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I'm trying to determine the slope of sampling points in QGIS 3.4, but the values are all ~89.99°, which I know is incorrect. Pretty sure it's an issue when creating the slope raster.

Based on the steps detailed below, is it clear to anyone what I'm doing wrong?

My points and ⅓ arc-second USGS DEM are set to CRS EPSG:4269 - NAD83.

This is my DEM (with points). enter image description here

I select Processing => Toolbox => Raster Terrain Analysis => Slope to generate the slope raster.enter image description here

This is the output slope raster (with points): enter image description here

Then I use the point sampling tool with the slope raster and points to generate the slope values. These are all ~89.99°, which isn't right.

enter image description here

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  • EPSG:4269 is a geographic coordinate system which uses degrees as the horizontal units and your vertical units are most likely metres (possibly feet). Project your DEM to an appropriate CRS that uses the same horizontal units (i.e. metres or feet) before running slope. Or calculate the appropriate z factor for your latitude.
    – user2856
    Dec 18, 2019 at 1:13
  • Thanks for the help. You're right. I calculated a z factor using the centroid latitude from my DEM and this formula: z=1/(111320*cos(latitude*pi/180)). It worked.
    – Brian Allen
    Jan 2, 2020 at 16:02

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