https://en.wikipedia.org/wiki/Ordnance_Survey_National_Grid#/media/File:Ordnance_Survey_National_Grid.svg https://en.wikipedia.org/wiki/Ordnance_Survey_National

https://en.wikipedia.org/wiki/Ordnance_Survey_National_Grid#/media/File:Ordnance_Survey_National_Grid.svg https://en.wikipedia.org/wiki/Ordnance_Survey_National_Grid#... It looks like there's a line where grid north and true north always agree. https://en.wikipedia.org/wiki/North_magnetic_pole#/media/File:IGRF_2000_magnetic_declination.gif https://en.wikipedia.org/wiki/North_magnetic_pole#/media/Fil... Looking at this, it looks like "places where magnetic north matches grid north" might not be a straight line, or might not be a line that goes straight up and down the grid? E: maybe the former is the border between the red and blue areas here? https://en.wikipedia.org/wiki/World_Magnetic_Model#/media/File:Wmm2010declination.png https://en.wikipedia.org/wiki/World_Magnetic_Model#/media/Fi...