[GRASSLIST:4330] Re: GIS visitor requests directions

[Eric G. Miller]

On Tue, Aug 20, 2002 at 01:36:37AM +0100, Glynn Clements wrote:
> 
> Nigel McFarlane wrote:
> 
> > > > As I understand this (based on the man pages and experimentation)
> > > > this ONLY works when projecting between NAD27 and NAD83.
> > 
> > so do you agree with Victor that generic coordinate transformations
> > aren't possible with GRASS 5?  That would make GRASS a no-option
> > for me (arghh - manifold, here I come).
> 
> The projection utilities currently ignore the datum, except for the
> special case where one of the locations uses NAD27 and the other
> NAD83. It wouldn't necessarily be that hard to add generalised datum
> shifts; GRASS already has the necessary code, it just isn't used by
> *.proj.
> 
> I don't necessarily agree with Victor; the facts that is reporting a
> problem with converting between locations which both use NAD83, and
> with errors of around 200 miles both suggest that something else is at
> play here.

My guess would be feet vs. meters for the units.  Also, GRASS doesn't
not handle State Plane 83 without some extra effort (that is, the
g.setproj routine does not offer state plane 83 as an option, so
"state plane" is always state plane 27 in it's "mind").  Probably
better to just get hold of the projection parameters for the AOI and
plug them in (then adjust ft. vs. US ft. to meters conversion, if using
survey feet).  With Roger's "fix", the NAD27 <-> NAD83 should be handled
well enough (using NADCON the error should be pretty low).

-- 
Eric G. Miller <egm2 at jps.net>