# UTM Interpolator?

Say what? UTM Interpolator?

This is a tool that allows the plotting or extracting of UTM coordinates from a USGS 7.5 minute topo map (or equivalent such as the custom printed maps from mytopo.com).  Each UTM grid on a 7.5 minute topo map is 1000 meters square. The UTM interpolator breaks that down to 100 meter squares.

Editor’s note: this article is intended as an introduction to the use of a UTM interpolator and not as an instruction on map/compass/GPS usage.  Further reading and training is always recommended. A good primer is UTM Using your GPS with the Universal Transverse Mercator Coordinate System by John Carnes.

How does it work? With familiarity and practice, UTM coordinates can be extracted from a map or plotted to a map in a matter of seconds. Below is an image of a map obtained from mytopo.com, where you can select to have the map printed with a UTM grid an no additional charge (see blue grid lines). The northing lines run up and down and the corresponding numbers are at the top of he map. Near the center there is a number of 311000.  for northing values, you need to add a zero to match the value you will see in your GPS device (311000 becomes 0311000). Next is easting, which the lines run side to side and the corresponding numbers are on the sides of your map. Take note of the lower value on the map, 3857000. Now locate the “x” drawn on the map in pencil. It is to the right of 311000 and above 3857000. It is this “X” that we will determine the approximate GPS coordinates to enter in your GPS device.

To calculate the UTM coordinates, locate the hole in the upper right corner of your UTM interpolator. Place this hole over the “x” and align the grid with the blue UTM lines on your map as illustrated in the photo below. Alignment with your northing and easting lines is critical.

Now lets first take the northing number. Start with writing down 0311_ _ _. To extract the final 3 values, count the numbers to the left of the “x” until you reach the blue line. In the image above, the blue line falls on 3. The northing coordinate is then  0311300.

Next lets take the easting value. Start with writing down 3857_ _ _. To extract the final 3 values, count up to the “x”. This is where your estimating skills come in. The UTM interpolator touches the easting blue line between 6 and 7, but almost at 7.  I am estimating this at 6.8 and this would translate with the easting value being 3857680 (some of you may think 6.9 would be more accurate–look at the placement of the interpolator over the “x”, it is not quite in alignment and if the interpolator was slid down to alight with the “x”, 6.8 would be correct. The error was added to illustrate the importance of proper placement!).

Since we are eyeballing the space between 6 and 7, the accuracy  is within a reasonable 10 meters with each square on your UTM interpolator being 100 meters. Your coordinates are now 0311300 over 3857680.

Interpolators can be purchased if you search long and hard on-line and often they come with books on this subject. You can also make them yourself and then laminate them so you can attach the interpolator to you compass lanyard as I have done below.

Nordicpatroller has 25 interpolators available to the first 25 who request one. Go to the contact page and provide your name and address for your free interpolator (limit 1 per person).