Using GPS in Geoscience Education

Consumer-grade GPS

Consumer-grade GPS receivers vary widely in their functionality and cost. Cost is usually determined by the amount of internal memory (most no longer accept external memory cards), additional functionality (e.g. digital compass, barometric altimeter), and color screen. The ability to add an external antenna to boost signal strength and improve accuracy is mostly determined by the size of the unit. Smaller units usually do not accept external antennas.

The basic functions required for geoscience instruction are the ability to record waypoints and tracks. Modern units advertize accuracy levels around 10 m, but under good circumstances (stationary receiver and unobstructed view of the sky) the accuracy can improve up to 2 m with an external antenna.

Useful features:

• WAAS-enabled - receives Wide Area Augmentation System to improve accuracy
• Accepts external antenna
• Ability to upload map data - gives real-time context to measurements
• Barometric altimeter - can be used with similar unit as base station to improve vertical resolution (elevation)
• Other navigation signals - some new units can receive LORAN signals, broadcast signals from GPS base stations, all of which are meant to improve accuracy.

Color screens are a costly, battery-draining feature that add little to the student experience.

Professional-grade GPS

Professional-grade units are generally classified as "survey-grade" or "geodetic" as a measure of their accuracy and designated purpose. Geodetic receivers are usually designed to be stationary and relatively permanent. Survey GPS receivers are designed to be used instead to traditional surveying instruments (e.g. theodolites, total stations, etc.). Most of these units have a separate antenna and receiver device.

The initial accuracy of both receivers is generally better than a consumer-grade receiver due to better antennas and by recording more signal information from the GPS satellites. Accuracy better than 1 m (1 cm for geodetic units) is obtained through post-processing of the data. This generally requires the use of commercial software (there are some online services) that is beyond the scope of this discussion.

Consumer-Grade GPS Units

Consumer-grade GPS units record two types of data: waypoints and tracks (sometimes called "breadcrumbs"). Waypoints are created by the user to identify a specific location. They are useful to store locations of specific observations/measurements, benchmarks, or other landmarks. The waypoint is stored in the GPS with a default or user-specified name, date, location, and elevation. Some units allow the receiver to average the waypoint location data while you occupy the site to reduce noise-related errors.

Tracks, or trails of breadcrumbs, are usually recorded automatically by the receiver to store the travel path of the user. These are useful to keep track of where you have been. Time-stamps recorded with each point of the tracks preserve a history of movement. Most receivers allow the user to specify the recording rate of track points or to set the recording method to "automatic." See the user documentation on how to set the recording method. Finally, most receivers can backtrack along track to retrace your steps.

External antennas are available for some consumer-grade GPS receivers (check the receiver specifications). These amplify the GPS signals and can be mounted on a car roof, survey stake, or even a hat so that the GPS receiver can be kept out of the elements. The improved signal/noise with an external antenna and a WAAS signal can improve horizontal GPS accuracy to less than 5 m, and in some cases to as little as 2 m. Relatively flat terrain and little or no obstruction of the sky is needed to obtain this kind of accuracy.

Higher-end consumer units may have a barometric altimeter intergrated into the unit. These can be calibrated manually or by the (relatively low-accuracy) GPS elevation. When paired with an identical GPS unit used as a stationary base station, these altimeters can produce relative elevation accuracy of about 2 m. Data from both receivers is downloaded to a PC and the base station altimeter data is used to remove meteorological trends in barometric pressure (e.g. storms) by subtracing the base station altimeter data from the roving GPS altimeter data. Time-stamps are used to synchronize the two data sets.

Professional-Grade GPS Units

If you are using a survey- or geodetic-grade GPS, chances are you know what you're doing. Recording methods vary widely with manufacturer. Note that point, line, and polygon files can often be created within the GPS unit itself. This can greatly ease the inclusion of such data into GIS software.

The great advantage of most professional-grade GPS systems is the improvement in measurement accuracy through better antennas (particularly the most recent generation), and through the ability to conduct post-processing differential corrections.

While differential corrections can be made using your own GPS base station, it is important to note that many regions have permanent base stations that regularly publish the correction data for anyone working in the area. Government agencies (e.g. transportation departments, natural resource/conservation agencies, universities, etc.) often post data on servers where it can be downloaded by date. The use of this public data can make set-up easier for a lab and allow those with limited receivers to make precise GPS measurements.