If your map has a printed UTM grid on it (which is a good idea for various reasons, covered on this tip) you can, with a little practice, be able to estimate your position down to about 100 meters or so. This should be good enough for most applications.
However, if you’re into precision navigation, or want to make your map positioning a bit more accurate, here’s a quick DIY compass hack to increase your accuracy.
You may know of a little clear plastic device (see left image) made to do this, which is known as a Romer, supposedly after the British military officer who invented them. These do allow you to do various bits of map and compass wizardry a little more easily, but they are small, easy to lose, only work for maps with a certain scale, and in general are not very suitable for civilian users, so I don’t recommend them for most people.
Let’s get to it.
a roll of athletic tape
a map with a kilometer scale bar divided into 100 meter increments
fine tip pen
(Note that if you print out a map from CalTopo, it should have a perfect scale bar for doing this.)
1 - Put a thin strip of athletic tape on the top and right hand edges of your compass base plate. Trim away any extra tape.
Place the top right corner of the baseplate on the one kilometer mark of the scale bar.
At each 100 meter increment, draw a small tick mark on to the tape.
Repeat this for the right-hand edge of your compass.
When you're done, your compass should look like this.
Now, you can measure UTM coordinates on your map. Put the top right corner of the base plate on the point you want to measure, and then read the tick marks in 100 meter increments to the left for easting and down for northing.
Example:, Put the top right corner of the baseplate on Sisson lake. We can see that the easting is about 500 meters, and the northing is about 300 meters.
(Note - If you change to a map with a different scale, you’ll need to start over and copy the new scale bar.)