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.