Introduction to DEMs
A digital elevation model (DEM) is a raster data set with a regular grid of elevations arranged by column and row. We use it as a generic term, and there are other more specific terms. DEM's have also been called DTM (Digital Terrain Models, with buildings and vegetation excluded) and DSMs (Digital Surface Models, including vegetation and buildings). Some agencies use DEM in a more restricted sense. USGS now calls their DTM a DEM.
At each location in the raster the elevation is recorded. Elevations were historically recorded as integers to the nearest meter, but most high resolution DEMs now use 4 byte floating point values.
|Two different representations of a DEM: as points on the left, and as areas on the right. For use in slope and many other operations, only the point representation on the left makes sense. The version using areas does not support interpolating between the grid postings.|
|View of grid data stored in a DEM,
available with "Edit
DEM grid" option on Edit
The most common convention for DEMs has the 0,0 point in the SW corner, with rows increasing northward and columns eastward. This is a regular Cartesian grid. This convention differs from the common convention for imagery or that used on computer graphics screens, but MICRODEM will handle all details for any data its supports.
|The simplest display of a DEM consists of a map, which can display Elevation, Slope, Reflectance, or Contours or other options shown in the Product Gallery. The Reflectance option is probably the most useful, at least to someone who thinks like a geologist.|
|The DEM can also be displayed as 3D views (Perspective, OpenGL), and merged with a wide variety of other map data. See Product Gallery for samples and links to directions.|
As a raster, DEMs store only the coordinates of one corner and the spacing in the x and y directions; all other coordinates can be computed from these and and row/column locations of the z values. This can be very efficient in terms of computer storage.
DEMS differ from TINs (Triangulated Irregular Networks), which use a variable point location strategy to put elevations at critical locations. TINs promise to decrease storage enough to make up for having to store x,y, and z coordinates, and to make up for the overhead of triangle indexing, but to date no significant number of TINs have been created to compare with the wide availability of DEM data.
With the rise of LIDAR, some applications are going to direct manipulation of the Point clouds rather than first transforming the data to grids or TINs.
|DEM grids for 30 m UTM spacing (triangles) and
1 geographic spacing (squares) for a location in
Wyoming. The UTM grid is square, while the geographic
grid is almost rectangular. This mismatch varies over the
map area, changing slowly in the north-south direction
and rapidly east-west.
The most common DEMs use arc second spacing (NED/3DEP, SRTM, ASTER GDEM). The prevalent spacing is 3", and that is often called 90 m or 100 m. The true spacing is close to 90 m in the NS direction, but EW it will be less as you move away from the poles. Now there are global data sets with 1" spacing, and some with 1/3" spacing.
|DEM Header information. You can display or edit this data.
Note that the header has information on the datum, size of the DEM, units for horizontal and vertical resolution, spacing, elevation range, the SW corner's coordinates, and whether the data is integer or floating point.
For many applications, DEMs can be regarded as unchanging and a constant layer in a GIS. However, DEMs are now being used to monitor change. Recent eruptions at Mount Saint Helens have been recorded with multiple LIDAR DEMs. SRTM DEMs differ from NED in areas of large open pit mines and the adjacent large spoil piles, which postdate the topographic maps used to create NED. NOAA has been running LIDAR surveys along beaches, recorded changes from coastal erosion after hurricanes or El Nino storms.
MICRODEM deals with practical application of the DEMs, for many users who will view the DEM and its creation as a black box. Users of the DEMs must be aware of the accuracy and resolution limitations of the data, detailed in the published descriptions. For a user without recourse to digitizing (and these large DEMs represent major investments in digitization), a DEM either exists covering the area of interest or does not. If the DEM exists, it is either accurate enough to use or not. That decision will be subjective; in some cases, "pushing" a DEM beyond its accuracy may be preferable to the alternative of using a paper map and doing the work by hand. The user must take responsibility for specific uses of the data. Perfect DEMs will never exist everywhere, and there will always be tradeoffs in their design and use.
Last revision 9/28/2017