[R-sig-Geo] Coordinate transformation for directions?
Barry Rowlingson
b.rowlingson at lancaster.ac.uk
Thu May 17 10:55:03 CEST 2012
spTransform does a great job of transforming points from one CRS to
another, but what's the best way of transforming directions?
For example, the wind at a point is blowing from the SW in lat-long
coordinates. That would be -3pi/4 radians if you consider lat-long as
a cartesian coordinate system. But suppose you want to work in
Ordnance Survey grid references. These coords are rotated slightly
compared to epsg:4326, and the rotation is different at different
points. Lat-long North is never exactly straight up on OSGrid maps.
My thinking is to consider a small vector (how small?) from (x,y) to
(x+dx,y+dy) in the required direction in lat-long, then transform
those two points to OSGrid, then compute the angle with the atan2
function.
Apart from wondering how small the vector needs to be, have I missed
any other subtleties? Has anyone written this already?
Note these are point-wise directions, not bearings of A from B. Great
circles are not involved!
Barry
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