Support QGIS .qpj files

[springmeyer]

Turns out QGIS writes its own projection file - some variant of OGC WKT with extras like EXTENSION["PROJ4_GRIDS". Got a hold of one from a user running QGIS 1.8:

PROJCS["unnamed",GEOGCS["Clarke 1866",DATUM["unknown",SPHEROID["clrk66",6378206.4,294.9786982139006],EXTENSION["PROJ4_GRIDS","@conus,@alaska,@ntv2_0.gsb,@ntv1_can.dat"]],PRIMEM["Greenwich",0],UNIT["degree",0.0174532925199433]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",35.83333333333334],PARAMETER["central_meridian",-90.5],PARAMETER["scale_factor",0.9999333333333333],PARAMETER["false_easting",500000],PARAMETER["false_northing",0],UNIT["Foot_US",0.3048006096012192]]

[springmeyer]

/cc @timlinux - can you help me understand whether a QGIS .qpj is intended for use outside of QGIS? In this case a report came in of data lining up correctly in QGIS but not in the tilemill. The difference is the .qpj leads to a proj4 string like:

+proj=tmerc +lat_0=35.83333333333334 +lon_0=-90.5 +k=0.9999333333333333 +x_0=152400.3048006096 +y_0=0 +ellps=clrk66 +nadgrids=@conus,@alaska,@ntv2_0.gsb,@ntv1_can.dat +units=us-ft +no_defs

while the .prj results in:

+proj=tmerc +lat_0=35.83333333333334 +lon_0=-90.5 +k=0.9999333333333333 +x_0=152400.3048006096 +y_0=0 +ellps=clrk66 +units=us-ft +no_defs

In TileMill I can start looking for an preferring .qpj files over .prj files if they exist. But want to make sure you think this is a good idea. Are .qpj files around for the long haul? Will their format change? They basically look like valid OGC WKT right now + extra details.

[timlinux]

@springmeyer Hi. Yeah I think you can reasonably expect the .qpj file to stick around for the long term. You can see the back story for adding .qpj here:

http://hub.qgis.org/issues/2123
http://hub.qgis.org/issues/2154

cc @jef-n in case he has any further comments to make.

[springmeyer]

Thanks @timlinux. So, @jef-n: is there any formal documentation of this file and its format I can link to if I were to document that TileMill supports it?

[jef-n]

It's what GDAL's OSRExportToWkt() returns and what can fed into OSRImportFromWkt() without hassle. While OGR morphs the WKT into an ESRI WKT format for the .prj and back - and that's not lossless IIRC.

[sgillies]

@springmeyer Review of node-srs issues is part of my getting up to speed on the project. Since we've got a test for this since cb6ce7c I'm closing.

[springmeyer]

great, thanks @sgillies - yes, nothing more to do here as node-srs handles without any changes. As far as preferring .qpj over .prj this would be handled by some application calling node-srs and in the case of TileMill that's millstone, so tracking over at tilemill-project/millstone#115