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We are currently having some issues with gdaldem slope. We'd like to create a slope map of a geotiff, but the result seems to be some degrees off to what it actually should be.

Our input geotiff is in EPSG 3857 projection. So it's meter based. Also the elevation data inside the geotiff is meter based. So we would expect that a -s of 1 (which is default) should be ok, right? Horizontal and vertical units are the same. If we provide a -s of about 0.8, it seems to be ok.

As reference we use: https://www.openslopemap.org/karte/

This is the gdalinfo for one of our files. We have a lot of them though. All in the same project and world-wide. So some at the equator and some very far away.

Would be great to know what we are doing wrong. If our projection is just not the right one for the job.

gdalinfo ktn_dgm5m_wm.tif
Driver: GTiff/GeoTIFF
Files: ktn_dgm5m_wm.tif
Size is 55022, 25272
Coordinate System is:
PROJCS["WGS 84 / Pseudo-Mercator",
    GEOGCS["WGS 84",
        DATUM["WGS_1984",
            SPHEROID["WGS 84",6378137,298.257223563,
                AUTHORITY["EPSG","7030"]],
            AUTHORITY["EPSG","6326"]],
        PRIMEM["Greenwich",0,
            AUTHORITY["EPSG","8901"]],
        UNIT["degree",0.0174532925199433,
            AUTHORITY["EPSG","9122"]],
        AUTHORITY["EPSG","4326"]],
    PROJECTION["Mercator_1SP"],
    PARAMETER["central_meridian",0],
    PARAMETER["scale_factor",1],
    PARAMETER["false_easting",0],
    PARAMETER["false_northing",0],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]],
    AXIS["X",EAST],
    AXIS["Y",NORTH],
    EXTENSION["PROJ4","+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs"],
    AUTHORITY["EPSG","3857"]]
Origin = (1407137.000000000000000,5965203.000000000000000)
Pixel Size = (5.000000000000000,-5.000000000000000)
Metadata:
  AREA_OR_POINT=Area
  DataType=Generic
Image Structure Metadata:
  COMPRESSION=LZW
  INTERLEAVE=BAND
Corner Coordinates:
Upper Left  ( 1407137.000, 5965203.000) ( 12d38'25.90"E, 47d 8'29.44"N)
Lower Left  ( 1407137.000, 5838843.000) ( 12d38'25.90"E, 46d21'49.72"N)
Upper Right ( 1682247.000, 5965203.000) ( 15d 6'42.77"E, 47d 8'29.44"N)
Lower Right ( 1682247.000, 5838843.000) ( 15d 6'42.77"E, 46d21'49.72"N)
Center      ( 1544692.000, 5902023.000) ( 13d52'34.34"E, 46d45'14.63"N)
Band 1 Block=256x256 Type=Float32, ColorInterp=Gray
  NoData Value=-3.4028234663852886e+38
  Metadata:
    RepresentationType=ATHEMATIC
Georg
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  • EPSG 3857 may use metres as the horizontal unit, but it's not a good CRS for distance calculations (slope = rise/run). It's designed for web mapping https://gis.stackexchange.com/a/242547/2856 – user2856 Mar 24 '22 at 08:08
  • Thanks for your answer! What projection would you then be using? Specially when our data is world-wide? Would be have to use different projections depending on how far away from the equator we are? – Georg Mar 24 '22 at 08:43

0 Answers0