I have a number of DEMs from the National Elevation Dataset (NED), that come standard in the NAD83 coordinate system. I need to project them in order to define linear units for analysis. Every time I project the DEM, the projected result contains a systematic pattern of error, where elevation values are distored up to 250 meters in either direction (higher/lower). The errors appear to occur on a horizontal scale of 200 meters or so. Not only are the errors apparent when calculating the difference in elevation between the projected and unprojected DEM, but they are obvious even just on visual inspection of the DEM and particularly in the hillshade. I tried two different projections (Transverse Mercator and UTM Zone 12), both based on NAD83, and got similar errors but in different patterns. I have attached a few images in hopes of illustrating what I may not be describing very well in words. These patterns of error occur regardless of the resampling technique used when projecting the raster.
I know that prjecting a DEM inherently introduces some error by "stretching" but I have not noticed errors like these in projecting other DEMs. I am hesitatnt to use these projected DEMs in terrain analysis as the results don't look right even to the naked eye.