First, you need to identify the types of applications that you will be using GPS to support. Will you be using GPS as a tool as an educational resource or perhaps to support your recreational activities (hunting, fishing, etc.). Perhaps you intend to use GPS to support natural resource management or local mapping. Or are you are going to use a GPS to support survey work? Each of these applications requires a different grade of GPS receiver.
How accurately do you need the unit to tell you where you are on the Earth? Pay attention to the manufacturer's claimed accuracy (such as plus or minus 10 meters, which means that this GPS will usually tell you where you are on the Earth to within about 10 meters). Manufacturers offer features that may help you increase the accuracy of the data you collect, such as the ability to use the WAAS signal (consumer-grade GPS units and above), or take advantage of a Trimble Virtual Reference Station network (high-accuracy GPS units).
How much data and what type of data will you need to record? GPS units will let you collect only a certain amount of data. Some will let you collect point locations or keep record lines or tracks about where you have traveled with your GPS. Others may let you collect areas or polygons around certain areas of interest, such as an agricultural field or your neighborhood.
The versatility of a GPS unit can also be important, depending on its intended use and the location in which you will be using it. For example, many handheld GPS units are waterproof, but only some float when dropped in water. Also, some GPS units allow the addition of an external antenna. The antenna can increase your unit's ability to receive satellite signals (which can be useful in wooded areas) and let you worry less about keeping your GPS unit held upright (keeping its internal antenna pointed toward the sky).
Some GPS units are sold with useful software that may help you plan when to collect data, or they have with maps preloaded onto the GPS that will help you navigate from Point A to Point B. More advanced (and expensive) GPS units let you customize software on the GPS unit to let you quickly record information about a location, such as the number of trees at a given forest plot or the number of buildings along a given street.
A variety of issues need to be addressed when selecting a GPS. For more information on this topic, refer to the article in the Map@syst eXtension Web site on "Selecting Handheld GPS Receivers."
