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# The Global Positioning System

### Fact Sheet 062-99 (February 1999)

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 || Using Topographic Maps With the GPS || GPS Accuracy Considerations ||  || Additional Information Sources || Information ||

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The Global Positioning System (GPS) is  a constellation of  navigation satellitescalled 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 anaccurate tool for determining their location on the terrain.  The GPS receiver helps determinelocations on the Earth's surface by collecting signals from three or more satellites through a processcalled triangulation.  Identifying a location on the Earth is more useful if you also know aboutthe surrounding topographic conditions.    Using a topographic map with the GPS receiverprovides important information about features of the surrounding terrain and can help you plot aneffective 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 thereceiver has been programmed correctly to account for variables such as the map datum, coordinatesystem, and units of measure.  Refer to the instructions that accompany the specific receiverbeing used to ensure that these variables are entered correctly.  Most USGS maps were compiledusing the North American Datum of 1927 (NAD27), although recently revised maps use the NorthAmerican Datum of 1983 (NAD83). You will need to refer to the map compilation notesin the lower left corner of each map to determine the correct datum to enter into the GPSsetup program.  You will also need to program the GPS receiver to use the correct coordinatesystem, either Universal Transverse Mercator (UTM) or Latitude/Longitude.  When using theUTM coordinate system, be sure to program the correct UTM zone into the setup parametersof the GPS receiver.  The UTM zone information is found in the map compilation notes foreach map.   The desired units of measure (meters or feet) will also need to be entered into the setupparameters.  You should select meters when using the UTM coordinate system toensure compatibility between the two parameters. 

#### Topographic Map Tools

Both the UTM and the Latitude/Longitude coordinate systems allow GPS positionsto be plotted on a map and map positions to be entered into the GPS.  The USGS publishescoordinate reading tools that make it easier to determine coordinate positions. 

The Metric Coordinate Reader is a tool used for determining points or positions onmaps that have a metric grid, such as the UTM grid. The CR-1 version is designed for use onmaps with scales of 1:50,000, 1:100,000, 1:250,000, 1:500,000, and 1:1,000,000, whereas theCR-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 youcan construct a grid by drawing lines that connect blue UTM ticks found in the margins ofthe map.  When a full grid is carefully drawn, metric coordinates of map points can be measureddirectly with the reader.  Both metric coordinate readers come with instructions and cost $2.50each, plus a $5.00 handling charge.

The Latitude and Longitude Reader can be used to interpolate this coordinatesystem on USGS 7.5-minute topographic maps.  This tool is designed for use with maps whoselatitudes fall between 10 degrees and 50 degrees, and it allows for the convergence of the lines oflongitude at the higher latitudes.  To use this reader, draw a grid on the map at an interval of 2minutes and 30 seconds, using the ticks found in the margin of the map.  After carefully drawing thegrid, you can use the reader to directly measure the latitude and longitude coordinates of mappoints. The latitude/longitude reader comes with instructions and cost $2.00, plus a $5.00 handlingcharge.

The above coordinate reading tools can be obtained by contacting the USGSat:

U.S. Geological Survey507 National CenterReston, VA 201921-888-ASK-USGS

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GPS Accuracy Considerations
Several things must be taken into account when determining the accuracy ofhandheld GPS receiver readings. 
Stand-Alone mode of operation
A GPS receiver used in a stand-alone (handheld) mode can provide accuracieswithin +/-100 meters of a true position.  Such variations in accuracy can be attributed to one or moreof the following factors:

***Selective availability*** (S/A) is the intentional degradation of the GPSsignal.  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, cancause errors from one to hundreds of meters.  The user should always be certain to select the datumthat corresponds with the map being used.

***Receiver errors*** from software or hardware failures can cause errorsof any size. 

***Map error*** on USGS topographic maps can be +/-12.2 meters at the1:24,000 scale as determined by National Map Accuracy Standards.  This factor should be takeninto account when comparing GPS readings to a topographic map.

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Additional Information SourcesPublications
USGS 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 thesymbols used on USGS topographic maps (Free).

USGS Fact Sheet 088-01 - Digital Raster Graphics - Provides information on theUSGS 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 itideal for use with GPS receivers.

The above USGS publications are available from:

USGS Information ServicesBox 25286Denver, CO 802251-888-ASK-USGS
Internet Sites
You can find information on GPS resources by doing keyword searches using oneof the many Internet search engines available. Shown below are some of these searchengines and their Internet addresses:

AltaVista - www.altavista.com/Yahoo - www.yahoo.com/Lycos - www.lycos.comWebCrawler -www.webcrawler.com/MonkeySweat -www.MonkeySweat.comgoogle.com - google.com

The following Internet sites have been found to be very useful:

The U.S. National Geodetic Survey - Principal U.S. Government agencyresponsible for providing geodetic resources, products, and services.  Site contains access to surveymonument control data sheets and coordinate conversion software. www.ngs.noaa.gov/

U.S. Coast Guard Navigation Center - Site contains GPS satellite almanac and orbitinformation.  www.navcen.uscg.gov/

National Geographic Maps - Trails Illustrated - Tutorials describing the use of GPSwith topographic maps. www.trailsillustrated.com/

Macintosh GPS Communications-The Global Positioning System (GPS) Library -Site contains numerous links to GPS-related Web sites. www.gpsy.com/gpsinfo/
Information
For information on other USGS products and services, call 1-888-ASK-USGS, or visitthe general interest publications website at erg.usgs.gov/isb/pubs/pubslists/.

Please visit the USGS home page at www.usgs.gov/.

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*This document has undergone official review and approval for publicationsestablished by theNational Mapping Division, U.S. Geological Survey. Some figures, have been modifiedor added to improve the scientific visualization of information.*

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