Linking GPS, Maps, and Coordinate Systems

Some people think that after they have a GPS receiver, they really don’t need a map, especially if the receiver has built-in mapping capabilities. This isn’t necessarily true. GPS receivers are best used in conjunction with maps, whether those maps are paper or digital. Here are some of the reasons why:

• Detail: Most maps on handheld GPS receivers don’t offer the detail of full-size paper or digital maps, especially topographic maps and nautical charts.
• Size: A GPS receiver’s screen is pretty darn small, and it’s just about impossible to get the big picture that a full-size map can give you.
• Backup: If you have a paper map with you and know how to use it, the map becomes an important backup if your GPS receiver’s batteries fail or if you encounter poor satellite coverage. Gadget lovers might consider a paper map and magnetic compasses primitive, but they don’t require batteries — and both are lightweight and cheap, to boot.
• Complementary: After you get back home or to the office, you might want to see where you’ve been on a map, based on the locations that you’ve stored in the GPS receiver.

With a digital map, you can easily plot the exact route that you took or identify the places you visited. All maps and GPS receivers use coordinate systems, which are grids on maps that enable you to find locations on a map. Because GPS receivers are designed for use with maps, they support a number of coordinate systems that correspond to those commonly found on maps. Thus, you can take a location that you recorded on your GPS receiver and precisely locate it on a map. By default, your GPS receiver displays positions in latitude and longitude. But you can change the settings to display locations in exotic-sounding coordinate systems such as the Finnish KKJ27 grid, the Qatar grid, or the W Malaysian R grid. You probably won’t need to switch to some of these obscure coordinate systems, so you can stick with latitude and longitude or Universal Transverse Mercator (UTM), which are used pretty much everywhere.

If you’re fuzzy on what latitude and longitude or UTM are, which gives an overview of the two coordinate systems. Suppose you have your GPS receiver set to latitude and longitude, and you record some locations. When you get home, you find out that your paper map doesn’t have latitude and longitude marks but uses only the UTM coordinate system. Not a problem. Look in your GPS receiver user manual for information on changing the coordinate system from latitude and longitude to UTM. Location points that you store in latitude and longitude appear as UTM coordinates. This is a quick and easy way of converting data between coordinate systems. Or you can visit the Graphical Locator Home Page at Montana State University (www.esg.montana.edu/gl/index.html) to perform online conversions of latitude and longitude and Township and Range. Although Township and Range is a popular coordinate system used on many maps in the United States, don’t expect your GPS receiver to support this system.

Unlike the latitude and longitude and UTM grid systems, which were mathematically derived, grids in the Township and Range system may not always be the same size, thus making it difficult for a GPS receiver to determine coordinates in this system. Most maps that have Township and Range information also have latitude and longitude or UTM marks. If you have a location reported in Township and Range, you’ll need to spend a bit of time looking at a map, manually figuring out the position that the coordinates point to, and then plot that position by using a different coordinate system.