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Your GPS translates signals from space into UTM or longitude/latitude coordinates. Among the best-selling handheld units nowadays are the Garmin GPSMAP 62S Handheld GPS Navigator and the Magellan eXplorist GC. (Click links to see product descriptions and the latest prices on Amazon.)


UTM longitudinal zones for the United States. The traditional longitudes (in degrees) are listed at the top.


UTM latitude zones are represented by letters.


Topo map with UTM grid. Note labeling of easters and northers on each side. Use these to plot your GPS coordinates and find your exact position. You can also reverse the process, looking up coordinates on locations you want to visit, then entering them into your GPS unit before your trip.

Some GPS units can upload topographical maps and generate a route based on the waypoints you enter into memory.

This GPS unit includes an electronic compass and can also provide approximate travel times, distance and speed (while you're moving). The extra features are nice but add to the cost of the unit. In any case, you should still bring your map and compass with you, since there are times when you won't be able to get a signal.

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Using a GPS Unit - The Basics

Continued from Page 1

If you’ve never used a hand-held GPS unit, here’s how it works: Imagine you’re out in the field and want to find your exact position on a map.   Alternatively, you might be looking for a certain spot and know its GPS position, but not know how to get there.

In either case, you'll turn on the unit and activate its satellite search function. After a few moments, signals from space respond to your call and some coordinates appear. For example, you might see 14T 0240615E and 5329341N on the screen. This is your current location. 

"14T", represents the UTM zone, you're in. (The map you're carrying should include this zone.) To understand what a zone is, check the two maps in the next column over on this page. One covers the numbers 10-19, which are longitudes while the lower map uses letters to indicate latitude. Thererfore, the number 14 on your GPS screen is the UTM longitude zone you're in, while the T represents your latitude from the Equator. Together the number and leter represent a square chunk of geography on the face of the earth. Note that longitudes are always idenfified by number, while latitude zones use letters.

Once you've got your zone identified, you can start honing down your position to an exact footprint. The first coordinate written to the right of of it of "14T", in our example, is "240615E". This is known as an easting.   The "E" stands for East and represents a longitudinal (vertical) coordinate. But the number 240615 is an actual measurement in meters. This is the distance that your position lies from the zone’s reference line on a map. Check the third map in the next column on this page, and you'll see how these big numbers are marked on the UTM east-west grid lines, at the top of the map, running left to right. (It's hard to read, but it starts at "12 S 72000" and climbs in 1000-meter increments.)

Here's how to plot the sample coordinate 14T 0240615E on the paper map of the area you're in (which should have the corresponding numbers):

  1. Find the grid line marked 240,000 at the top. 
  2. Now you've got to move 615 meters to the right of that line to mark your exact longitude. To figure out how far that is, find the conversion scale provided in the legend or elsewhere on the map.
  3. Use a string or pencil to carry the measurement over to the line at the top and mark the spot. (This works just like a mile scale on a road map.)

Next, you'll find the latitude on the map and locate that spot as it lines up with your first mark. The second number listed on the GPS screen in our example, "5329341N", is called a norther.  This is your horizontal coordinate. The "N" stands for North. And the figure "5,329,341" equals the number of meters your position lies north of the Equator. To find the spot:

  1. On the map, follow the range of numbers marked on the left and right borders. In the sample map on the right, you can see the north to south UTM grid lines and the number range. 
  2. Now you can locate the exact latitude in the same manner as the longitude.
  3. Mark that latitude spot on the map where it intersects with the mark you made for the Easter earlier.   This is the exact position where you’re standing, within a meter or so.

Just as you use GPS to find out where you’re at on a map, you can identify an easter and norther on the map first, then enter those coordinates into a GPS unit to get directions. The display will even provide a map and compass bearings.

Any location (i.e. UTM or longitude/latitude coordinates) entered into a handheld GPS memory is known as a waypoint.  It might be a mountain peak, trailhead, campsite or parking lot. Before setting off on a trip, it's always a good idea to enter a waypoint for each place of interest or importance. That saves you time messing around with a map. (Of course, if for some reason the GPS stops functioning you may still need it.)

Veteran hikers in the backcountry program in locations where two baselines meet, or where a trail meets a bridge, or other easily identifiable landmarks to help them stay oriented. Even before they reach one of these spots, they can use their GPS unit to tell them how close they're getting. Most GPS units provide compass bearing, and some even furnish the compass!

A sequence of waypoints entered into a GPS unit is known as route.  You can retrieve information about a waypoint by using the "GoTo" or "Find" function. Once the waypoint comes up, ask the unit for a bearing to that location from your current position.  Some units also calculate travel time, as well as the rate of elevation change between your current location and the waypoint. 

A GPS unit is huge asset in situations of low visibility, featureless terrain, a snow-covered landscape or an open sea. Moreover, latitude and longitudinal markings on topo maps are provided only at 2.5 minute intervals, whereas the UTM grid makes calculating distance a walk in the park. That's because those thousands of meters can be condensed into a few kilometers simply be moving your decimal three digits the the left.

Unlike a compass, you can’t simply point a GPS receiver at a landmark and retrieve any useful information. However, in unfamiliar terrain, some units will allow you to record your footsteps.  This is known as recording a “track”. Keep the device steady by hanging it on your pack, rather than carrying it in your hands.  Then, on the return trek, retrieve the track from memory. Now you can navigate your return trip using the LCD screen. This feature is also useful if you have to make your return trip in the dark.

Most GPS models provide additional features, such as a barometric altimeter, electronic compass, the ability to upload and view topo maps, and wireless data sharing (including photos of your trip). Garmin is the top manufacturer of handheld units in the United States. Like all electronic gadgets, GPS should be considered a convenience rather than a survival tool. Make sure the batteries are fully charged and always carry an extra set. 

In bad weather, thick forest cover, alongside a high cliff wall, down in a canyon, or when other overhead obstructions are present, you may not be able to get a signal from four satellites. In recent years, a new application called the Wide Area Augmentation System, or WAAS, has improved the ability of GPS units to retrieve signals even when obstructions exist.

Weather in space can also affect signal strength, since GPS satellites orbit high above the earth.  For best results, take readings whenever you’re in a clearing or high point of elevation. If the temperature drops below freezing, the LCD screen may stop working, so keep the device insulated and warm.  If water penetrates into the GPS unit, it could short-circuit the electronics, so you should also pack it in a waterproof container.

And before you jump on Ebay to purchase somebody's old 1995 model for what seems like a steal of a price, don't forget that the kinks in this new technology are still being ironed out, so later models are your best bet.

The UTM Grid in more detail

When examining a world map, you'll notice that the distance between meridians and parallels begins to shrink as they converge at the two poles.  This creates distortion in global mapping, since the planet is fat in the middle but skinny at both ends. To get around this problem, a grid system (or dadum) called the Universal Transverse Mercator was developed in the 1940s.  It’s included on USGS topographical maps and is nowadays a standard feature of handheld GPS units. UTM coordinates are derived from a mathematical grid of 60 longitudinal-like zones, each 6 degrees across, for a total of 360 degrees, and a lesser number of latitude zones measuring 8 degrees high.  UTM coordinates are more exact than traditional latitude/longitude readings.

All recently published USGS maps, as well as standard GPS software, conform to a UTM standard known as WSP84.  Older USGS maps use the 1927 North American Datum, abbreviated as NAD27, or a later version known as NAD83  Check to see which dadum is listed on the map you’re using. If it's not the default WSP84, you'll have to change to it in the preferences or settings menu of your GPS unit.  Otherwise, your coordinates will be slightly off. You also have the option of selecting traditional longitude and latitude coordinates.

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