[OpenLayers-Dev] Projection Support

[Christopher Schmidt]

Mike Adair worked on some great projection support stuff for OpenLayers
at the code sprint, and I've been working on making the underlying
infrastructure for his changes come together. 

The basic principle is that you should not need projection support for
anything, but if you want it, you can include the proj4js library, and
it will be there.

To that end, there is:

 * OpenLayers.Projection class, a wrapper around proj4js projection, 
   and provider of transform function. (#1035)
 * displayProjection support for controls, so that MousePosition, 
   Permalink, and ArgParser can display coordinates in a different
   projection than the projection of the map. (#1036)
 * geometry.transform function, which transforms a geometry from one
   projection to another in place. (#1037)
 * Format internalProjection and externalProjection support, so that
   geometries are transformed in/out as requested. (#1039)
 * Vector Layer support for reprojection using format classes (not yet
   complete)

There are still some API discussions happening here -- Tim had a
different vision in mind for transformation, and is going to comment at
some point. Other feedback would also be appreciated: I don't do much
with transformations, so I don't know what I'm missing here.

To actually do any work, OpenLayers depends on proj4js, a library which
is, at the moment, in a state of flux. 

Thus far, in order to create code which doesn't need things like proxies
or remote access, I've been including .js files and projection
definitions manually. This seems to work relatively well. In order to
have projection support, you simply include proj4js.js in your page,
along with the projection:
    
    <script src="../lib/proj4js/proj4js.js"></script>
    <script src="../lib/proj4js/defs/EPSG900913.js"></script>

It seems likely that we will want to assemble tools that let you build a
customized projection library with all the bits you need put together.

Mike Adair has been working on proj4js, based on Richard Greenwood's
cscs code, as I understand it. I'm looking at it, and I have a couple
questions.

 * Where is the current canonical home for the code? There is definitely a 
   version of it in Mapbuilder's cscs/trunk, but the version in
   http://svn.openlayers.org/sandbox/madair/lib/proj4js/ seems to have
   most of the remote loading stuff added on to it, and so I think
   that's the newer version of the code. 

 * Where should the code live? I'm happy putting it anywhere, but right
   now it feels scattered and I'm lost as to where improvements should
   be done. I'd like to start hacking on the library, as well as
   documenting it better. I think we need to be looking at making it a
   full on library in and of itself, which (again) can live wherever.

   Eventually, I think it seems likely that we will want to move this
   into an OSGeo code repository. In the short term, I think that
   identifying a single SVN repository and keeping it there, and 
   documenting where 'there' is, is the first step forward.

   Mike, where would you like to keep this code? For the purposes of
   provenance review and the like, I'd like to keep it out of OL SVN as
   a primary home -- you have commit access to the Mapbuilder SVN, and
   I'm happy to maintain it there while pursuing possible future
   arrangements. 
 
 * How is the code managed? I don't know much about the code, but I do
   know that right now, there are things like API naming and the like
   that bother me. Who do I take that to? 

   Specifically, I don't like the '_reportError' style naming, and I'd
   like to do a number of cleanups to make proj4js more
   "OpenLayers-like" in style. (Dropping _, picking a standard for
   whitespace, stuff like that.) I don't mind leaving things the way
   they are if someone else is going to be maintaining the library, but
   right now, it feels like it's not usable in OpenLayers, and I'd like
   to change that.

 * Creating a service where you can build your own projection javascript
   library based on the appropriate pieces from proj4js and the EPSG
   data seems like a good thing that we should work on. I'm happy to
   take the lead on that and building other similar tools for managing
   the proj code based on the OpenLayers stuff. 
 
Mike, I think most of these are directed at you: can you offer feedback
when you get a chance? (And if you're the wrong person, direct me in the
right direction?)

In the short term, I'm going to copy the proj4js code into a seperate
sandbox, which anyone can feel free to hack on, and attempt to build
what I think I would like to see and help maintain for OpenLayers users.   

Regards,
-- 
Christopher Schmidt
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