The method you've described relies on the assumption that the Mag North and True North on the map doesn't vary through time. You wouldn't want to rely on this method using an old map for the Alaskan region for example as the magnetic pole will have shifted quite considerably depending on the age of the map.

With Grid mapping systems it is even easier. You work out the magnetic declination (time varying) to true north which will also have a true north to grid north adjustment angle as well (this being fixed and not subject to time change). Add or subtract to give the Magnetic variation with respect to the map grid then use the magnetic variation with respect to the grid north lines to compensate for measured azimuths from the compass and visa versa. This ensures more accuracy and when using a baseplate compass very little pencil and protractor work is required i.e. line up the edge of the baseplate from where you are on the map to where you want to go, rotate the inner bezel until the internal lines within the compass bezel are running parallel with the map grid northing, read of the compass bezel angle and adjust for the Mag to Grid variation. When the compass needle is pointing north on the compass bezel you move off in the direction the baseplate is pointing.

You can usually get this whole process done within a few seconds with practice.


BTW depending on the scale of the map with the grid system. The variation of Grid North to True North may be slightly different depending on which part of the map you are working on. The map should indicate this so that you can accurately determine the Mag North to Grid North Variation.

And if using a GPS system always using the appropriate GPS datum system conversion to match the map grid system you are using. i.e OSGB, WGS-84 etc. Getting the wrong datum can lead to significant errors in navigation. Also the GPS needs to know the Grid to Mag variation as well. Some GPSs do not calculate this variation automatically for you.

Even with a GPS to tell you where you are (very accurate), GPS systems are not really to great at telling you which direction you should head of in (not very accurate). A map and compass is more accurate in this respect.