Convert magnetic azimuths to grid azimuths

Determining magnetic azimuths using a lensatic compass

This Web course consists of four lessons and four student activities Lesson 1 - Determine magnetic azimuths using a lensatic compass. Lesson 2 - Convert magnetic azimuths to grid azimuths. Lesson 3 - Convert grid azimuths to back azimuths. Lesson 4 - Plot back azimuths and determine location on a topographical map.

Declination is the angular difference between true, grid, and magnetic North The declination diagram is located under the lower margin on most topographic maps The declination diagram shows the angular relationship between the three Norths True North is represented on the declination diagram with a star Grid North is represented on the declination diagram with the letters GN Magnetic North is represented on the declination diagram with an arrow The Grid-Magnetic angle (G-M angle) value is the angular size that exists between grid North and magnetic North The G-M angle is expressed to the nearest half degree The grid convergence is an arch on the declination diagram, giving the difference between magnetic North and grid North To determine grid azimuths from magnetic azimuths your must add or subtract the G-M angle. If the magnetic North arrow on the declination diagram is the right of the grid North line (marked with an E in the G-M angle) add the G-M angle to the magnetic azimuth. If the result of this addition is larger than 360 degrees you must subtract 360 degrees to determine the grid azimuth. If the magnetic North arrow is to the left of the grid North line (marked with a W in the G-M angle) subtract the G-M angle to determine the magnetic azimuth. If the result of this subtraction is a negative number you must add 360 degrees to determine the grid azimuth.