[josm-dev] Proj4J - New plugin supporting 3000+ projections

[Frederik Ramm]

Hi,

On 02/25/2011 05:10 AM, Josh Doe wrote:
> Perhaps my sensationalist subject line turned people off from this.
> Any chance this could get added to the svn? I'd like to submit patches
> against that rather than continually updating the zip on the ticket.
> I'd do so myself, but I don't want to be presumptuous and ask for an
> svn account.

Go ahead and request an OSM SVN account, it's nothing presumptuous.

I see some trouble coming up when this plugin interacts with other 
stuff, like the tile/WMS backgrounds and the ImportImage or PDF plugins.

These problems already exist - your plugin doesn't create them; it's 
just that your plugin opens the door to people using all kinds of 
strange projections and believing that was "supported".

Problems I suspect may arise:

* Selecting a projection that is only valid for a small part of the 
world (indeed, most of the "3000+" will not work for the whole planet) 
and then zooming out - what will happen? Do you actually include the 
validity bounding boxes for each and switch back to plain Mercator 
outside of that area?

* Imagery subsystem (tile/WMS) is likely to, at best, squish or stretch 
images, but will not properly reproject them, which will lead to 
noticeable inaccuracies especially on low to medium zoomlevels. The kind 
of inaccuracy depends on the projection chosen. For example, if you set 
JOSM ot 4326 then request tiles from a Mercator tile server, the middle 
section of each tile will be displaced towards the equator. This effect 
is probably more pronounced with other source/destination combinations.

* ImportImage plugin contains its own projection library which (a) may 
be partly unnecessary if your plugin is loaded - maybe ImportImage could 
be made to depend on yours? - and (b) it tries to reproject loaded 
images to the display projection, one should try if this really works 
with the usual projections.

* Gauss-Krueger proejctions commonly used in Germany are not processed 
by proj.4 accurately unless an extra grid file (BETA2007.gsb) is used, 
see 
http://osgeo-org.1803224.n2.nabble.com/gdal-dev-Re-ogr2ogr-BeTA2007-grid-td4900463.html 
- will that be possible with your plugin?

* I have no clue what the PDF import plugin does if your viewport is 
projected. I assume that there's a whole host of plugins assuming that 
you can just scale an image to the viewport and it will somehow 
magically work.

Don't let these points discourage you from continuing your work, it is 
most appreciated. I would just advise some caution when saying that 
certain projections are now "supported" as this will naturally lead 
users to believe that "everything still works" if they select that 
projection - which may not always be the case :)

Bye
Frederik