.2
  • MICRODEM: version 2024.8.18 
  • SAGA: version 9.3.0
  • WhiteBox: version 12/7/2022
DEMs used
  • Started with 10x10 km, 1 m DTM from USGS in Glacier National Park
  • MICRODEM used mean aggregation to create 30 m and 1 arc second versions

Created slope maps.

The tables show the number of pixels in each of the slope grids

  • Full grid shows the size of the grid, including missing data.
  • Valid elevations shows the number of elevation in the aggregated DTMs.  For the UTM version, the data extends to the edge of the grid.  For the geographic, there are triangular regions on each side due to the rotation between UTM and geographic grids.
  • Valid slopes shows the number of slopes calculated by each program
    • SAGA and Whitebook compute a slope for every pixel with an elevation, requiring special code along the edges.
    • GDAL and GRASS compute a slope for some of the edge pixels.
    • MICRODEM does not compute slopes for any pixel that do not have eight adjacent neighbors.


This map shows the geographic grid.  In green are edge pixels to the valid elevation portion of the grid where GDAL and GRASS computed a slope but MICRODEM did not.  In purple all of the other software computed a slope where MICRODEM did not.




Final note.  GDAL, GRASS, and SAGA all changed to grid definition for their slope maps.  Instead of using pixel-is-point to match the original DTM, they output pixel-is-area.  They also did a 1/2 pixel shift of the coordinate of the corner of the grid, so the grids have the similar SRTM pixel orientation model as the original DTM.  This required a smarter test in MICRODEM to determine if two grids are identical (or alternatively, only use pixel-is-area if you have to use multiple GIS programs.


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