What
is a DEM (Digital Elevation Model)?
Digital Elevation Models are data files that contain the
elevation of the terrain over a specified area, usually at a fixed grid
interval over the surface of the earth. The intervals between each of the grid
points will always be referenced to some geographical coordinate system. This
is usually either latitude-longitude or UTM (Universal Transverse Mercator)
coordinate systems. The closer together the grid points are located, the more
detailed the information will be in the file. The details of the peaks and
valleys in the terrain will be better modeled with a small grid spacing than
when the grid intervals are very large. Elevations other than at the specific
grid point locations are not contained in the file. As a result peak points and
valley points not coincident with the grid will not be recorded in the file.
The files can be in either ASCII or binary. In order to
read the files directly you must know the exact format of the entire file
layout. Usually the name of the file gives the reference location to some map
corner point in the file. The files usually contain only the z value (elevation
value) and do not contain the actual geographical location that is associated
with that point. The actual location associated with that elevation data is
calculated by software reading the actual DEM file, knowing the precise
location of the data value inside the DEM file. In addition, there will be some
needed reference information in the header (first part) of the file. When an
elevation is calculated at locations other than the actual grid points, some
method of interpolation from the known grid points is used. Again, this is done
in software that is external to the actual DEM file.
The DEM file also does not contain civil information such
as roads or buildings. It is not a scanned image of the paper map (graphic). It
is not a bitmap. The DEM does not contain elevation contours, only the specific
elevation values at specific grid point locations.
Some companies chose to encrypt their DEMs, thereby
prohibiting you from making your own files, converting data from other sources
or allowing you access to data files that were provided from anyone other than
that software vendor. SoftWright maintains an open architecture on all our data
files. Details for all DEM file formats that SoftWright supports are available
to any of our customers.