# validating the calculation of slope from DEM

my purpose is to calculate and validate slope in percentage of southern california using DEM and landsat data.

step 1)i used a DEM file and changed with the float type file to integer type step 2) i then change the projection from degree to meter. what does the value signify? step 3) i calculated the slope in percentage and changed the float type into integer type. the value calculated using spatial analyst tool> surface>slope ranges from 0-420!!!. is it possible that the percentage rise in slope is 420% ? step 4) using landsat data i tried to see at what percentage slope the land is getting developed. i find that it is getting developed at more than 50% slope also!! is that logically possible or my calculation of percentage slope is wrong?

is there any way i can calculate the distribution of developed land based on the % slope for the entire study area using arcgis?

• (1) A good place to begin is with the metadata for the DEM: that will tell you what units the elevations are in. If you don't have metadata, just compare the DEM pointwise with another DEM of the same area (even a crude one will do), because the units are almost always either meters or feet. If the units are actually feet, the slopes will be less than 1/3 of what you have calculated. (2) You can introduce errors by converting floats to integers in the values. Avoid that. (3) Your last question is answered in many threads here in many ways; review the raster-based questions. – whuber Sep 30 '13 at 19:33
• step 2) the projection is in meters so does that mean that the values are in meter. there is no such metadata available to check though. i need to convert that into integers because i need the attribute table and to use it for further statistical analysis such as querying for slopes more than 20%. what can be the other way out to avoid the error? – sayan de sarkar Sep 30 '13 at 19:46
• "The projection is in meters" means the position coordinates are in meters. You do not need to convert to integer representation to query the values (and even if you did, you would not want to do that before computing the slopes). – whuber Sep 30 '13 at 20:16
• A basic question is, where did the DEM come from? If you don't have metadata for it, go to the source and ask the questions about the units, both horizontal and vertical. Once you know that, it will give you the basis to solve all your questions about slope. If nothing else, there are ample sources for DEM data of southern California. The most well known would be the USGS National Map. You could compare data from here, with your DEM, to determine the units. – Get Spatial Oct 1 '13 at 0:06