The Global Positioning System - USGS

1 The Global Positioning System (GPS) is aconstellation of navigation satellites calledNavigation Satellite Timing And Ranging(NAVSTAR), maintained by the of Defense. Many outdoorenthusiasts recognize that a handheld GPSreceiver can be an accurate tool fordetermining their location on the terrain. TheGPS receiver helps determine locations on theEarth's surface by collecting signals fromthree or more satellites through a processcalled triangulation. Identifying a location onthe Earth is more useful if you also knowabout the surrounding topographic a topographic map with the GPSreceiver provides important information aboutfeatures of the surrounding terrain and canhelp you plot an effective route from onelocation to another. Using Topographic Maps With theGPSWhen using a topographic map with a GPSreceiver, you need to ensure that the receiverhas been programmed correctly to account forvariables such as the map datum, coordinatesystem, and units of measure.

2 Refer to theinstructions that accompany the specificreceiver being used to ensure that thesevariables are entered correctly. Most USGS maps were compiled using the NorthAmerican Datum of 1927 (NAD27), althoughrecently revised maps use the NorthAmerican Datum of 1983 (NAD83). You willneed to refer to the map compilation notes inthe lower left corner of each map todetermine the correct datum to enter into theGPS setup program. You will also need toprogram the GPS receiver to use the correctcoordinate System , either UniversalTransverse Mercator (UTM) orLatitude/Longitude. When using the UTMcoordinate System , be sure to program thecorrect UTM zone into the setup parametersof the GPS receiver. The UTM zoneinformation is found in the map compilationnotes for each map. The desired units ofmeasure (meters or feet) will also need to beentered into the setup parameters.

3 Youshould select meters when using the UTMcoordinate System to ensure compatibilitybetween the two parameters. Topographic Map ToolsBoth the UTM and the Latitude/Longitudecoordinate systems allow GPS positions to beplotted on a map and map positions to beentered into the GPS. The USGS publishescoordinate reading tools that make it easier todetermine coordinate positions. The Metric Coordinate Reader is a tool usedfor determining points or positions on mapsthat have a metric grid, such as the UTMgrid. 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 the CR-2 version is designed for useon maps with scales of 1:24,000, 1:25,000,1:62,500, 1:125,000, and 1:250,000. SomeUSGS maps do not depict the completeprinted UTM grid, but you can construct agrid by drawing lines that connect blue UTMticks found in the margins of the map.

4 Whena full grid is carefully drawn, metriccoordinates of map points can be measureddirectly with the reader. Both metriccoordinate readers come with instructions andcost $ each, plus a $ handling Latitude and Longitude Reader can beused to interpolate this coordinate System onUSGS topographic maps. Thistool is designed for use with maps whoselatitudes fall between 10 degrees and 50degrees, and it allows for the convergence ofthe lines of longitude at the higher use this reader, draw a grid on the map atan interval of 2 minutes and 30 seconds,using the ticks found in the margin of themap. After carefully drawing the grid, youcan use the reader to directly measure thelatitude and longitude coordinates of mappoints. The latitude/longitude reader comeswith instructions and cost $ , plus a $ above coordinate reading tools can beobtained by contacting the USGS Geological Survey507 National CenterReston, VA 201921-888-ASK-USGSGPS Accuracy ConsiderationsSeveral things must be taken into accountwhen determining the accuracy of handheldGPS receiver readings.

5 Stand-Alone mode of operationA GPS receiver used in a stand-alone(handheld) mode can provide accuracieswithin +/-100 meters of a true position. Suchvariations in accuracy can be attributed to oneor more of the following factors:Selective availability(S/A) is the intentionaldegradation of the GPS signal. TheDepartment of Defense controls S/A to limitaccuracy for military users. S/Acan introduce errors of up to 100 meters. User mistakes, including incorrect geodeticdatum selection, can cause errors from one tohundreds of user should alwaysbe certain to select the datum thatcorresponds with the map being errorsfrom software or hardwarefailures can cause errors of any size. Map erroron USGS topographic maps canbe + at the 1:24,000 scale asdetermined by National Map AccuracyStandards. This factor should be taken intoaccount when comparing GPS readings to atopographic Department of the Geological SurveyUSGSFact Sheet 062-99 February 1999 The Global Positioning SystemAdditional Information SourcesPublicationsUSGS Fact Sheet 142-97 - The UniversalTransverse Mercator (UTM) Grid - Coverstopics on map projections, the UTM grid, anddetermining a UTM grid value for a mappoint (Free).

6 USGS Topographic Map Symbols - Apamphlet that depicts and describes thesymbols used on USGS topographic maps(Free).USGS Fact Sheet 122-95 - Digital RasterGraphics - Provides information on the USGS digital raster graphic (DRG). A DRG is ascanned image of a USGS topographic image is georeferenced to the Earth inthe UTM coordinate System , which makes itideal for use with GPS above USGS publications are availablefrom:USGS Information ServicesBox 25286 Denver, CO 802251-888-ASK-USGSI nternet SitesYou can find information on GPS resourcesby doing keyword searches using one of themany Internet search engines below are some of these searchengines and their Internet addresses:AltaVista - - - - - following Internet sites have been foundto be very useful:The National Geodetic Survey -Principal Government agencyresponsible for providing geodetic resources,products, and services.

7 Site contains accessto survey monument control data sheets andcoordinate conversion Coast Guard Navigation Center - Sitecontains GPS satellite almanac and Geographic Maps - Trails Illustrated- Tutorials describing the use of GPS withtopographic GPS Communications-The GlobalPositioning System (GPS) Library - Sitecontains numerous links to GPS-related information on other USGS products andservices, call 1-888-ASK-USGS, use theEARTHFAX fax-on-demand System , which isavailable 24 hours a day at 703-648-4888, orvisit the general interest publications websiteat visit the USGS home page