DEM to Slope Error

[MaryM]

I have 1 meter DEMs.  I went to make a slope map and it looks like a contour map, not a slope map, I used Sptail Analyst-Slope.  Since it did not work out, I converted a DEM to Mass Points and made a Terrain and used Surface Slope, again it turned out like a contour map.  Why would this happen?  I need to know how to make a Slope map out of the DEMs I have.
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[XanderBakker]

Hi Mary,

It sounds to me that the DEM may be a rather flat area and perhaps the DEM is an Integer (rounding all values). If this is the case, only when the DEM "jumps" to the next value a slope will be detected. Did you use a "Z factor"?

Is it possible to post a small part of a DEM you have to see if I can detect the problem?

Kind regards,

Xander

[MaryM]

A portion of the DEM is attached.  I used what the default was for the Z factor.

[XanderBakker · Accepted solution]

Hi Mary,

The DEM is integer and as an integer does not have the detail te create a decent slope map. I tried to approximate what it should be; a float with decimal values by applying a Focal Statistics Mean with a Circle radius of 25 cells. If you calculate the slope on this dataset, you will find something more similar to what it should be. See the image attached (left slope on the original DEM and right slope on smoothed DEM displayed in 3D).

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The important thing to do is, to see if you can get the data that was used to create the DEM yourself or ask the provider if they have a better (floating) version of the data. The data at this point is not useful to derive Slopes.

Kind regards,

Xander

[MaryM]

We have the orignal LiDAR the DEM was created from, FEMA Region 3-MD-VA-WV, it is for free download from USGS.  It looks like they have recently posted new slope maps too, I may try to use theirs.  Can you explain to me why integer makes such as different as opposed to floating point DEM?  Does this effect point location data in the DEM?

[XanderBakker]

Hi Mary,

Imagine a graduate slope in an near flat area (the green line in the attached image). When translating this data to integer, it will create a "jump" at an elevation of 0.5 (see blue dashed line); all decimal values from 0 to 0.5 will be 0 and all values from 0.5 to 1 will be 1. At this "jump" location, the slope will be high, while the rest of the slope will be 0.

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Kind regards,

Xander