# DEM Elevation Units

I'm fairly new to the GIS world, and have a quick question. I'm currently using ArcGIS 10.1. I do not know much about rasters, so does transforming a DEM from one projection to another automatically convert the units of the cell elevations?

To expand a little on Ryan's answer, a raster DEM is just like many other rasters, it contains a numeric value for each cell. That value could be elevation in feet, temperature in degrees C, or anything else that can be represented numerically. Reprojecting it to another coordinate system would not change the values in the raster cells, because GIS doesn't "know" that it's an elevation value that should be changed (unless you tell it).

Also, horizontal and vertical coordinate systems are two different things, just because your raster is in UTM Zone 17 North, NAD83, Meters (or whatever), doesn't mean it will have elevation units in meters as well, or even that it will have elevation values at all.

• +1 -- but there's an important caution: reprojecting a DEM almost always changes the values in the cells, because they must be resampled. The only exceptions are reprojections that amount to rotations by multiples of 90 degrees along with any uniform rescaling. Such changes can be profound--mountain tops can shrink by a hundred meters, for instance--but they of course are not systematic changes in units of measurement. – whuber Jan 23 '14 at 20:11
• If you plan analyze the topography of your projected DEM, it's a good idea to make the Z units (i.e., the DNs, cellvalues, etc) the same as the XY units. For instance, slope values, derived as rise/run, would be confusing if "rise" was in meters while "run" was in feet. – Roland Jan 24 '14 at 0:19
• Also a big caution against using the zfactor: that applies one multiplier to all cells, when in fact the projection is a distortion by definition, i.e. one unit at the northernmost part of the projected DEM is not the same size as one unit at the southernmost part of the projected DEM - yet, zfactor assumes that the scale is identical over the entire projected raster. Maybe someone can explain it better? See more details here: gis.stackexchange.com/a/80109/19306 – Tom Grundy Jan 29 '14 at 1:18

No this should not change with a projection.
The numeric value of the cell is the Dn (digital number) which generally is represented as units above sea level.
You need to know what the original data was created in (ie: feet, meters, etc).
With that information you could convert to another format and transform the z factor also.

• When converting from one raster format to another (arctoolbox> To_Raster) you have the option of specifying a z factor. This would allow you to convert from Meters to Feet let's say. You can also transform in that dialog by changing the output projection in the environments settings section. – Brad Nesom Jan 23 '14 at 18:09