[MetaCRS] "FOSS Maintained" Source of CRS Definitions

[Frank Warmerdam]

Martin Desruisseaux wrote:
> There is already an existing model, which is an international standard 
> approved both by ISO and OGC: ISO 19111. Would it be an acceptable model?

Martin,

Can you point us to a public document on this model?  I'm frankly somewhat
intimidated by it, and concerned that a lot of stuff it describes is not
practically useful.  But I really need to do a current review before I
spout off much more.

>>> There is already a "standard" and open format for CRS definitions,
>>> namely WKT, correct? Couldn't this be used?
> 
> It can be used for "minimal" CRS definition. But the OGC standard WKT 
> can not express Bursa-Wolf parameters targeting any other datum than 
> WGS84, and can not contain useful metadata like area of validity. Yet it 
> still applicable when there is nothing better. But when a choice is 
> possible, I rather push for the EPSG database.

I'd be interested to hear from the CS-MAP folks if they model datum
shifts between arbitrary datums, as opposed to mapping everything through
WGS84 (as PROJ4 is inclined to do).

A few limitations to WKT that I am concerned about:
  * There is no way to express datum shift methods other than 3/7 parameter
    transformations to wgs84 (via TOWGS84).  In particular, how am I
    supposed to document use of grid shift files in WKT?
  * There is no way to express that there are more than one possible
    datum shift approximation available for use.
  * No way to mark coordinate systems as deprecated, categorize them for
    display to the user and such.  If we produce a "super dictionary" it
    would be nice if user interfaces could provide a useful navigation
    mechanism to user based on it.

I can imagine picking the ISO model as our central CRS data model, and
distributing definitions in some format that maps well to that model, like
GML, but I'm concerned we make our dictionaries so complicated that they
become very difficult to exploit.

> As far as I known the EPSG database is the most widely accepted at least 
> at OGC. Some other sources like the PostGIS "spatial_ref_sys" table are 
> actually generated from a subset of the EPSG database.

The EPSG database will certainly be a large portion of a super-dictionary.
However, I would like to be able to capture other coordinate system
dictionaries too.  A few of interest to me:

  * The datum list from NGA, as used in GeoTrans.
  * The ESRI "coded" extensions to EPSG.
  * The ERMapper projection and datum definitions.
  * The MapInfo datum list.
  * The IAU (International Astronomical Union) definitions for other planets.
  * OGC authority space definitions like urn:ogc:def:crs:OGC::CRS84

I should also point out http://spatialreference.org/ as a point of interest
in our discussions.  This web site maintains it's own list of CRS definitions,
many user submitted, though starting from the EPSG set.  It effectively stores
them internally as WKT (I think) but endevors to return them in a variety of
formats (using OGRSpatialReference facilities to do the translation).

>> - CRS interface spec. (not necesary but would be nice)
> 
> GeoAPI ?

When talking about dictionaries I would prefer not to get hung up on
API specifications.  I'd prefer to manage and provide a dictionary in
some form suitable exploitation by different packages and leave it to
them to talk about APIs.

I'm keen on GeoAPI as a basis for shared Java coordinate system libraries
but I would like to keep that discussion distinct from the dictionary
discussion if possible.  Obviously starting from an ISO model (or perhaps
a subset of the ISO model) makes it easier to map the dictionary to
something like GeoAPI.

Best regards,
-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org