Datum Transformation

[markb]

I think that proj4 can transform between datums by adding the correct
parameter string (e.g. '+towgs84='... or some grid string) to the
'proj4text', but I haven't done this before, and I don't want
introduce any inaccuracies beyond the limitations of the datum
conversion itself (I'm pretty sure proj4 can be compiled with datum
support). I'm always careful to transform my datums correctly, I've
just been spoiled (not to my benefit of course) with a transformation
selection option in other software. I've seen that PostGIS's
Transform function appears to work the same as SpatiaLite's does. The
process for selecting a particular datum conversion for PostGIS seems
to involve creating a custom SRID with that additional parameter
appended to the spatial reference definition. Have any of you been
through this process before?

One possible example would be transforming from NAD_1927 to NAD_1983
for the Continental US.

I see that datum transformations are also registered with the EPSG, so
they also have an SRID-like code/id (the above example would be
1241). Would it be possible to have a datum transformation table,
similar to the 'spatial_ref_sys' table, whose SRID-like value could be
specified as an optional third parameter (or overloaded) within the
Transform function. I'm sure I'm way over simplifying this, it is
just that I'm so close to not having to open any proprietary software
for certain tasks now, projecting vector data is one of them. If I'm
way off base let me know. Thanks for your input.

-Mark

[markb]

I've been messing around with this some yesterday and today, and well
I'm stumped. Its not urgent or anything, I have other ways to
reproject data, it is just perplexing and a little limiting. I've
visited numerous resources relative to Proj4, PostGIS and datum
transformations in general with no forward momentum. And I've
certainly tried my hand at many modified/custom SRID's with no avail.
I figure I have been doing this in such an abstracted and GUI'ized way
up to this point (I'm sure I'm not alone here), that I could never
have fully appreciated the complexities until now (like just about
everything when you scratch the surface and see the many layered onion
below). I see why this isn't easier from PostGIS's or SpatiaLite's
Transform() function. I think people have baked-up some custom
solutions for PostGIS, but none appear to be very integrated.

Mostly for now, I just think it is important for people to know that
datum transformations are important (just a reminder), and that
generally (exceptions exist) Transform()ing doesn't convert datums
over (i.e. your coordinates don't change {maybe units}, but they
probably should, though you would have to choose a method/area).
Still doesn't change the fact that SpatiaLite rocks though. Thanks
for your time everyone. -Mark

[Keith]

Hi Mark,

I've used OGR and proj4/cs2cs to project from nad27 to nad83. This is,
of course, outside Spatialite.

I agree Sandro has done an amazing job developing Spatialite -
documentation, forum, GUI, routing, raster, binaries for Mac, Linux,
Windows, etc.

Keith