Plot a GPS Fix on a Paper Topographic Map

A phone or GPS receiver tells you where you are as a pair of numbers. A paper map tells you what is around you. Joining the two is a basic navigation skill, and it is what search-and-rescue teams ask for when someone calls for help: a grid reference they can plot in seconds. This guide takes one real position, the summit of Mount Washington in New Hampshire, from a phone screen to a pencil mark on a USGS 1:24,000 quadrangle.

The starting point

A phone mapping app shows the summit as 44.2706, −71.3033 in decimal degrees. The minus sign on the longitude means west. Before going further, check what the map margin says about its grid and datum. Recent USGS US Topo maps are printed on NAD83 with a UTM grid in metres; NAD83 and WGS84, the GPS datum, agree to about a metre in North America, which is less than the width of a pencil line at this scale. Older quadrangles printed on NAD27 can be offset by tens of metres; if your map says NAD27, set your receiver to NAD27 or expect the mark to be slightly off.

Step 1: convert to UTM

The coordinate converter gives 19T 316175 m E, 4904508 m N. Zone 19 covers 72° W to 66° W, and band T covers 40° N to 48° N. The same point in other notations is 44°16′14.2″ N, 71°18′11.9″ W in degrees, minutes and seconds, or 44° 16.236′ N, 71° 18.198′ W in degrees and decimal minutes, the format a handheld GPS would show. Write down the UTM reference; it is far easier to plot than latitude and longitude, because the grid on the map is printed in the same metres.

Step 2: find the 1 km square

UTM grid lines on a 1:24,000 sheet are usually printed or ticked every 1,000 m, labelled with the leading digits in large type, such as ³16 for 316,000 m E and ⁴⁹04 for 4,904,000 m N. Our point lies in the square whose south-west corner is 316000 E, 4904000 N. Find the vertical line labelled 316 along the top or bottom margin and the horizontal line labelled 4904 along the sides; the square to the right of the first and above the second is the one you want. A common mistake is to read the square to the left or below, which puts the mark 1 km out.

Step 3: measure within the square

The remainder is 175 m east and 508 m north of the square’s corner. The map scale calculator shows that 1 km at 1:24,000 is 4.17 cm, or 1.64 inches, on the map. So 175 m is 0.73 cm, about 0.29 inch, to the right of the 316 line, and 508 m is 2.12 cm, about 0.83 inch, above the 4904 line. A plastic romer or a ruler with millimetres makes this quick; many compasses have a 1:24,000 or 1:25,000 romer on the baseplate. The rule for reading and writing grid references is “along the corridor, then up the stairs”: easting first, northing second.

Step 4: write a short grid reference

For radio or a phone call, rescuers often use a shortened reference. Taking the hundreds of metres from each coordinate gives a six-figure reference, 161 045, which identifies a 100 m square, the hundred-metre digits from 316175 and 4904508. An eight-figure reference, 1617 0450, identifies a 10 m square. Always state the zone and the 100 km context the first time, or the map sheet name, because the same short digits repeat every 100 km. Here a full reference would be 19T 316175 4904508, read as “one nine tango, three one six one seven five, four nine zero four five zero eight”.

Common mistakes

• Swapping easting and northing. The easting always has six digits and the northing seven in most of the world; if the numbers do not fit the grid labels, they have been swapped.
• Dropping the minus sign. A positive longitude of 71.3033 is in Kazakhstan, not New Hampshire. Converters cannot guess the hemisphere.
• Ignoring the datum. A NAD27 map plotted with WGS84 numbers can be off by more than the width of a trail.
• Trusting the decimals. A phone fix is typically good to 3 to 5 m in the open and worse in forest or canyons. Plotting to the nearest 10 m is as precise as the position deserves.
• Using a resized printout. If the map was enlarged or reduced, use its printed scale bar, not the 1:24,000 in the margin, to measure within the square.

Checking your work

Look at the mark: does the terrain match what you see? A summit mark should sit inside the highest closed contour, a stream crossing on a blue line. If the mark is in a valley while you are standing on a ridge, re-check the zone, the corner of the square and the direction of your measurements before relying on it.

Plotting checklist

1. Read the grid, datum and scale from the map margin.
2. Convert the GPS position to UTM in the same datum.
3. Find the 1 km square from the leading digits of easting and northing.
4. Measure the remainders east, then north, with a romer or ruler at the map’s scale.
5. Mark the point and check it against the terrain around you.
6. Write the full reference with zone and band before shortening it.