Yes Yes, Magnetic Variation is a No No, so here’s the Real Angle
Drifting off course just might not be your fault on a long compass leg. This year I happened to notice a few orienteering and rogaining maps with the wrong correction angle applied or specified. There may well be a few more like that.
Intrigued, I did a bit of sleuthing and came up with a convenient culprit – the internet. If required to orient a grid based map to magnetic north, an amateur mapper or course planner/setter will likely head to the internet. There are a number of really useful calculators in the cloud and our mapper quickly selects the well known magnetic variation as the required angle for the locality and all is well. S/he thinks.
Unfortunately for the mapper, magnetic variation is the angle between True North and Magnetic North. The angle we need is the grid magnetic angle (GMA), which is between Grid North and Magnetic North.
So what? Well the difference between the two angles is roughly 2 degrees where I live. On a 200m orienteering compass leg, you are shifted 7m, perhaps additional to your normal error. Just enough to miss seeing the control. Rogainers may have a 300m compass leg giving a 10.5m shift. We really don’t need that disadvantage do we?
For more detail refer to this VicMap item.
Solutions to the Crime
So can you use the internet calculators to get the GMA? I assume yes if they specify, or if you can select, GDA94 or WGS84 co-ordinate systems upon which our maps are based. You could easily check a few calculations against these useful lookups.
ViCmap offers a downloadable spreadsheet and a text file that contain the current year angles for all areas of Victoria – necessary since the GMA varies across VIC by over 6°. Both reference the 1:30k topo series for location and appear to be updated every 1 to 2 years)