After reading all the information Arc provides on slope I am still left wondering which option to use. If I have a DEM and want to calculate all areas in the DEM that have a 10% or greater slope there are two options, for example:

Slope in Percent the range of values are: 0-440

Slope in Degrees the range of values are: 0-77

In both cases I go to: layer properties, click on symbology, and then the classified option. With in the classified option I click "classify". In both cases I see the range of values mentioned above. If I click on "%" button next to break values it appears to give me the percent of slope based on the extent of my DEM, no matter what the range values of my DEM are, the top number is always 100%. My question is, how do you calculate the true slope, as in, what areas of the DEM are greater than 10%?

Slope in Percent the range of values are: 0-440

Slope in Degrees the range of values are: 0-77

In both cases I go to: layer properties, click on symbology, and then the classified option. With in the classified option I click "classify". In both cases I see the range of values mentioned above. If I click on "%" button next to break values it appears to give me the percent of slope based on the extent of my DEM, no matter what the range values of my DEM are, the top number is always 100%. My question is, how do you calculate the true slope, as in, what areas of the DEM are greater than 10%?

How is the slope defined or attributed int he DEM itself? Is it in Degrees or Percentage of slope? Or is that the problem; it doesn't really tell you?

If memory serves, the tangent of a slope (in degrees) * 100 will give you the percentage of slope. Inversely, the arctangent of the percent-of-slope/100 gives you the angle in degrees.

If you are seeing numbers of less than 1 (something like .75) take the arc tangent to get the degrees. In this case you'll get about 37 degrees. Anything steeper than a 1 is greater than 45 degrees ( remember that a 45 degree slope is a 100% slope: rise/run = 1/1).

Not sure if this helps, but it was fun (in a gray-haired geek sort of way) to think about trig again...:o