Was this ever resolved? I'm having the same problem, but I'm still confused about what I'm supposed to do with the nice fresh NED tif I just downloaded from the USGS Seamless Server. I've never had trouble calculating slope etc. from DEM files downloaded elsewhere.
Without doing anything to the NED, ArcCatalog says that the Spatial Reference for it is GCS_North_American_1983. Is that the same as the projection? When I downloaded it, USGS specified the output format as "ArcGRID NAD 83 Geographic." What am I supposed to project it to? Everything else in my map is in NAD_1983_UTM_Zone_12N (it's northern Arizona).
I understand that the horizontal coordinates are probably in decimal degrees and the vertical are probably in meters. I'm not sure how to convert these to something usable (and I'm not sure where to look to confirm the units on these).
I tried projecting the NED to the 12N projection, then doing a raster calculation to convert the elevation (z) from meters to feet, then tried getting slopes (via both 3D and Spatial Analyst, is there a difference?) but the % slopes were still in the thousands. After all that I downloaded a fresh NED in case I messed up the original in the process somehow (superstitious, maybe but this is driving me crazy!).
So . . . how do I get from an NED from USGS to slope? Thanks in advance!!
-Laura