Measuring a Bearing on a Map (Lat/Lon)

The main reason for measuring a bearing on a map is to help with navigation, for example:

  • You know where you are on the map and you want to go towards the Incident Command Post (ICP), which is on the map, but it is several miles away over hilly terrain.
  • You know where you are on the map and you want to go to a higher elevation for a better view of a potential safety zone, but you can’t get there directly because of a wetland.
  • You know where you are on the map and you want to go to the lookout, but you can’t see it because of morning fog.

A protractor or compass can be used to measure a map bearing; a protractor is easier to use and more accurate than a compass. There are different types of protractors, but a common one is made out of flat clear plastic and is in the shape of a semi-circle with degree marks.

Using a Protractor to Measure a Bearing on a Map

Steps to measure a bearing on a map using a protractor:

  1. Center the protractor over the starting point and orient 0 degrees with the true north (the 0 degrees line needs to be parallel to the edge of the map)
  2. Draw a line on the map from the starting point towards the destination. If you don’t want to draw a line on your map, align a string or ruler with the start point and the destination point.
  3. Read the bearing where the line, string or ruler intersects the protractor. This is the direction of travel to your destination.
  4. Before using this bearing in the field, adjust the compass for declination.

navigation_pms6-3

 

Using a Compass to Measure a Bearing on a Map

A compass can also be used to measure a bearing on a map. One way to measure a magnetic bearing is described below:

  1. Adjust compass for map declination. Disregard the magnetic needle.
  2. Place the compass on the map where one edge of the base plate touches both the start and destination point. The direction of travel arrow needs to point towards the direction of the destination.
  3. Turn the compass housing so that the orienting arrow points to the true north, parallel to map edge and longitude lines.
  4. Read bearing at the index line. This is the direction of the magnetic bearing.

navigation_pms6-4
Steps to measuring a true bearing on a map using a compass.

  1. Set compass at 0 degrees declination. Disregard the magnetic needle.
  2. Place the compass on the map where one edge of the base plate touches both the start and destination point. The direction of travel arrow needs to point towards the direction of the destination.
  3. Turn the compass housing so that the orienting arrow points to true north, parallel to map edge and longitude lines.
  4. Read the true bearing at the index line.

 

[Continue to Plotting Points on a Map using Lat / Lon]

**Information adapted from the National Interagency
Incident Management System Basic Land Navigation
Manual, PMS 475 dated June 2007.