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I have a DEM data set of Nepal and want to create a percent slope map of that data. When I use the slope function in ArcToolbox I am getting extremely high values (I know Nepal is very mountainous but i am sure that 500,000,000+ is incorrect).

  • could it be something like this? In that question it was SAGA GIS, but it might be something similar, x and y in degrees and elevation in meters? – Steven Kay Dec 11 '16 at 19:42
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Often when you perform a slope analysis and get very high values, the units are in percentages, not degrees. In theory, a slope analysis on a DEM will calculate slopes of anywhere between 0 and 90 degrees. However, with percentages, anything over a 45 degree slope will be greater than 100%. If slope is rise / run, then a 45 degree slope will have a rise of 1 and a run of 1 which equates to a 100% slope.

See the ESRI documentation on how slope works.

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With this in mind, a very large rise over a very small run will give you extremely large values. This is often seen at cliffs. As Nepal is very mountainous, one should expect to see slopes like this. Try running the slope again with outputs of degrees instead of percentages.

One more thing to think about is the coordinate system of your DEM. You should be using a projected coordinate system with units in metres (or feet). Using a DEM in degrees is nonsensical.

  • I agree that using DEM in degrees is nonsensical, at least for analyses. However, the the error described by @Elizabeth could be caused by DEM that has degrees as an unit measure. – Jan Caha Dec 12 '16 at 10:08
  • I have checked and you are right, it is in degrees. How to i convert this to meters? I have tried reprojecting the raster but am a little unsure what I should be reprojecting - geographic or coordinate system. (I am aware I may also just be doing the wrong thing all together! – Elizabeth Dec 14 '16 at 12:16
  • @Elizabeth you need a projected coordinate system. There are plenty of posts on this site on reprojecting rasters to a projected coordinate system. – Fezter Dec 14 '16 at 19:54
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It is hard to say what could be wrong without seeing the data. I guess, however, that the problem will be that X,Y coordinates are not in the same units as Z values. For example your X,Y coordinates could be in degrees and your Z values are in meters or something like that. Solution can be either projecting the raster and/or setting Z factor in Slope tool.

  • I have been leaving the Z value to a default of 1 as I am not really sure what this does. Could you explain it? – Elizabeth Dec 11 '16 at 21:06
  • try setting it to 1/110000 (try 0.000009 or 0.00001) and see if that works. – Steven Kay Dec 11 '16 at 21:31
  • The Z value helps you specify ratio of Z units to X and Y units. If your Z units are feets and X,Y are in meters then the Z value should be 0.3048 to unify the measures. – Jan Caha Dec 12 '16 at 10:05
  • It appears that my Angular Unit is Degree (0.0174532925199433). what should my z value be? – Elizabeth Dec 14 '16 at 12:55
  • You have to reproject the raster to Projecte Coordinate System. You need to choose one suitable for your data and area. You can not really have Z values in degrees that would not make sense. – Jan Caha Dec 14 '16 at 15:13

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