Details Ticket 744


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Serial Number 744
Subject v.proj does not do datum conversions!
Area wish
Queue grass
Requestors russo@swcp.com
Owner none
Status resolved
Last User Contact Never contacted
Current Priority 30
Final Priority 70
Due No date assigned
Last Action Wed Aug 8 07:10:32 2001 (7 yr ago)
Created Fri May 25 19:03:38 2001 (7 yr ago)

Transaction History Ticket 744


Fri, May 25 2001 19:03:38    Request created by guest  
Subject: v.proj does not do datum conversions!

Platform: FreeBSD 4.3-STABLE
Xwindows version: Xfree 3.3.6
Xwindows manager: fvwm2
TclTk version: tcl/tk 8.3
grass downloaded at: Baylor site
grass binary for platform: I compiled the sources myself
grass sources source: no, I got a source code package from the server, grass5_beta11
c compiler name: gcc

v.proj does not do conversion between geodetic datum.  This "bug" report stems
from a discussion on GRASSLIST in which it was pointed out that this isn't a
bug, but a "shortcoming."  I submit it here to get it in the queue for possible
development.

If I create a location using USGS SDTS files referenced to the NAD83 datum (GRS80
ellipsoid) and try to move them into another location that was created using
files in the NAD27 datum, it seems that the projection function merely does a
UTM->lat/lon->UTM conversion, with no intervening datum shift.  This leads to
erroneous data import, because the process should go:

NAD83 UTM->lat/lon on GRS80 ellipsoid->lat/lon on clark66 ellipsoid->NAD27UTM
PROJ.4 comes with a program called nad2nad that does the conversion, and it may
be the case that some or all of the appropriate routines for this are in the
proj library of grass.  

I will try to look at v.proj and the projection library (which, as I understand,
is based on PROJ.4, with which I am somewhat familiar) and see what it would
take to get v.proj to recognize the difference in datum and do a datum conversion
using the NADCON files.  I'll pass on any possible modifications I make back
to the GRASS developers.  May take a few weeks, though.

At a minimum, if there is no manpower to implement datum shifts in v.proj, v.proj
should issue a warning that the two locations use different data.  The man page
should also be updated with all that extra spare time you all have :-)

This thing is a fairly serious issue for North America --- the USGS has been
releasing old files using NAD27 coordinates, but newer ones with NAD83.  USGS
DEM files are mostly in NAD27, most DLGs are in NAD27, but all the new stuff
is in NAD83.

Wed, Aug 8 2001 07:10:24    Comments added by egmiller  
This is a known problem but won't be fixed for GRASS 5.0.  Therefore, closing
this bug report.
Wed, Aug 8 2001 07:10:32    Status changed to resolved by egmiller  
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