[Proj] NAD27 and WGS84 woes

[Hamish]

Michal Migurski wrote:
> There certainly could be some error in those files. They do
> have a strange rotation going on; when I render bitmaps I
> can see slices of blank data around the edges, illustrated
> here:
> 
> 	http://teczno.com/tmp/oakland_east.dem.jpg
> 
> Does that apparent rotation mean anything to you? I've
> seen it with quadrangles from other sources as well.


the rotation is due to the difference between true north and grid north
at your site.

hints for calculating this Convergence Angle using proj or grass can
be found here:
  http://grass.osgeo.org/wiki/Ps.map_scripts#Creating_a_fancy_North_Arrow


Hamish