[GRASSLIST:4329] Re: GIS visitor requests directions

[Glynn Clements]

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.

AFAIK, differences between datums are typically less than 1km, and
differences between international and US survey feet are around 2ppm
(an error of ~40 metres between antipodes). An error of 200 miles
(i.e. ~2.9 degrees) would have to be due to something major, e.g. 
using the wrong projection parameters.

-- 
Glynn Clements <glynn.clements at virgin.net>