Wiggo
Well-Known Member
Bit of an obscure one, this. Now, I'm perfectly happy with all the various chart datums and the whole issue of magnetic variation, but I have recently turned up a bit of a poser: Grid Convergence. The grid north on OS maps does not entirely line up with True North, although officially the difference can be ignored for practical purposes.
I believe that this convergence angle comes from the fact that the OS system is based on 25 500km squares arranged in a 5 x 5 grid overlaid on the UK, and that laying a square grid on a (vaguley) spherical earth means that the edges can't all point to true north. Seems reasonable. It also implies that somewhere on the OS Grid the Grid Convergence angle will be zero, and logic tells me this should be the Greenwich Meridian. Can anyone confirm this? By inference, west of the meridian Grid North should be west of True, and vice versa.
I believe that this convergence angle comes from the fact that the OS system is based on 25 500km squares arranged in a 5 x 5 grid overlaid on the UK, and that laying a square grid on a (vaguley) spherical earth means that the edges can't all point to true north. Seems reasonable. It also implies that somewhere on the OS Grid the Grid Convergence angle will be zero, and logic tells me this should be the Greenwich Meridian. Can anyone confirm this? By inference, west of the meridian Grid North should be west of True, and vice versa.
axis of the chart being there with the Easterly latitude/ right (x) axis. The deviation increases to the north and east as you porgress further away from the origin.