the boundaries are on a lat-lon grid, but the datapoints are in
UTM, resulting in irregular sizes, e.g. 370 x 465 data points
these 10m DEMs are the best, most accurate data available from the
USGS
non-USGS DEMs
there have some other formats over the years which use the ".DEM"
file extension but are a different, incompatible format
for example, there was a popular landscape rendering program
called "VistaPro" which defined it's own, different DEM format, so
you may occasionally encounter a "VistaPro DEM"
There is a format called "Japanese DEM" with the file
extension ".mem"
i've encountered a non-standard DEM, very similar to USGS DEM but
with the following differences:
header is 893 bytes, instead of the standard 864 or 1024
some high-res DEMs for individual states are available free online -
see the Locations Page
Utilities
DEM2DXF (290k) is a small
Windows program for people in the CAD world who are stuck using DXF
USGS DEM - Technical Details:
Classification Levels
Root-mean-square error (RMSE)
is a measure of how closely a data set matches the actual world
Level-1 DEMs
reserved for 7.5-minute DEMs created by scanning NAPP photography
vertical RMSE of 7 meters is the desired accuracy, 15 meters is
the maximum permitted.
Level-2 DEMs
have been processed or smoothed for consistency and edited to
remove identifiable systematic errors, derived from hypsographic and
hydrographic data digitizing
RMSE of one-half contour interval is the maximum permitted.
Level-3 DEMs
are derived from DLG data from both hypsography (contours, spot
elevations) and hydrography (lakes, shorelines, drainage)
RMSE of one-third of the contour interval is the maximum
permitted.
sometimes you will encounter both level 1 and 2 DEMs for the same
area; the higher level number is much better data and should be
used
File format
type A record: header information, one per file
type B record: made up of data from one-dimensional arrays called
profiles, one per line of elevation data
type C record: contains statistics on the accuracy of the data in the
file
Logical record size of 1,024 bytes
more than one record is usually required to store a single record
type B