topic Re: DEM to Slope in ArcGIS Spatial Analyst Questions

DEM to Slope

PeterMelle — Tue, 01 Mar 2011 11:24:59 GMT

Hello,

I'm trying to create a slope map using a DEM file. I used the spatial analyst> surface> slope function. I wanted the map to be in percent slope/rise and not degrees. When I do this, the map it creates looks right, but the units are way off and the ranges for values are extremely high, all the way to 440? I tried to make sure that the DEM and the layer I was working both had the same coordinate system, but at this point I am lost and not sure what to do. My other question is that would it matter if I use 3D Analyst or Spatial Analyst, just out of curiosity?

Re: DEM to Slope

PatrickTaurman — Tue, 01 Mar 2011 12:38:42 GMT

Hi Peter,

Is your raster in a geographic or projected coordinate system? If geographic, please re-project to a projected coordinate system using the Project tool, then run the Slope. If your raster is currently in a projected coordinate system please see the following Web Help:

The range of slope values in degrees is 0 to 90. For percent rise, the range is 0 for near infinity. A flat surface is 0 percent, a 45 degree surface is 100 percent, and as the surface becomes more vertical, the percent rise becomes increasingly larger.

http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//009z000000v2000000.htm

Oh, and it doesn't matter if you use Slope for 3D Analyst or Spatial Analyst.

Patrick

Re: DEM to Slope

LauraShaffer — Sun, 10 Apr 2011 23:13:39 GMT

Have you had any luck with this? I am having a similar problem with NED's (type of DEM) downloaded from USGS. Even after reprojecting to UTM_12N I still get really high values for percent slope. I've calculated slope successfully many times before, but never with these NEDs so I wonder if that's the problem.

Thanks and good luck

-Laura

Re: DEM to Slope

LauraShaffer — Mon, 11 Apr 2011 21:58:59 GMT

Thanks for your response. Someone at work helped me figure it out today. I was getting values of 500%+ which I thought was a problem. It turns out there are areas of my site that really do have slopes over 500%. Sorry it turned out to be a silly question - I just didn't expect to see numbers above 100%.

Thanks again for your input!

-Laura

Re: DEM to Slope

PatrickTaurman — Tue, 12 Apr 2011 12:47:37 GMT

Hi Laura,

Glad to hear your colleague was able to assist in verifying that slopes over 100% did exist in your data. Here is the math for converting from degree to percent (in case you ever want to check your numbers).

tan (0 degrees) = 0 * 100 = 0%
tan (45 degrees) = 1 * 100 = 100%
tan (90 degrees) = infinity

Patrick

Re: DEM to Slope

RajendranVijayalakshmi — Mon, 08 Jul 2013 10:50:45 GMT

Hi
I need to get slope value in percentage, could anyone tell me how to change the value of slope from degree to percentage in arcgis, is there any tool for calculating directly to percentage ( not manual). i calculated slope from dem.

Re: DEM to Slope

KristyBruce — Fri, 12 Jul 2013 20:38:25 GMT

I am having the same problem. I downloaded a NED from the USGS and my percent slope is 0 to 13,000,000. If I understand the thread, it looks like I need to project the Raster. Do I project the original DEM before creating the slope or do I project after creating the percentage slope......OR do I have another problem entirely? I used a Z factor of .3048 because my original NED is a 30 meter DEM
any help is appreciated. Thanks

Re: DEM to Slope

EricRice — Sat, 13 Jul 2013 00:10:34 GMT

Greetings,

13 million means your data is in a geographic coordinate system. You should project the DEM (making sure to NOT use Nearest Neighbor Resampling) to a projected coordinate system that ideally uses the same linear unit as your Z unit so your Z factor can simply be one. I assume the data (Z's) is in meters, so UTM is a very reasonable choice here. You basically want X,Y,Z to all be the same unit, unless you like setting a z factor or have bias/requirements towards certain projections.

Rajendran,
To get slope in % choose Percent_Rise on the parameter Output Measurement.

Best,
Eric

Re: DEM to Slope

KristyBruce — Mon, 15 Jul 2013 18:36:08 GMT

That was very helpful, thank you!

Re: DEM to Slope

davidmullen — Wed, 04 Sep 2013 20:27:36 GMT

Why isn't there a function to do a simple rise/run slope calculation? That is what most people understand when we talk about slope.

Re: DEM to Slope

myname — Sat, 11 Feb 2017 04:31:48 GMT

ArcGIS support in percent slope/rise and degrees, toturial here: how to calculate slope map from dem in arcgis - YouTube

Re: DEM to Slope

JennieCatalano1 — Tue, 16 May 2017 22:01:20 GMT

Link leads to dead end.

Re: DEM to Slope

Robert_LeClair — Tue, 16 May 2017 22:05:06 GMT

Here's an alternative that may work better - https://www.youtube.com/watch?v=4cZwCx5R4pE

Re: DEM to Slope

JennieCatalano1 — Tue, 16 May 2017 22:15:30 GMT

Thank you! I’ll check it out.