Grid convergance

It may be necessary to know the angle, at any point in the projection, between the meridian and the grid line (that is between true north and grid north). The difference in direction between the grid and the graticule lines is known as convergence; the value of the convergence is zero along the meridian 2° W but varies over the rest of the projection. The grid bearing, true bearing and convergence are thus connected by the following relationship. True bearing = Grid bearing + convergence. True bearings are measured clockwise and C has the sign as defined above. I.e. convergence is +ve to the east of the meridian 2° W and -ve to the west of it.

A spreadsheet that calculates grid convergence at any location - as well as performing other map projection functions - is available from here.

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