[Gdal-dev] issue warning to user when performing datum shift (OGR, GDAL)

[Ed McNierney]

Dylan -

I understand the concern, but I agree with Frank on the implementation
issues.  May I suggest that another approach would be to avoid the
"default" behavior and require an explicit statement by the user of the
datum shift method to be used?  I realize that backward compatibility
requires accepting current transformations "as is", but a statement
like:

ogr2ogr -s_srs 'prj=latlong datum=NAD83' -t_srs '+proj=utm +zone=11
+datum=NAD27 +method=NADCON' output.shp input.shp

Might help avoid any mystery about what "default" behavior is.  A
compile-time "strict" option might be required to turn this feature on.

I don't think that a command like the one above should warn the user
that a datum shift is being performed, any more than the user should be
warned that the coordinates of the points will be projected.  The
command line asks for a datum shift, so the user shouldn't be surprised
if one is performed.

	- Ed


Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
Phone: +1 (978) 251-4242
Fax: +1 (978) 251-1396
ed at topozone.com

-----Original Message-----
From: gdal-dev-bounces at lists.maptools.org
[mailto:gdal-dev-bounces at lists.maptools.org] On Behalf Of Dylan
Beaudette
Sent: Monday, July 31, 2006 4:41 PM
To: gdal-dev
Subject: [Gdal-dev] issue warning to user when performing datum shift
(OGR,GDAL)

Hi everyone,


In light of some recent thought, experimentation, and discussion [1] I
am wondering if it would be at all possible or even desirable to warn
the user when a datum shift operation is about to be performed. Within
the US, and assumming that the NADCON grid was installed with Proj4,
transforming between NAD27 and NAD83 should work just as expected.
However, in other parts of the world, this may not be the case.
Therefore, it might be a good idea to follow the model used in GRASS,
where the user is specifically prompted for datum transform parameters
whenever defining a new locatio, in these situations.

Any thoughts / reasons not to do this?

Cheers,

1. http://casoilresource.lawr.ucdavis.edu/drupal/node/259


Dylan
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