[postgis-devel] Issue 89 in postgis: transform() grid-shift 2nd chance logic defective

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Status: New
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New issue 89 by william.... at acm.org: transform() grid-shift 2nd chance  
logic defective
http://code.google.com/p/postgis/issues/detail?id=89

What steps will reproduce the problem?

select x(transform(pointfromtext('POINT (173 52)',4269),4267)),
y(transform(pointfromtext('POINT (173 52)',4269),4267));
WARNING:  transform: -38 (failed to load NAD27-83 correction file)
ERROR:  transform: couldn't project point: -38 (failed to load NAD27-83
correction file)
<SELECT aborted; no output rows>

What is the output as corrected?

select x(transform(pointfromtext('POINT (173 52)',4269),4267)),
y(transform(pointfromtext('POINT (173 52)',4269),4267));
WARNING:  transform: -38 (failed to load NAD27-83 correction file)
WARNING:  transform: -38 (failed to load NAD27-83 correction file)
   x  |        y
-----+------------------
  173 | 51.9999999990852
(1 row)


Please use labels and text to provide additional information.

The PostGIS transform() function contains logic that intends to substitute
a "2nd chance" non-shifted transformation for an unavailable grid-shift
transformation (e.g., NAD83 to NAD27 transformation of a point outside of
the area covered by the NAD27 grid-shift table, or in the absence of that
table).  The logic handling this situation is located in the
transform_point() function of lwgeom_transform.c but seems outdated with
respect to how recent versions of proj.4 handle the situation.

This discord causes a "SELECT ... transform(geom,srid) ... FROM mytab" to
abort and be rolled back where even a single row of table mytab contains a
geometry geom that lies outside the bounds of an applicable grid-shift
table.  Running on Postgres 8.3.5, PostGIS 1.3.3, and proj.4 4.6.1, such a
SELECT aborts with the twin messages shown in the problem reproduction  
above.

Attached is a patch that seems to be a reasonable solution for this problem.

Although this patch works for me, please scrutinize it carefully.  This is
my first look at PostGIS and proj.4 source code.  Worse, I code in C very
rarely and am especially error-prone when using C pointers and indirection.
  I have not been able to fabricate data that exercises every branch of this
patch, so it may still contain errors.

Thanks so much for PostGIS!  It's a tremendous resource that contributes
materially to the public good.

Attachments:
	lwgeom_transform-1.3.3.diff  3.1 KB

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