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
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