How important is this information? Assuming we drop all the citation
strings, or authority names and codes, but retain the important
parameters of each projection (in CF and proj format), are we missing
anything important numerically?
In other words, are there any show-stopper issues that would keep us
from using proj4 instead of wkt?
Generally not. You indeed mentionned a significant word,
"numerically", which
indicates that you are interested more in the information for
reprojection
purposes, than strict (not to say pedantic ;-)) identification of
the SRS. If
the software that process the dataset all use proj4 and the proj4
string is an
adequate representation of the SRS, then it is a perfect fit.
There are situations where 2 distincts SRS from the EPSG database will
translate into identical proj4 string. For example, just looking at
the very
beginning of the epsg file provided with proj4, I can see :
# TWD97
<3824> +proj=longlat +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +no_defs<>
# IGRS
<3889> +proj=longlat +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +no_defs<>
The corresponding WKT (given by using gdal utility 'testepsg
EPSG:XXXX') are :
GEOGCS["TWD97",
DATUM["Taiwan_Datum_1997",
SPHEROID["GRS 1980",6378137,298.257222101,
AUTHORITY["EPSG","7019"]],
TOWGS84[0,0,0,0,0,0,0],
AUTHORITY["EPSG","1026"]],
PRIMEM["Greenwich",0,
AUTHORITY["EPSG","8901"]],
UNIT["degree",0.0174532925199433,
AUTHORITY["EPSG","9122"]],
AUTHORITY["EPSG","3824"]]
GEOGCS["IGRS",
DATUM["Iraqi_Geospatial_Reference_System",
SPHEROID["GRS 1980",6378137,298.257222101,
AUTHORITY["EPSG","7019"]],
TOWGS84[0,0,0,0,0,0,0],
AUTHORITY["EPSG","1029"]],
PRIMEM["Greenwich",0,
AUTHORITY["EPSG","8901"]],
UNIT["degree",0.0174532925199433,
AUTHORITY["EPSG","9122"]],
AUTHORITY["EPSG","3889"]]
Are the Taiwan_Datum_1997 and Iraqi_Geospatial_Reference_System
really the
same, or is it because we haven't captured the difference, or it is
too subtle
to be modelled with proj4, or it would require some grids that are not
available, etc... I have no idea.
Also you should be aware that some SRS have no translation into
proj4 strings
due to unsupported projection methods for example.
Using proj4 strings is not a total heresy however. Postgis is a
famous and
widespread example of the use of proj4 strings. Although in the
Postgis case,
the spatial_ref_sys table includes authority names and code columns,
and the
SRID is used as a key in the geometry_columns table. So you can fix
proj4
strings in a centralized way if needed.
Another point to have in mind is that the proj4 strings (at least
the ones
provided in the epsg file that comes with proj4) are generally
derived from the
EPSG database, and there can be imperfections in the way they are
translated,
or later improvements. A recent proj4 change has added for example
TOWGS84
statements in a lot of cases where they were missing before.
You can look at the following page which gives details on how GDAL,
proj4,
libtiff and postgis SRS databases are built from the EPSG database :
http://svn.osgeo.org/metacrs/geotiff/trunk/libgeotiff/csv/README
Some could also say that proj4 strings are not a very formal
definition, and
that WKT has a better potential to act as a standard. However, reading
http://home.gdal.org/projects/opengis/wktproblems.html might be
enlightning.
(Note : apparently the page has been written quite some time ago,
and some
statements do no longer apply, for example the one about PostGIS.
PostGIS has
used proj4 strings and not WKT for a long time. But most of the rest
still
apply however.)
Not sure to really have helped you. There are issues with either
using WKT or
proj4 strings.
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