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UTM Explained

Contributed by Mike Melnick

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Navigating in an adventure race is probably the most important discipline in the sport. Keeping your team on track and choosing efficient routes starts and finishes with plotting the points given in the racer instructions.

Background

Most adventure races use the Universal Transverse Mercator (UTM) system to define locations on their maps. The UTM system of coordinates are plotted similar to cartesian coordinates in units of metres. In other words, coordinate values increase going east and north. Technically, the UTM system has many projections around the globe in an effort to keep the grid lines square and to keep the grid sizes about 1 km on a side. For almost all adventure races, only one grid projection will be used. Looking at the corners of the map will indicate the projection and the location in it's entirety. For example, in Figure 1, you can see that the upper right corner, the easting is 707000 and the northing is 5666000. If you stood at the intersection of those two grid lines with a GPS, the device would give you your location to the nearest metre or even centimetre.

Figure 1. Close up of corner of map

Get to the Point

Consider Figure 2. For adventure races that use UTM grids (which is every one that I've ever raced) CP7 would be reported as 070650 and CP8 would be 070660. Because they are exactly one grid square apart, the distance between them is 1 km.

To put it more formally, any coordinate that you receive in your racer instructions, you can dissect into an Easing and a Northing. Let's consider the coordinate 367946. Please refer to Figure 3

First, split the 6 digit number into two 3 digit numbers: 367 and 946 Next, find 36 across the top or the bottom of the map (whichever is most convenient) Use a ruler and tick mark off 4 tenths of a grid space to get the east of 367 Now repeat for 94, but use the left or right numbers From 94, tick mark up 6 tenths of a grid square to get the northing of 946 Where those two coordinates intersect is your point

Figure 3. Sample plotting of UTM 367946.

Gotchas

One of the obvious problems is that the coordinates given in an adventure race are to the nearest 100 m. That leaves an awfully large area of uncertainty. When plotting your point, pay attention to clues in the instructions, such as "parking lot" or "trail intersection". Then use the UTM coordinates given to get close and circle the likely actual location. Keep your circle wide so you don't let yourself get too much tunnel vision while looking for a point

UTMs are plotted in two different datums, North American Datum 27 (NAD27) and NAD83. It's happened where a race director recorded the UTM coordinates off of a GPS using a different datum than what was plotted on the race map. Racers then navigated to a location that was 220 m to the west and about 90 m to the north. If you're certain you're at the right location and there's no sign of the CP, hunt around a little.

Wrap up

Getting around a race course using only coordinates and a map is a challenge, but definitely something that's achievable. It goes without saying that getting the points on the map in the correct location is imperative. Which route looks good changes drastically depending on the start and end locations.