The needle deviation (engl. Grivation ) denotes the angle between Grid North a geodesic card and magnetic north . It is negative for western deviations (magnetic north west of grid north) and positive for eastern deviations.
Since the needle deviation is calculated from the (changeable) declination and the (unchangeable) grid north, the change in this value (usually annually) is usually noted on the map. The deviation from grid north and the declination from true north can also be specified individually.
The main advantage of the needle deviation is that each grid line running north-south on the respective map can be used as a reference for directions in relation to true north without having to fall back on a separate north line.
The following applies:
The signs must be observed, i.e. the directions of the deviations.
- Declination (geography) is the deviation from true north and magnetic north
- Meridian convergence is the deviation between true north and grid north .