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GPS is an excellent, multidisciplinary, inquiry-driven, field-based, standards-based tool applicable to many subjects, including mathematics, geography, earth science, environmental studies, and more.  Ideas for using it in the educational curriculum follow.

Begin the GPS discussion with a base of familiar objects and concepts: Ask the students, "If I were to tell you that I was 10 feet from the far wall in the room, where could I be?" The answer is anywhere on a straight line that is 10 feet from the wall.

Next, "If I were 10 feet from this wall and 5 feet from this wall, now where could I be?" The answer is a single point, at the intersection of the 2 lines. Next, "If I were 10 feet from [Maria]" - answer - circle with a radius of 10 feet, from Maria. Next, "If I were 10 feet from [Maria] and I could be floating in space?" Answer: a sphere with a radius of 10 feet, centered on Maria. If they understand THAT, then it is not a big leap to the following: We can determine that we are 11,002 miles from GPS satellite A, 10,887 miles from satellite B, and so on. The intersection of all those spheres is our current location.

Base the discussion on mathematics. GPS is one of the best real-world examples of the equation distance = rate x time. Time is the critical component, which is why GPS was made possible by accurate time pieces, since the time from the satellite to the receiver is miniscule and needed to be accurately measured. The rate is the speed of light. So the distance is computed by the time difference between the GPS receiver and the satellite.

Use GPS coordinates to help explain the latitude-longitude and the UTM coordinates on any USGS topographic map of your area of interest. Walk with the GPS and pose questions such as, "why do the northings on UTM decrease as you're walking south?"

Have students, in teams, determine the coordinates and elevation of the school grounds (or wherever you're collecting) on a USGS 1:24,000 scale topo map (7.5 minute map) using manual interpolation in both latitude-longitude and UTM coordinates. Then go out and collect the coordinates using a GPS receiver. Which team had the closest coordinates? What about the vertical elevation? Don't be too hasty to drop the manual interpolation for the GPS reading, especially on the vertical. This is because GPS readings for elevation may be less accurate than what students can interpolate from the topographic map. It lends itself to a good discussion on being critical of the data you are working with; know where it came from, and know its benefits and limitations.

Discuss the vertical and horizontal datums in conjunction with GPS and tie the lesson to the mathematics standards.

Set up a virtual geocaching course around the campus or off campus. Make the geocaches virtual so that (1) you are not littering the landscape with boxes; (2) there is no danger that someone else could take your geocache, and (3) students find things that exist permanently on the landscape - reading a number off of a sign, visiting trees, bicycle racks, buildings, wetlands, and so on. Build the whole activity into a fun and educational cohesive theme.

Load the ArcExplorer Java Edition for Education from ESRI on your laptops (or ArcGIS / ArcView or other GIS or mapping software) with an aerial and topo map of the neighborhood where you are conducting the activity.  Then have the students upload the points they collect on the ground. You might have them WRITE their names on the ground with GPS waypoints (with 100 meter high letters, for example) and then upload them on top of the aerials/topos.  Then in your GIS session, have a contest:  Whose name is the most legible?

Import the GPS coordinates into a geographic information system (GIS). Overlay the points on a digital orthophoto of the area or a scanned digital raster graphic (topographic map). Then hyperlink ground photos to the GPS coordinates.

There are 2 methods to do this: 

1. Collect points, write them down, and manually enter via a text editor.
2. Collect points, store inside GPS unit, and with cable, do an upload into computer.

Method 1: 

Collect points and attributes.
In text editor, line 1 should be the header line, such as "lat, long, pH, groundcover, O2, etc"
Line 2 begins your data, separated by commas, such as "site1, 39.7022, -107.4832, 5.7, grassland, 38"
Save as Text such as "gps.txt" OR use Excel and save as DBF.
Access ArcGIS or other GIS software.
Tools --> Add XY Data.
Find your tabular data; TXT or DBF.
Click on layer to make visible. 
Change legend to make graduated symbol map based on attributes you collected.

Method 2:

Use the Minnesota Garmin DNR Utility or another utility to bring in the coordinates automatically from your GPS receiver into your computer. Once you have it stored on your computer, you can map it as above.