[gdal-dev] WKT EXTENSION - any docs or reference

[Etienne Tourigny]

Hi all,

The WKT EXTENSION nodes are used in a few contexts, however I have not
been able to find any documentation, aside from random web pages and
mailing list comments about them.

Could anyone point me to any existing documentation, and inform me on
any other EXTENSION parameters that are supported by OGR?

1)
PROJ inside GEOGCS nodes

for example, Google mercator:

PROJCS["Google Maps Global Mercator",
    GEOGCS["WGS 84",
        DATUM["WGS_1984",
            SPHEROID["WGS 84",6378137,298.257223563,
                AUTHORITY["EPSG","7030"]],
            AUTHORITY["EPSG","6326"]],
        PRIMEM["Greenwich",0,
            AUTHORITY["EPSG","8901"]],
        UNIT["degree",0.01745329251994328,
            AUTHORITY["EPSG","9122"]],
        AUTHORITY["EPSG","4326"]],
    PROJECTION["Mercator_2SP"],
    PARAMETER["standard_parallel_1",0],
    PARAMETER["latitude_of_origin",0],
    PARAMETER["central_meridian",0],
    PARAMETER["false_easting",0],
    PARAMETER["false_northing",0],
    UNIT["Meter",1],
    EXTENSION["PROJ4","+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0
+lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext
+no_defs"],
    AUTHORITY["EPSG","900913"]]

2)
PROJ4_GRIDS inside DATUM nodes

for example:

$gdalsrsinfo IGNF:LAMBE

PROJ.4 : '+proj=lcc +lat_1=46.8 +lat_0=46.8 +lon_0=0 +k_0=0.99987742
+x_0=600000 +y_0=2200000 +a=6378249.2 +b=6356514.999978254
+nadgrids=ntf_r93.gsb,null +pm=2.337229167 +units=m +no_defs '

OGC WKT :
PROJCS["unnamed",
    GEOGCS["unnamed ellipse",
        DATUM["unknown",
            SPHEROID["unnamed",6378249.2,293.466021],
            EXTENSION["PROJ4_GRIDS","ntf_r93.gsb,null"]],
        PRIMEM["unnamed",2.337229167],
        UNIT["degree",0.0174532925199433]],
    PROJECTION["Lambert_Conformal_Conic_1SP"],
    PARAMETER["latitude_of_origin",46.8],
    PARAMETER["central_meridian",0],
    PARAMETER["scale_factor",0.99987742],
    PARAMETER["false_easting",600000],
    PARAMETER["false_northing",2200000],
    UNIT["Meter",1]]

other example from
http://www.osgeo.org/pipermail/gdal-dev/2011-August/029856.html

    VERT_CS["NAVD88 height",
        VERT_DATUM["North American Vertical Datum 1988",2005,
            EXTENSION["PROJ4_GRIDS","g2009conus.gtx"]],
        AXIS["Up",UP]]	

which is generated by the following PROJ.4 string
"+proj=longlat +datum=WGS84 +no_defs +geoidgrids=g2009conus.gtx"

Also, the +geoidgrids PROJ.4 ṕarameter is not documented...

thanks
Etienne