Universal Transverse Mercator (UTM) is a grid system that describes a person’s geographic location in the backcountry. It is simple to understand and use because:
1. It is intuitive – it’s concepts can be understood quickly,
2. It can be easily self taught,
3. Young hikers grasp this system easily,
4. A location on a map can be quickly determined, and;
5. It is a selection option for Global Positioning System (GPS) receivers.
A navigation grid is a reference system developed by cartographers that can be used to plot a geographic position on a map. There are many grid systems available for use such as Latitude and Longitude and Universal Transverse Mercator (UTM). Several countries have their own national grid system.
The UTM system is just like a Cartesian Grid. For example, the grid position below is just 2.2 units over and 2 units up.
To understand the complete grid we will start by observing that the globe is divided into 60 zones. Each zone is 6° of longitude wide. Each zone runs north and south; 84° north to 80° south.
The image below highlights the UTM zones in the continental United States.
UTM grid consists of Northings and Eastings. The image below highlights the complete layout of a UTM zone. Notice the Central Meridian that runs north and south through the zone. Like longitude, this meridian runs from pole to pole. All values for measuring position are in meters. At the equator the zone is 500,000 meters wide. The width of the zone is described by Eastings. Northings run north or south from the equator; again all values are in meters.
UTM coordinates are presented such that the zone is listed first, followed by the Easting and then the Northing.
10 0524120 E 4891555 N
The hiker should think of a grid as a series of defined squares on a map (see below.) On a 1:24,000 scale (7.5 minute topographic quadrangle) the grid lines are 1000 meters apart; north or south the spacing between grid lines is 1000 meters.
The coordinate values are known Easting’s (vertical lines) and Northing’s (horizontal lines.)
Easting values increase moving from left to right and Northing’s increase from bottom to top. Coordinate values are always positive.
Every location will have a zone identifier. On the map above the zone is linked to the Easting value and is the first set of numbers. In this case the zone identifier is the number 10.
The letter “T” seen above is a secondary, horizontal (east-west) identifier. I personally pay little attention to it in my backcountry trips.
All USGS maps identify the zone in the title block at the bottom left of the map. Note that on some commercial maps the UTM zone identifier may not be in the title block and can be hard to find.
The UTM coordinate can now be refined to a meter. Again the spacing between the grid lines is 1000 meters (1 kilometer). On the maps below, the tick marks between gridlines are in increments of 100 meters. The hiker can then interpolate the distance between the tick marks.
The position of the large X on the map above would be described as:
10 5 25 270 East (the green line)
47 91 180 North (the red line)
The final three places will always be expressed (10 5 25 270 East.) The value 2 is in units of hundreds, the 7 is in units of 10 and the 0 is in units of 1’s. Thus, 50 meters would be written as 050.
Every point on a map (e.g., a mine, an intersection, a camp site, etc) can be described using UTM coordinates to the accuracy of one square meter.
I recommend consider carrying a small plastic ruler or other suitable straight edge when accuracy is important. For general hiking and backpacking, one can quickly estimate a current position in the backcountry without other map tools.
UTM coordinates of a destination taken from a map can be easily saved on a GPS receiver. For example, to do this the hiker “marks” a waypoint and then moves the backlit bar (yellow shaded area) from “save” to the “location” data field. The “location” data field is then edited per the receiver’s instruction manual.
A fine reference for more practical information about UTM grid is Lawrence Latham’s book GPS Made Easy. Chapter 5 has an easy to understand tutorial on this grid system; that’s how I learned it.