[GRASS5] [bug #744] (grass) v.proj does not do datum conversions!

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this bug's URL: http://intevation.de/rt/webrt?serial_num=744
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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.



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