[GRASSLIST:4348] Re: GIS visitor requests directions

[Eric G. Miller]

On Tue, Aug 20, 2002 at 10:27:33AM -0700, Victor Wren wrote:

> > 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).
> 
> This is getting a little over my head, now.  I have not tried to add new 
> projections to GRASS before.  Is there anywhere instructions to do this is
> recorded on the web?

All the state plane projections should be supported.  I meant, just
specify the type, parameters, etc. to g.setproj (run when creating a new
location).  Common state plane projections are lambert conformal conic
and transverse mercator (not UTM, but related).  Up to you to figure out
what the projection parameters should be (hint, look through
grass5/etc/state83).  The part about the conversion factor is important.
Iff your state plane data is in US Survey Feet (or will be), change
the <LOCATION>/PERMANENT/PROJ_UNITS entry "meters: 0.3048" to
"meters: 0.30480061".  Seems like a small number, but the error adds up.
This conversion factor will be used in projections.

Clear as mud ?

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