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This question already has an answer here:

After the DEM creation, I wanted to create slope layer.

However, the result has not satisfied me. I don't think the slope map is correct. Have you ever encountered false calculated terrain map?

This is the DEM:

DEM

And this is the slope layer.

SLOPE

marked as duplicate by Andre Silva, xunilk, MaryBeth, aldo_tapia, whyzar Jan 5 '18 at 14:02

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    That could happen if the units of the map are in degrees and height value in meters. The you would need to use a proper scale value (-s) gdal.org/gdaldem.html. – user30184 Jun 13 '16 at 14:46
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    What is the CRS of the DEM layer? if the layer is in a latlon CRS you should: "ratio of vertical units to horizontal. If the horizontal unit of the source DEM is degrees (e.g Lat/Long WGS84 projection), you can use scale=111120 if the vertical units are meters (or scale=370400 if they are in feet)" or reproject the dem to a CRS that uses the same measuring units for the coordinates and the elevations. In the later case the ratio of vertical units to horizontal should be set to 1. see also gis.stackexchange.com/questions/194511/slope-analysis-in-qgis/… – Gerardo Jimenez Jun 13 '16 at 14:47
  • you already had two useful answers. I just wanted to add that I had exactly the same problem and indeed converting the CRS of the DEM to a projected reference system did help. Good luck! – Carina Jun 13 '16 at 15:01
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    @user30184 Please post your comment as an answer – underdark Jun 14 '16 at 11:00
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    @underdark, I made a compilation from comments and linked canonical Q/A to both alternatives: scaling x and y units of CRS; or reprojecting it. – Andre Silva Jan 4 '18 at 14:30
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As mentioned in comments by user30184 and GerardoJimenez this is due to the horizontal coordinates x and y having a different unit of measurement from the elevation coordinate z.

For example, having latitude and longitude in degrees and elevation in meters.

So either:

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