The Global Positioning System
Fact Sheet 062-99 (February 1999)
|| Using Topographic Maps With the GPS
|| GPS Accuracy Considerations ||
|| Additional Information Sources ||
Information ||
The Global Positioning System (GPS) is a constellation of
navigation satellites called Navigation Satellite Timing And
Ranging (NAVSTAR), maintained by the U.S. Department of Defense.
Many outdoor enthusiasts recognize that a handheld GPS receiver
can be an accurate tool for determining their location on the
terrain. The GPS receiver helps determine locations on the
Earth's surface by collecting signals from three or more
satellites through a process called triangulation. Identifying a
location on the Earth is more useful if you also know about the
surrounding topographic conditions. Using a topographic map with
the GPS receiver provides important information about features
of the surrounding terrain and can help you plot an effective
route from one location to another.
Using Topographic Maps With the GPS
When using a topographic map with a GPS receiver, you need to
ensure that the receiver has been programmed correctly to
account for variables such as the map datum, coordinate system,
and units of measure. Refer to the instructions that accompany
the specific receiver being used to ensure that these variables
are entered correctly. Most USGS maps were compiled using the
North American Datum of 1927 (NAD27), although recently revised
maps use the North American Datum of 1983 (NAD83). You will need
to refer to the map compilation notes in the lower left corner
of each map to determine the correct datum to enter into the GPS
setup program. You will also need to program the GPS receiver to
use the correct coordinate system, either Universal Transverse
Mercator (UTM) or Latitude/Longitude. When using the UTM
coordinate system, be sure to program the correct UTM zone into
the setup parameters of the GPS receiver. The UTM zone
information is found in the map compilation notes for each map.
The desired units of measure (meters or feet) will also need to
be entered into the setup parameters. You should select meters
when using the UTM coordinate system to ensure compatibility
between the two parameters.
Topographic Map Tools
Both the UTM and the Latitude/Longitude coordinate systems
allow GPS positions to be plotted on a map and map positions to
be entered into the GPS.
The Metric Coordinate Reader is a tool used for determining
points or positions on maps that have a metric grid, such as the
UTM grid. The CR-1 version is designed for use on maps with
scales of 1:50,000, 1:100,000, 1:250,000, 1:500,000, and
1:1,000,000, whereas the CR-2 version is designed for use on
maps with scales of 1:24,000, 1:25,000, 1:62,500, 1:125,000, and
1:250,000. Some USGS maps do not depict the complete printed UTM
grid, but you can construct a grid by drawing lines that connect
blue UTM ticks found in the margins of the map. When a full grid
is carefully drawn, metric coordinates of map points can be
measured directly with the reader.
The Latitude and Longitude Reader can be used to interpolate
this coordinate system on USGS 7.5-minute topographic maps. This
tool is designed for use with maps whose latitudes fall between
10 degrees and 50 degrees, and it allows for the convergence of
the lines of longitude at the higher latitudes. To use this
reader, draw a grid on the map at an interval of 2 minutes and
30 seconds, using the ticks found in the margin of the map.
After carefully drawing the grid, you can use the reader to
directly measure the latitude and longitude coordinates of map
points.
The above coordinate reading tools can be obtained
here
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GPS Accuracy Considerations
Several things must be taken into account when determining
the accuracy of handheld GPS receiver readings.
Stand-Alone mode of operation
A GPS receiver used in a stand-alone (handheld) mode can
provide accuracies within +/-100 meters of a true position. Such
variations in accuracy can be attributed to one or more of the
following factors:
Selective availability (S/A) is the
intentional degradation of the GPS signal. The Department of
Defense controls S/A to limit accuracy for non-U.S. military
users. S/A can introduce errors of up to 100 meters.
User mistakes, including incorrect
geodetic datum selection, can cause errors from one to hundreds
of meters. The user should always be certain to select the datum
that corresponds with the map being used.
Receiver errors from software or
hardware failures can cause errors of any size.
Map error on USGS topographic maps
can be +/-12.2 meters at the 1:24,000 scale as determined by
National Map Accuracy Standards. This factor should be taken
into account when comparing GPS readings to a topographic map.
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Additional Information Sources
Publications
Fact Sheet 077-01 -
The Universal Transverse Mercator (UTM) Grid - Covers topics on
map projections, the UTM grid, and determining a UTM grid value
for a map point (Free).
USGS Topographic Map Symbols
- A pamphlet that depicts and describes the symbols used on USGS
topographic maps (Free).
Fact Sheet 088-01 - Digital Raster Graphics - Provides
information on the USGS digital raster graphic (DRG). A DRG is a
scanned image of a USGS topographic map. The image is
georeferenced to the Earth in the UTM coordinate system, which
makes it ideal for use with GPS receivers.
Internet Sites
The following Internet sites have been found to be very
useful:
The U.S. National Geodetic Survey - Principal U.S. Government
agency responsible for providing geodetic resources, products,
and services. Site contains access to survey monument control
data sheets and coordinate conversion software.
www.ngs.noaa.gov
U.S. Coast Guard Navigation Center - Site contains GPS
satellite almanac and orbit information.
www.navcen.uscg.gov
Information
For information on these and other products and services,
contact one of customer service representatives at
800-248-6277 or
Sales@GeoMart.com
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