Step 1- Choose the function of your GPS unit.
Non-Mapping GPS Receivers-They show no map detail on screen, but include a "plotter screen" that displays an overhead view of your location.
Basemap GPS Receivers- will usually display major roads, lakes and rivers, railroads, coastlines, cities, airports, and exit information for the federal highway system.
Mapping GPS Receivers- The maps displayed on your receiver's screen can show business and residential streets, restaurants, banks, gas stations, and much more. A mapping receiver can let you look up any street address, and navigate to it using an electronic map.
Step 2- Choose the GPS options that you want.
Built-in Memory- The more internal memory your GPS receiver has, the more data you can download into it. If you are using it in your car you should have a lot of memory.
Communications- incorporate 2-way radio technology, so you can stay in touch with other users while navigating.
WAAS Capability- A GPS receiver with WAAS is capable of accuracy to less than 3 meters (the standard GPS signal gives you accuracy to about 6-12 meters).
Compass and Altimeter- Most GPS receivers can't tell you in which direction you're pointing when you're standing still. You'll need a true electronic compass to get a bearing while standing still.