First, measure the distance from the location to the nearest coordinate line. Take this distance, in inches, and multiply it by the number of seconds per inch (N to S=19.8 and E to W=24.4.)

This will give you the number of seconds from the map location to the nearest coordinate line. Then, take this number of seconds and add or subtract (East or South of line, subtract--West or North of coordinate line, add) it to or from the value of the nearest coordinate line. This will give you your new coordinate in degrees, minutes and seconds. Now, take the number of seconds you have and divide them by 60 and this will convert your seconds to thousandths of a minute.

Note: Coordinates of Longitude & Latitude increase in value, in a "left and up" direction!

Finding A Location From A Coordinate, On 7.5 Minute USGS Topo Map

Take the thousandths of a minute and multiply it by 60. This will give you the number of seconds. Then, take the coordinate and convert it from degrees, minutes and thousandths of a minute, to degrees, minutes and seconds.

Next, take the value (in degrees, minutes and seconds) of the coordinate line, on the map, that is closest in value to, but lower than the converted coordinate and subtract that coordinate from your converted coordinate.

The number of minutes and seconds you get here, all has to be converted to seconds. Take this number of seconds and divide it by the number of seconds per inch (N to S=19.8 and E to W= 24.4). This will give you the distance in inches you need to measure from your chosen coordinate line.

Since you subtracted the lower coordinate line on the map from your converted coordinate, the location you are looking for will always be either North or West of the coordinate line on the map.