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I have an elevation raster map of the world, in Lat/Long. When I make a slope map, the results of the raster cells will be either 0, or values close to 90. The problem seems to be that the Slope function does not understand the horizontal units (which is obviously degrees, but should be meters). How can I solve this? I am using ArcGIS.

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You have to reproject the raster file to a metric coordinate system, e.g. Lambert azimuthal equal-area projection. As ArcGIS always uses the same units (degree or meters) for vertical and horizontal measures.

You can use the toolbox:

Data Management Tools -> Projections and Transformations -> Raster -> Project Raster
  • Thank you for your answer. Where can I find the Lambert projection? Would Mercator also be fine? – Yvonne Sep 28 '15 at 11:28
  • I guess so, as long as it has metric units. You could also use Cylindrical Equal Area for example. After you're finished you can project back to whatever your previous reference system was, I am guessing WGS84 – Generic Wevers Sep 28 '15 at 11:32
  • I would NOT use Mercator for calculations BY ITSELF. Global Slope calculations are not so simple. I would read through the second answer found here gis.stackexchange.com/questions/14750/… before moving forward. – Tangnar Sep 28 '15 at 12:58
  • That's why I originally suggested Lambert azimuthal equal-area. My answer was not focussing on the best reference system needed for global slope calculation it mainly focused on here error and the solution to it. But thanks for the great link! – Generic Wevers Sep 28 '15 at 13:06

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