How to Read
the Universal Transverse Mercator (UTM) Grid

From GPS World,
April 1996, pp. 32, Advanstar Communications,
Eugene, OR.

N.G. Terry, Jr.

The Universal Transverse Mercator (UTM) grid is most appropriate for scales of 1:250,000 and larger. On large-scale maps such as U.S. Geological Survey (USGS) 1:24,000, 7.5 minute quadrangles, the simple numbers of the UTM grid make plotting precise locations easier than with the complex degrees, minutes, and seconds of latitude and longitude. For example, you might readily use the UTM grid while hiking to report the location of an emergency by cellular phone to 9-1-1 or to record the location of a favorite campsite or trailhead. The UTM grid has tremendous value for emergency service organizations. In Figure 1, an example of industrial use is shown.

The UTM grid is of particular interest to anyone using a GPS receiver because most models offer UTM as a coordinate system option. UTM coordinates simply measure in meters east and north from two perpendicular reference baselines. All USGS 1:250,000 and 1:100,000 topographic maps carry a full UTM grid. On those maps, grid lines are 10,000 meters (10 kilometers) apart. On 1:24,000 maps, grid lines are spaced 1,000 meters apart. Coordinates are written along the sides of a map designating specific grid lines or tic marks. The two larger numbers are known as principle digits (see Figure 2). To plot coordinate values, read right, then up. Abbreviated coordinates can be used for locally defined areas (not larger than 100 kilometers x 100 kilometers). Abbreviated coordinates are always given as an even number of digits (as in Figure1, 0491) so you know where to separate easting and northing coordinates. This allows you to abbreviate to the degree of precision you require, as will be shown later. A full UTM coordinate value defines a worldwide unique postion ("map address").

Reading UTM Coordinates. In our example, we will locate an isolated feature, a drill hole. Think of UTM grid coordinates as the drill hole's (geospatial) address. First check the bottom left corner of your map for the map's datum and for its grid. The default datum for most GPS receivers is the World Geodetic System of 1984 (WGS 84). A mismatch between datums on your map and GPS receiver can cause errors of several hundred meters. On the USGS 7.5 minute, 1980 edition Sierra Madera NE, Texas map sheet (Figure 1), the note reads like this:

1,000-meter Universal Transverse Mercator grid, zone 13 1927 North American Datum

While most USGS topographic maps use the 1927 North American Datum (NAD 27), maps are being slowly revised to NAD 83. In this example, set your GPS receiver for NAD 27. When building a waypoint or writing a grid coordinate, the first part is the grid zone. In this case, we see in the note it is Zone 13.

To find a complete UTM coordinate, read right to the grid intersection before your place of interest. In this case, it's line 704, also known by its principle digits as line 04. Then count grid lines up to the intersection (for example, 3391 or 91). The abbreviated coordinate 0491 (think 04/91) gives the location to within 1,000 meters. Measuring right in meters from line 04, we find the drill hole is another 250 meters. The complete easting component is 704250E.

Measuring up in meters from grid line 91, the drill hole is another 520 meters. The complete northing component is 3391520N. The complete grid coordinate for the drill hole is Zone 13 704250E 3391520N or abbreviated as 04259152 (think: 0425/9152).

This coordinate defines a location of the drill hole to within 10 meters, or about the size of a modest home. The abbreviated coordinates can be seen embedded in the full coordinates:

04259152 = 0425 9152 Zone 13 704250E 3391520N

In the field, an expedient grid reader can be made by tearing off a corner of the map and marking both edges out from the corner in 100-meter increments. Use the map's bar scale as a guide. Plastic grid readers are available from USGS and commercial vendors. With these you can easily measure coordinates to 10 meters, or near the precision of the USGS 7.5-minute map. One of the great features of UTM coordinates is the ability to provide a more precise location by simply adding a pair of digits to abbreviated coordinates. For example:

4 digits     0491 accurate to 1,000 meters

6 digits    042915 accurate to 100 meters, a little more than an acre

8 digits   04259152 accurate to 10 meters, approximately the size of a house.

10 digits 0425091520 accurate to 1 meter.

Another Example. Hilltop 3705 is located at grid 0577591395 or Zone 13 705775E 3391395N.

Hilltop 3774 is located at grid 04309021 or Zone 13 704300E 3390210N.

Make an expedient grid reader using the metric bar scale in Figure 1 and answer the following: What is the grid for hilltop 3726? (The answer is at the end of this sidebar.)

The Big Picture. (See Figure 3.) Let's see how our previous example fits into the world as a whole. Our example was referenced to Zone 13. The world is divided into 60 zones of latitude, each 6o wide, that extend from 84o N to 80o S. These zones begin at 180o longitude and are numbered consecutively eastward. The conterminous United States is covered by 10 UTM grid zones (see Figure 4). In the northern hemisphere each zone's northing coordinate begin at the equator as 0,000,000 and are numbered north in meters. Thus our drill hole is located 3,391,520 meters north of the equator. For the southern hemisphere, the equator is labeled 10,000,000. The easting coordinates are measured from an artificial reference line drawn parallel and 500,000 meters to the west of the zone's central meridian. (Thus each central meridian is numbered 500,000.) In the case of Zone 13, the central meridian is 105o W.

Answer: Hilltop 3726 is at grid 05589068 or Zone 13 705580E 3390680N.

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