1 October 2003
geometrically corrected. The removal of ground relief adds to the accuracy meas-
urement of distances on the ground. DOQs are available over the internet through
the USGS or state level natural resources and environmental agencies. They
come in black and white and color infrared. These digital aerial photographs come
in a variety of scales and resolutions (often 1-m GSD). Due to the ortho-correc-
tion process, DOQs are typically in UTM, Geographic, or State Plane Projection.
The images typically have 50 to 300 m overlap. This overlap simplifies the mo-
saic process. DOQs work well in combination with GIS data and may aid in the
identification of objects in a satellite scene. It is possible to link a DOQ with a
satellite image and a one-to-one comparison can be made between a pixel on the
satellite image and the same geographic point on the DOQ.
(2) Digital Elevation Models (DEM). A Digital Elevation Model (DEM) is
a digital display of cartographic elements, particularly topographic features.
DEMs utilize two primary types of data, DTM (digital terrain model) or DSM
(digital surface model). The DTM represents elevation points of the ground, while
DSM is the elevation of points at the surface, which includes the top of buildings
and trees, in addition to terrain. The DEM incorporates the elevation data and
projects it relative to a coordinate reference point. (See
on DEM, DTM, and DEMs.
(3) DEM Generation. Elevation measurements are sampled at regular in-
tervals to form an array of elevation points within the DEM. The elevation data
are then converted to brightness values and can be displayed as a gray scale image
(Figure 5-24). The model can be viewed in image processing software and su-
perimposed onto satellite image data. The resulting image will appear as a "three-
dimensional" view of the image data.
(a) DEMs come in a variety of scales and resolutions. Be sure to check
the date and accuracy of the DEM file. DEMs produced before 2001 have as
much a 30 m of horizontal error. As with other files, the DEM must be well reg-
istered and in the same projection and datum as other files in the scene. Check the
metadata accompanying the data to verify the projection.
(b) The primary source of DEM data is digital USGS topographic maps
and not satellite data. Spaceborne elevation data will be more readily available
with the processing and public release of the Shuttle Radar Topography Mission
(SRTM) data. Some of this data is currently available through the Jet Propulsion