Measuring Lat Lon Coordinates with a Map Ruler
Using a map ruler designed specifically for the USGS 1:24,000 scale map series, you can easily determine the latitude longitude coordinates of a point on the map.
Start by locating the 2.5 minute grid that contains the point of interest, in our example the red star.
Remember that the United States is in the northern and western hemispheres. Thus latitude values increase from south to north, and longitude values increase from east to west.
The right to left increase of longitude is often counterintuitive to beginning navigators.
In our example, the red star is north of the 38° 2' 30" parallel by a few minutes. It is not as far north at the 38° 5' parallel.
It is also a few minutes west of the 119° 55' meridian, but not as far west as 119° 57' 30".
We will start by measuring the latitude, which is the easiest of the two to measure.
You will measure how much farther north the point is from the known parallel to the south.
Place the zero end of the map ruler on the southern parallel. The 2.5 minute end should touch the next parallel to the north.
Note that one edge of your ruler is marked in minutes and seconds, while the opposite edge is marked in decimal minutes. Use the edge that matches the coordinate format you are using.
Slide the ruler east or west until it touches the point of interest.
Measure the number of minutes and seconds further north to the point of interest.
In our example, the point is 1' and 20" north of the southern parallel.
Add this measurement to the latitude of the southern parallel to get the latitude of the point of interest.
In our example the southern parallel has a latitude of N 38° 2' 30". So the latitude of our point is N 38° 3' 50"
Measuring the longitude is not quite as easy. If you place the ruler horizontally in the grid, the ruler extends past the edge of the grid. It's too long. This is because the meridians of longitude move closer together as you move away from the equator.
The solution is to place the ruler diagonally between the two meridians. This effectively scales the ruler to fit.
Keep the ends of the 2.5 minute ruler on the meridians, and move the ruler vertically until it touches the point of interest. Read from the ruler where the point touches it, to determine the number of minutes and seconds west of the eastern meridian.
Add this measurement to the longitude of the eastern meridian to get the longitude of the point of interest.
In our example, the point of interest is 1' 50" west of the eastern meridian at W 119° 55', so the resulting longitude is W 119° 56' 50"
To reach some points in the grid it may be necessary to use the ruler on the other diagonal or to extend the end of the ruler above or bellow the grid, while still keeping the ruler ends on the 2.5' meridians.
We now have the latitude longitude coordinates of the point of interest, N 38° 3' 50" W 119° 56' 50"
We could now proceed to store these coordinates into our GPS unit, and use it to assist us in navigating to the point.