How to Measure Distance Along a Curved Line. The actual distance your feet travel on a hike is usually greater than the distance measured from the map. Still, you can get a fairly accurate measurement - even along a curved line - using this technique.
The sum total of distances marked on the paper straightedge, from first slash to last, is an approximation of the trail's distance. Be precise when measuring a curvy distance; the more twists of the straightedge and corresponding marks you make, the more accurate your measurement will be.
Locate and mark on the map the start and end points of the trail you'd like to measure.
Align the straight edge of a piece of paper on the map along the start of the trail and the first section of your planned hike.
Make a slash mark with a pencil at the start of the trail, leaving a tick mark on both the straightedge and the map.
Mark another pencil slash on the paper and map at the first point where the trail either bends away from the paper's edge or disappears underneath it.
Keep the second slash mark on the paper lined up with the second mark on the map, and pivot the paper until its edge is aligned with the next section of trail.
Make another slash across both the straightedge and map at the next place the trail bends away from the paper.
Pivot the paper again until it lines up with the successive section of trail, and make another slash.
Continue until you've reached the end of the distance you want to measure.
Measure that distance against one of the bar scales at the bottom of your map.