This page is particularly useful for the purpose of planning marine charting expeditions.
MagVar's basic calculation
First you need to convert from latitude and longitude to your map projection.
The table on the next page shows the relationship between longitude and latitude in degrees and in degrees.
For the calculations presented in this section we shall assume that you are using the Mercator projection.
(In the table, Lat is the latitude in degrees, Lon is the longitude in degrees and Off