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.

  • Welcome to GIS SE. As a new user, please be sure to take the Tour. Please also remember to include the version of GIS software in use with each question.
    – Vince
    Commented Sep 28, 2015 at 10:40

1 Answer 1


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
    Commented Sep 28, 2015 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 Commented Sep 28, 2015 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
    Commented Sep 28, 2015 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! Commented Sep 28, 2015 at 13:06

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.