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Using 10.0 I've got a raster of a fairly flat area and i'm trying to calculate the slope. Values for elevation are accurate and I am expecting slope values no more than 5 degrees over the raster.

After running the Slope (3D Analyst) I am getting slope values of 89 - 85 degrees and can't figure out why.

According to http://webhelp.esri.com/arcgisdesktop/9.2/index.cfm?TopicName=Calculating_slope the slope (θ) should be from the horizontal to the grade, but it looks like its being calculated from the zenith to the grade (90-θ off of what i'm expecting).

Any ideas on what I'm doing wrong?

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    Have you warped your DEM to a projected CRS? – Trevor J. Smith Oct 25 '16 at 20:01
  • Nope, no warping. DEM projection is in GC_NAD83_CSRS98. – cbannon Oct 25 '16 at 20:06
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    Perhaps your DEM has degrees as units for lat/lon, and meters for height but your process believes that x, y, and z are using the same units. – user30184 Oct 25 '16 at 20:06
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    @cbannon I think that might be your issue. If you're working with a DEM in a GCS, you should be converting your it to a projected CRS before running any slope calculations to ensure you have metres on the ground. – Trevor J. Smith Oct 25 '16 at 20:12
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I think your error is a result of performing a slope calculation on a DEM that isn't in a projected Coordinate Reference System (CRS). The reason for performing a transformation/warping of your DEM is to ensure that the X (Longitude) and Y (Latitude) units correspond with your Z units (elevation in metres).

Since you mentioned that your data was in the Canadian NAD83(CSRS) projection, I'll refer you to Natural Resource Canada's Canadian UTM Grid Reference. This will give you an idea of the UTM projection you need depending on the Municipal Region/Province/Territory that you're trying to project.

For more information: https://en.wikipedia.org/wiki/Map_projection

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Needed to convert it to a projected CRS before running any slope calculations to ensure you have metres on the ground.

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