[Proj] How to use +towgs84

[Aaron Friesen]

Jose' (et al),

> -----Original Message-----
> Any coordinate system must have a "towgs84" or any other form of
> conversion to a global datum. In particular WGS84 itself has
> "+towgs84=0,0,0". You should use:
> 
>   cs2cs +proj=latlong +ellps=mod_airy +towgs84=xxxxx +to +proj=latlong
> +ellps=WGS84 +towgs84=0,0,0
> 
> where xxxxx is the set of 7 parameters you have. They should have signs
> according to the conversion from local datum to WGS84 and rotations
> must agree with the "position vector rotation" method (not "coordinate
> frame rotation"). For the conversion from WGS84 to local datum just
> change the two coordinate system strings, not the signs.
> 
> Instead of  using
>     +proj=latlong +ellps=WGS84 +towgs84=0,0,0 you can use
>     +proj=latlong +datum=WGS84
> which is equivalent.

Is +towgs84 actually a misnomer?  Is it not in general the means to
apply a 3 or 7-parm transform?

Aaron