Using GPS in Geoscience Education
GPS = Global Positioning System
With the advent of 12-channel consumer GPS receivers that are WAAS-enabled (Wide Area Augmentation System ( This site may be offline. ) ), it is now possible to obtain horizontal positions to better than 5 meter accuracy for under $250. A number of WAAS-enabled handheld GPS receivers are available for $150 or less with a horizontal accuracy of about 5-10 meters. The addition of an external antenna to amplify the signals and reduce so-called "backplane reflections" can reduce the error to about 2-5 m. A general rule of thumb is that vertical GPS accuracy is roughly three times the horizontal accuracy.
An often-forgotten feature of GPS is its usefulness as a clock. The receiver clock is updated automatically by the atomic clocks in the satellites. This makes it a highly accurate time resource that can be used with other measurements.
Types of GPS Receivers
GPS receivers can be broken down into two basic classes:
- consumer-grade or handheld receivers ($150 to $600)
- professional grade receivers ($3500 and higher)
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.
- 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.
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.
Recording GPS Data
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.
Downloading GPS Data to a Computer
Most consumer- and professional-grade GPS manufacturers provide or sell connection cables and software to download GPS data from the receivers to a computer. Since each manufacturer, and often each receiver, uses different communication calls, it is necessary to find software that is compatible with your receiver model. Consumer-grade software will usually export text files that can be loaded into GIS software after some minor editing.
MN DNR shareware extension for Garmin GPS downloads. (more info) -- Works as ArcView extension or stand-alone Windows application. Data can be imported into a variety of projections and exported to text files or ESRI shapefiles.
Windows 2000/XP Note: These operating systems use multi-tasking and multi-threading techniques that can create communications problems with standard serial (RS232) ports. The result is that data may be lost while downloading from the GPS, but with no error messages from the software. One way to bypass this problem (other than going back to a Windows98/ME machine) is to use a Serial-to-USB adapter. These devices allow you to use a USB port on your computer rather than the serial port. The drivers for the device will create a "virtual" serial port that you set in the GPS software. While this will prevent data losses, it will not increase the communication speeds to USB levels. Some newer GPS receivers are using USB connections, hopefully a new trend.
Macintosh Note: Since most GPS receivers communicate over PC serial (COM) ports, there are currently few Macintosh-native solutions (e.g. GPSy (more info) ). However, there are a number of work-around solutions posted online (search "GPS download macintosh" or specify the brand of your receiver), but most report that the configuration of your hardware and operating system greatly influence the behavior of the connection. All seem to require running a Windows emulator and a version of the Windows operating system on your Mac. As newer receivers move to USB connections it may become simpler to connect to Macintosh computers.
Examples of Using GPS in Geoscience Education
- GPS field exercise to trace words in a field to explore GPS accuracy
- Floodplain lab modified to utilize GPS and GIS
- What is GPS? ( This site may be offline. ) - from Garmin
- What is WAAS? ( This site may be offline. ) - from Garmin
- All about GPS (more info) - from Trimble
- GPS Tutorial ( This site may be offline. ) - from Lowrance
- FAA Navigation Programs ( This site may be offline. ) - information about GPS, WAAS, and aircraft navigation
- US Naval Observatory Time Service Dept. (more info)
- US Air Force NAVSTAR Joint Program (more info)
- GPSy (more info) - a Macintosh-native program for GPS communications and simple GIS.
- Herrstrom, E.A., 2000, Enhancing the Spatial Skills of Non-Geoscience Majors Using the Global Positioning System, J. of Geosci. Edu., 48, p. 443.