[gdal-dev] Which Proj.4 transforms are available in GDAL?

[Even Rouault]

> 
> The shapefile clearly states DATUM["D_North_American_1927", and the
> download page (from the official agency)
> http://www.cnr.gob.sv/geoportal-cnr/ tells the same.

Hum, actually looking at the full .prj, it looks similar to EPSG:5460

See:

$ gdalsrsinfo ESRI::dptoA_Lambert_NAD27.prj

PROJ.4 : +proj=lcc +lat_1=13.31666666666667 +lat_2=14.25 +lat_0=13.783333 +lon_0=-89 +x_0=500000 +y_0=295809.184 +datum=NAD27 +units=m +no_defs 

OGC WKT :
PROJCS["IDGES_rev",
    GEOGCS["GCS_North_American_1927",
        DATUM["North_American_Datum_1927",
            SPHEROID["Clarke_1866",6378206.4,294.9786982]],
        PRIMEM["Greenwich",0.0],
        UNIT["Degree",0.0174532925199433]],
    PROJECTION["Lambert_Conformal_Conic_2SP"],
    PARAMETER["False_Easting",500000.0],
    PARAMETER["False_Northing",295809.184],
    PARAMETER["Central_Meridian",-89.0],
    PARAMETER["Standard_Parallel_1",13.31666666666667],
    PARAMETER["Standard_Parallel_2",14.25],
    PARAMETER["Scale_Factor",0.99996704],
    PARAMETER["Latitude_Of_Origin",13.783333],
    UNIT["Meter",1.0]]

$ gdalsrsinfo EPSG:5460

PROJ.4 : +proj=lcc +lat_1=13.78333333333333 +lat_0=13.78333333333333 +lon_0=-89 +k_0=0.99996704 +x_0=500000 +y_0=295809.184 +ellps=clrk66 +towgs84=205,96,-98,0,0,0,0 +units=m +no_defs 

OGC WKT :
PROJCS["Ocotepeque 1935 / El Salvador Lambert",
    GEOGCS["Ocotepeque 1935",
        DATUM["Ocotepeque_1935",
            SPHEROID["Clarke 1866",6378206.4,294.9786982138982,
                AUTHORITY["EPSG","7008"]],
            TOWGS84[205,96,-98,0,0,0,0],
            AUTHORITY["EPSG","1070"]],
        PRIMEM["Greenwich",0,
            AUTHORITY["EPSG","8901"]],
        UNIT["degree",0.0174532925199433,
            AUTHORITY["EPSG","9122"]],
        AUTHORITY["EPSG","5451"]],
    PROJECTION["Lambert_Conformal_Conic_1SP"],
    PARAMETER["latitude_of_origin",13.78333333333333],
    PARAMETER["central_meridian",-89],
    PARAMETER["scale_factor",0.99996704],
    PARAMETER["false_easting",500000],
    PARAMETER["false_northing",295809.184],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]],
    AXIS["X",EAST],
    AXIS["Y",NORTH],
    AUTHORITY["EPSG","5460"]]

So beside the datum difference, they are equivalent. The .prj one uses a
LCC2SP formulation of LCC, and the EPSG one a LCC1SP one. One can easily
check they are equivalent with:

$ gdaltransform -s_srs "+proj=longlat +ellps=clrk66" -t_srs ESRI::dptoA_Lambert_NAD27.prj
-88 14
608032.297052867 320003.359813003 0

$ gdaltransform -s_srs "+proj=longlat +ellps=clrk66" -t_srs EPSG:5460
-88 14
608032.297913122 320003.32045602 0

(the slight difference in northing might come from the .prj having a limited number
of digits for Latitude_of_origin compared to the EPSG one)

So the NAD27 datum might be just an artifact of something in their production
chain not being able to handle Octopeque 1935. Or just a matter of habit. My
knowledge of El Salvador geodesy is rather limited ;-)

Or if it is really NAD27 datum, I can indeed see that they are 3 NAD27->WGS84 transforms available
for central America.

$ ogrinfo pg:dbname=epsg -sql "select c.*, a.area_code, a.area_name from epsg_coordoperation c join epsg_area a on c.area_of_use_code = a.area_code where c.source_crs_code = 4267 and c.target_crs_code=4326 and area_south_bound_lat > 0 and area_north_bound_lat < 20 and area_west_bound_lon > -95 and area_east_bound_lon < -75"

Two of them are specific of Panama. The remaining one is EPSG:1171 that applies to
"Central America - Belize to Costa Rica"

Its TOWGS84 params are 0.0, 125.0, 194.0, 0.0, 0.0, 0.0, 0.0


Even

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