Barry Rice

- doing science on a terrestrial planet -

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The acronym GPS stands for Global Positioning System. GPS navigation involves using a handheld or backpack mounted transmitter/receiver (i.e. GPS unit) that is in communication with satellites at two transmission frequencies (1575.42 MHz, 1227.60 MHz). By precisely measuring the time lags of signals being sent between the GPS unit and the satellites, the geographic location of the GPS unit can be determined.

GPS satellites have been in operation since the launching of a NAVSTAR satellite in February 1978. After the Korean airliner 007 was shot down over the Soviet Union in 1983, reportedly due to navigational errors, President Reagan confirmed that continued access to GPS capabilities by civilians--then in its infancy--would be encouraged.

In order for a GPS unit to determine its geographic location, it must be able to communicate simultaneously with at least four satellites. (To understand why, pester a physicist or mathematician--the explanation is relatively simple but too peripheral to the discussion to include here.) There are twenty-four GPS satellites in orbit, and these are spaced so at least five are above the horizon at any time. (At any given time, more than 24 satellites may be operational if replacements for older satellites have been launched and the older units remain in service.) If trees, mountains, or other objects block the line of sight to the satellites, the GPS unit is reduced in accuracy. Clouds do not affect the accuracy of GPS units. Because of orbital errors, disturbances from the troposphere and ionosphere, and technological limitations, the accuracy of GPS units is approximately 10-20 meters. Differential correction data can be obtained for fancier GPS units to improve their accuracy to 1-3 meters. Prior to 1 May 2000, GPS units (without differential correction) were accurate to only approximately 100 meters. This is because the USA military imposed random error signals of about 340 nanoseconds to satellite signals for GPS units. This error signal was euphemistically called selective availability, and may be resumed at the military's discretion at any time.

A good GPS unit costs less than $200. I like the models made by Garmin. If you want a unit that can incorporate differential corrections, expect to pay a great deal more.

The acronym GIS stands for Geographic Information System. GIS refers to the computer hardware, software, data, and personnel system that can capture, manipulate, analyze, and display geographical coordinate data.

If you consider GIS to be a way to analyze and work with your GPS data, you are only thinking about one part of GIS's functionality. GIS can incorporate multiple layers of data, including information derived from sources such as tabular data (i.e. lists, such as zip codes or counties), seasonal or climatic variables such as rainfall or stream flow rates, or other data such as population growth rates, topo maps, etc. Incidentally, GIS was around before GPS.

Commonly used software packages for analyzing GIS data are ArcView and ArcInfo, both marketed by ESRI (Environmental Systems Research Institute). ArcView is a popular desktop GIS and mapping software package. ArcView v8.2 for Windows costs approximately $1500 for a single use license. Multiple use licenses (i.e. to load on more than one computer) can be purchased directly from ESRI. If you are interested in actually manipulating and modifying your geographic data, however, you will probably want to use ArcInfo. To buy ArcInfo, you must contact ESRI directly ( They do not reveal the price of ArcInfo on their web site, and it is probably most economical for you to buy a multiple use license with someone else. Staff of The Nature Conservancy should investigate with the GIS Enterprise program for discounts and training opportunities.


4 February 2009