[Proj] [proj4] beginner questions

[Frank Warmerdam]

On Mon, Feb 25, 2013 at 3:25 PM, John Lilley <john.lilley at redpoint.net> wrote:
> Greetings,
>
> We are getting started with proj4 and I am after some fairly basic things to
> being, I was hoping you could help me get started.  Our basic need is:
>
> * Reading the datum specification from ShapeFiles, TAB, MID/MIF.

John,

This is not a task that PROJ.4 does.  You might want to use a library
like GDAL/OGR (http://www.gdal.org) which reads a variety of raster
and vector file formats and has support to convert the coordinate
system definitions into a variety of formats including OGC WKT and
PROJ.4.

> * Converting an arbitrary datum to WGS84 for internal processing.

There are lots of issues with datum conversions.

First, keep in mind for some datums you need grid shift files to be
findable and they aren't in the default PROJ.4 build.  There are notes
on the PROJ.4 main page and FAQ.

Second, for datums where a TOWGS84 3/7 parameter transformation is
appropriate it is not always readily found in the source coordinate
system definition (for instance shapefiles will not have this).   On
the other hand when working with EPSG datums there are often several
shifts available and selecting the most appropriate is challenging.
An attempt is made in the PROJ.4 "epsg" init database to pick
something appropriate by default.

> * Presenting user with a pick list of datum for output.

Neither PROJ.4 nor GDAL/OGR offer particularly wonderful ways of
browsing a list of datums or coordinate systems in a decent way.  The
EPSG dictionary is one (long) list.  If you just want datums (or
perhaps geographic coordinate systems) you could examine the GDAL
gcs.csv and/or datums.csv files for lists.

> * Converting from WGS84 back to user-chosen datum.
>
> I suppose it is never that simple, but if you can point me in the right
> direction, that would be much appreciated.

Definitely not simple.

Best regards,
Frank

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