DEM to Slope

[PeterMelle]

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?

[PatrickTaurman]

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

[LauraShaffer]

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

[LauraShaffer]

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

[PatrickTaurman]

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

[RajendranVijayalakshmi]

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.

[KristyBruce]

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

[EricRice]

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

[KristyBruce]

That was very helpful, thank you!

[davidmullen]

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.