Drag the marker and get values of Grid Convergence and Magnetic Declination for that location.

Grid Convergence is defined as the difference between Grid North and True North for UTM projection (East Convergence is positive).

Magnetic Declination is defined as the difference between Magnetic North and True North (East Declination is positive).

Grid Magnetic Angle is defined as the difference between Magnetic North and Grid North (East Angle is positive) and it is calculated as 'Magnetic Declination' minus 'Grid Convergence'. This is also the value to be inserted in the 'Real World Coordinates - Angle' section of Software OCAD to have an orienteering map properly oriented to Magnetic North in case you have an UTM* georeferenced base map.

*Grid Convergence value is for UTM projections only, but it works also for other Transverse Mercator projections having the same central meridian as their UTM equivalent. So it works for instance for GaussBoaga Italy West - given that you are inside UTM32
or for Slovenia D48 (inside UTM33).

Viewer made by Remo Madella (Rem Maps).
Huge inspiration from Måns Beckman at
Grid declination is taken from geodesy.
Magnetic declination is taken from geomagJS with data from NOAA's World Magnetic Model.