Plotting Lat Lon Coordinates with a map ruler

Plotting latitude longitude coordinates onto a map is essentially the reverse of the process used to measure the coordinates of a point of interest.

As an example, suppose at our current position, our GPS receiver displays coordinates of N 38° 3' 50" W 119° 56' 50".

First we need to locate the 2.5 minute grid that contains this coordinate.

The latitude falls between the 38° 2' 30" and the 38° 5' parallels and the longitude falls between the 119° 55' and the 119° 57' 30" meridians.

Subtract the latitude provided by the GPS from the latitude of the southern parallel to determine that the point is 1' and 20" further north.

Subtract the longitude provided by the GPS from the longitude of the eastern meridian to determine that the point is 1' 50" further west.

It's this subtraction of minutes and seconds that makes using the decimal minute notation favorable.

Using the map ruler measure up from the southern grid line 1' 20". Make this measurement at the left and right edges of the grid, placing a small tic mark on each. Then draw a light line across the grid at the desired latitude. The point to be plotted lies along this line.

Use the map ruler to measure 1' 50" west of the eastern meridian. The map ruler will need to be placed on a diagonal and moved up or down until the 1' 50" mark crosses the line of latitude you drew in the previous step. This is the location described by the GPS coordinates.