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

[Andre Joost]

Am 05.09.2017 um 16:56 schrieb Even Rouault:
>> The problem is that there is no EPSG code for the projection,
>
> Ah, the -s_srs syntax is not limited to EPSG:XXXX. You can use a proj.4 string, inlined WKT, a
> filename that has WKT or proj.4 string in it, etc.
> See http://gdal.org/gdal_utilities.html
>
>> and NAD27
>> datum is handled badly outside the US and Canada. The country in
>> question is El Salvador, where they adopted NAD27, but it's not part of
>> the NAD27 datum shift files. EPSG suggests a Ocotepeque datum, which is
>> shifted to NAD27.
>
>
> Isn'it the datum used by ?
>
> $ gdalsrsinfo EPSG:5451
>
> PROJ.4 : +proj=longlat +ellps=clrk66 +towgs84=205,96,-98,0,0,0,0 +no_defs
>
> OGC WKT :
> 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"]]
>
> The TOWGS84 parameters are the ones of transformation http://epsg.io/6891 that is valid
> for El Savador (and neighbouring countries). There are other transformations from
> Octopeque 35 to WGS 84, but none have an area of use that include El Savador.
> They are also transformations from Octopeque 35 to NAD 27.
>

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.

In fact, I have not found any Ocotepeque based GIS data available from 
El Salvador, only from the surrounding countries.

Greetings,
André Joost