Several types of topographic maps are available. You can order nice, printed maps from USGS. I've been using maps on computer CDs and am completely addicted to them. They allow you to select the area where you are riding and how close you want to zoom in and then print out custom maps. The software is not cheap at $100 per state, but this is the slickest way to make maps.
Reading topographic maps requires some practice. You need to constantly consult the map and orient it using your compass and/or GPS in order to track your location, or you could easily become confused. One ridge begins to look a lot like the next, and you can end up miles off course if you don't pay close attention.
Once you have the right maps, you can augment them with a GPS unit. Note a GPS unit is not a replacement for a good map. You could destroy your GPS in a crash, run the batteries down, lose it or ride through areas with poor GPS reception (though these are fairly rare and usually limited to deep, narrow canyons or heavy foliage). So bring a map and your GPS.
On the most-basic level, a GPS will give you your location using latitude and longitude. Imagine a giant grid spread out over the Earth. Latitude indicates how far north or south of the equator you are. Anything north of the equator is positive, and positions south of the equator are negative. The south pole is at -90 degrees latitude, and the north pole is at +90 degrees latitude.
Longitude is the distance from the prime meridian, a line running through Greenwich, England. Locations west of that line are a negative number, and anything to the east is a positive number. This runs 180 degrees in each direction.
The finer measurements in latitude and longitude are known as minutes and seconds. For example, the following coordinates show a location expressed in latitude and longitude (the spot is Moonshine Gulch, a terrific little bar in South Dakota): 44 degrees 7 minutes 470 seconds north, 103 degrees 43 minutes 178 seconds west. This shows up on maps and GPS units as: N 4407.470', W 10343.178'. Note that the lines of latitude and longitude are marked on the sample maps shown here.
In the backcountry, GPS units work best when you download topographic maps. You can then use landmarks on the GPS, match them to your map and figure out your location.
Here's where two key GPS features come into play: waypoints and track logs. Waypoints are simply precise locations you can mark on your GPS unit. You press "mark," and the GPS unit marks the spot where you are standing. On more-advanced units, you can name the waypoint.
For example, you are going to want to mark the point at which you park your tow vehicle, the trailhead and significant intersections. As you ride, you'll build a list of these points, and you can use them to figure out where you are.
The GPS unit can map those locations on the screen. You then can use the "goto" function to figure out how far you are from a particular waypoint. Say you are on the trail and want to know how far you are from your starting point. Select "goto," and select the waypoint for the starting point. The GPS unit will display an arrow pointing toward the point and a reading of how far you are from that spot. Bear in mind that the built-in compass on GPS units requires that you are moving to read it correctly. When using "goto," you'll want to ride or walk a few yards before trusting the direction shown on the screen.
This feature is particularly useful in mountains, where your view is obscured by ranges and it's hard to figure out where valleys connect. We used a GPS unit on an elk hunting trip and were able to hike cross-country among hunting areas by looking at waypoints from the previous day on the screen and using the "goto" feature to find them.