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

[Andre Joost]

Am 05.09.2017 um 19:43 schrieb 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

Yes, projection parameter values are the same, and the datum shift is 
not stored inside the Shapefile definition.

>
> 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

... and that's the pitfall for Proj.4, resulting in zero datum shift 
because the grid shift files don't cover El Salvador.

>
> 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]]
>

>
> So beside the datum difference, they are equivalent. The .prj one uses a
> LCC2SP formulation of LCC, and the EPSG one a LCC1SP one.

Except that they have a scale factor with LCC 2SP, but it gets silently 
dropped by GDAL/PROJ.4.

>
> 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 they keep a kind of mistery on it. El Salvador should have moved 
to SIRGAS-ES2007.8 by now, but I have not found any projected CRS based 
on that.

>
> 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
>

... and that looks nothing similar to the Ocotepeque datum shift. They 
are about 400 meters apart.

Greetings,
André Joost