[OSRS-PROJ] Units used in Expressing Projections

[Frank Warmerdam]

Paul Ramsey wrote:
> Has OGC completely abandoned WKT? I see work in the offing on an XML
> form of coordinate expression, but no explicit spec coming to iron out
> the many gaps in WKT which have been filled in by inference over the
> past several years.

Paul,

WKT is still the only coordinate system description mechanism for simple
features, and is now deployed (with varying degrees of interoperability) in
ESRI, Autodesk (at least Mapguide), Cadcorp and FME software.

The original WKT is defined in the simple features specification and it was
refined for the CT spec.  Currently WKT is the only way of defining coordinate
systems in OGC interfaces other than the use of simple EPSG numbers for some
of the web services (WMS, etc).

There is an effort to build an XML coordinate system description mechanism
going on, and it will be utilized in GML and likely some of the other
interfaces over time as well.  When it is ready I believe it will be the
preferred format for coordinate systems and will likely eclipse WKT in
time, however:

  o It still isn't specified.
  o It is going to be pretty damn complicated by what I can see ... way more
    so than the WKT.
  o It will be some time before it rolls out in real software.
  o I don't anticipate folks like ESRI quickly dropping thier use of WKT.

So while the XML will eventually (I think) become the preferred coordinate
system format, it will be a while for it to be available, much less widely
used.  In the meantime there is only WKT.

Note that the CT spec was much more specific about WKT than the original
simple features spec.  It isn't clear to me there are a lot of gaps in the
CT WKT though it isn't clear how to work out the interoperability problems
between existing WKT and CT WKT.

I will continue to evolve the OGRSpatialReference classes to support
translation between various forms of WKT, OGC XML coordinate systems, PROJ.4
and any other representations of wide interest.

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    | Geospatial Programmer for Rent


----------------------------------------
PROJ.4 Discussion List
See http://www.remotesensing.org/proj for subscription, unsubscription
and other information.