Topographic maps and GPS

Don’t voyagers use nautical charts exclusively? Why would a voyager be interested in topographic maps? Actually there are good reasons to use topo maps when voyaging.

When you read the manual accompanying a GPS unit, it tends to focus on the coordinate system of latitude and longitude. The manual may well mention “other” coordinate systems, but the thrust is on the most widely used system by mariners, latitude and longitude, since this is the coordinate system used on nautical charts.

There are times, however, when we mariners use topographic maps with another coordinate system such as Universal Transverse Mercator (UTM). UTM is common to topographic maps produced by the U.S. Department of the Interior Geological Survey. It is the coordinate system every soldier learns in map reading classes since military maps use this system. Few mariners I’ve met are aware of the existence of such a coordinate system, and even fewer know how to use it, especially with a GPS.

This was brought to my attention on our arrival in Palau and the discovery that nautical chart coverage of this area is extremely poor, and the charts that do exist are fraught with errors. Widely used by the mariners here are topographic maps. While these do not always show water depths and hazards, making “eyeball navigation” a necessity, their accuracy and the detail of the numerous islands make them extremely useful and much better than their nautical cousins. But these maps use the UTM coordinate system and, though a few cross-reference points to latitude and longitude are marked on the maps, it’s difficult to correlate a latitude/longitude position with a position on the UTM map.

A grid based on meters

But what is UTM? Basically it’s a coordinate system with a 1,000-meter grid square overlying a topographic map. A point on the map is represented by two groups of digits, the first equal to the number of meters to the right of the origin and the second equal to the number of meters up from the origin. It is further defined by a zone number and letter designation for delineating a sub-zone. So my position as I write this would be fully described as 53N 0438403-0808235. This corresponds to a lat/long position of 07° 18.789′ N, 134° 26.639′ E. There is no simple conversion technique between the two coordinate systems, but your GPS (if it’s worth is salt) should be as readily useable for topographic maps as it is for nautical charts.

A set of UTM coordinates is used or plotted with the convention of “reading right, then up.” The first group of digits define a point on the map reading to the right (similar to the longitude scale east of Greenwich) and the next group reading up (similar to the latitude scale in the northern hemisphere). The convention of right then up is universally applied regardless of the side of the equator or the Greenwich meridian you’re on. It’s quite unlike the lat/long system which has increasing quantities going away (up or down on the chart) from the equator and either east or west (left or right on the chart) of Greenwich. Later I’ll describe the UTM plotting technique in detail.

Programming your GPS

The legend on a topographic map gives the information needed to program a GPS for use with the map. As an example, I am using a topographic map of Oreor, in the Republic of Palau, Caroline Islands. The legend says, among other things, that it is based on UTM and Guam Datum 1963. These are the two essential items of information needed to enter into the GPS in order to obtain UTM readouts.

Not all GPS units have the selection you need, so looking for this feature before buying is advisable. The Garmin models going back to the Garmin 45 do have the selections you need. My older Micrologic Explorer doesn’t – it only displays latitude and longitude.

My Garmin GPSII+, however, has 12 grid position selections: three for latitude and longitude (different formats), and nine other grid selections (UTM/UPS, User Grid, Taiwan Grid, Swiss Grid, Swedish Grid, Maidenhead, Irish Grid, German Grid, and British Grid). So for my case I select UTM/UPS.

The Datum of Guam 1963 is one of 107 datum selections in my Garmin GPS (another is the familiar WGS84). So with the two necessary selections entered in the navigation portion of the system menu, I obtain position locations displayed on the GPS in UTM format. This allows for direct plotting of the positions on my topographic map.

Plotting with UTM coordinates

On a topographic map using UTM coordinates and a 1,000-meter grid, the lettered sub-zones are 10,000,000 meters (6,213.7 stature miles) on each side of the square. The grid lines, spaced 1,000 meters apart, are labeled within each lettered sub-zone from 0 to 9,999. For ease of reading and using the map, the grid lines are labeled with abbreviated numbers. The last three digits are omitted (these are the number of meters up to 999 between the grid lines) and the significant digits preceding the fourth and fifth significant digit are labeled in superscript.

Using the plotting device on a topographic map of Palau in the Caroline Islands to plot the position 53N 0438910 0810700.

For example, the 1:25,000 map of Oreor, Republic of Palau, has labels along the vertical axis such as 40, etc., corresponding to actual grid lines of 0438, 0439, 040, etc.

A position of 53N 0438910 0810700 is plotted by finding the point 910 meters to the right of the grid line labeled 10. An example of plotting this position will follow.

Valhalla’s plotting device

To find the number of meters between grid lines, a plotting instrument is very convenient and these were (and probably still are) in use by the military. They allow direct reading for interpolation between the grid lines. Different scales match the map in use, such as 1:25,000 or 1:50,000.

Lacking one of the production devices, you can make one by copying mine (on page 104). The figure shows the layout (to scale). Using transparency film (available from any office supply store for either printers or copiers), make a photocopy of this figure. Either cut out the inner square or drill a small hole, big enough for a pencil point, at the circle labeled “Plotting Point.” Note that the scales are reversed, which makes plotting in the upper-right-hand corner easy – simply move the device to the right and up the number of meters beyond the grid lines on the map.

Using the plotting device

The illustration on the opposite page illustrates an example of using the plotting device to plot the position 53N 0438910 0810700. Remember that you must read right then up.

But what about on land?

Since that’s what UTM was designed for you’re in great shape. In addition to using the GPS for voyaging here in Palau, as we hike over the islands we have found the handheld GPS to be quite useful. In the 14 years since the topographic maps of Palau were printed, there have been some changes that make finding the trails, caves, and other interesting historical sites difficult without the benefit of a GPS that can display UTM coordinates.

By Ocean Navigator