How to Measure a Straight Line Distance Using a Topo Map

Line up the straight edge of a piece of paper along the start and end points of the distance you're measuring on the map, and make a pencil mark at the two points.

Find the bar scales in the topo map's margin. They're the long, thin bars labeled miles, feet and kilometers at the bottom center on United States Geological Survey (USGS) maps.

Choose which of the three measurements interests you.

Line up the left edge of the marked distance on the paper with the left edge of the appropriate bar scale.

Note that the left edge of the bar scale doesn't start at zero, but is subdivided to measure to the nearest tenth of a mile, thousand feet or hundred meters. Zero is found at the right edge of the subdivisions.

Look at the right-hand mark on the paper, and slide the paper to the right, along the bar scale, until the right pencil mark is aligned with the next higher whole number on the bar scale.

Note that the left edge of the distance to be measured should now be in the subdivided part of the bar scale at the left.

Note the distance on the bar scale that corresponds to the right-hand mark on the paper.

Add the number of tenths of a mile, thousands of feet or hundreds of meters (depending on which bar scale you chose) that falls to the left of the zero mark on the bar scale.

Understand that this total is the actual ground distance between the two map points.