All the data in my system is based on NAD27. I was wondering if I
digitize a map based on NAD83, how hard would it be for v.digit to
automatically convert the data to NAD27.
Bob Atnip
Camp Atterbury
Hi, Bob!
I'm definitely not the expert on this subject, but I know enough to say
that this is not painless. First of all, v.digit is not going to do a
thing for you. If you were interested in doing a simple projection
change, from, say, State Plane to UTM, you would use v.proj, _as_long_as
you_were_working_in_the_same_datum_!! Datum transformations are much more
complicated; they are not based on simple mathematical models. The latest
version of the program "proj", which is used by GRASS to do projection
transfromations, includes a module called "nad2nad" which is specifically
for converting between NAD27 and NAD83. The problem is that it is not
seemlessly hitched to GRASS! Definitely a "roll you own" proposition. You
will need to do a "v.out.ascii" to get ASCII coordinate pairs to use with
the program. Perhaps someone else out there would like to describe their
own experience, as I have never done this in GRASS... (I cheat and use
other GIS packages 
).
-Malcolm Williamson
Center for Advanced Spatial Technologies
University of Arkansas, Fayetteville AR
On Thu, 18 May 1995, Robert Atnip wrote:
All the data in my system is based on NAD27. I was wondering if I
digitize a map based on NAD83, how hard would it be for v.digit to
automatically convert the data to NAD27.Bob Atnip
Camp Atterbury