Why doesn my DEM data line up with the hard copy topos that I have from the USGS?
Another good question. Your DEM, NED, and topo elevation data may all be 1:24,000 source data sets. This generally means that the accuracy of all 3 data sets are the same. The complication comes from how your particular software interprets or interpolates the point data of the DEM or NED data sets. Very rarely do 2 different software systems have the same interpolation algorithms (a complex set of computer-based rules on how to interpret a data file) in order to output the same contour lines. As a result, you may have one data set that is off by 5 feet to the north, and another that is off 5 feet to the south. Both data sets are only off by five feet, but there is a 10 feet difference when you compare the two.
