[gdal-dev] gdal commands and alternate datum (Old Hawaiian) handling

[Even Rouault]

Matthew,

> Good morning,
>   I've got some raster images for Hawaii, and I've found that running any
> command (gdalinfo, translate, warp) will return the following error in my
> Ubuntu environment:
> 
> Warning 1: Unhandled (yet) value for Datum : OHA-M. Defaulting to WGS84...

I guess this comes from a geospatial PDF ? You're lucky I remember having put 
that warning message when writing the driver... So please always specify which 
format/driver you're talking about.

> 
>   I have seen that the files located in /usr/share/gdal/1.10 do seem to
> mention the Hawaiian datums, but I'm not sure how to enable support of
> those when I run my regular commands.  I'm running a script in Python, so
> I'm wondering if it may be possible to work around this, or if this is
> just a future development that hasn't happened yet.

The driver only handles a dozain of common datum codes from the list of 
hundreds of datums given at annex A (Predefined Datums) of 
http://portal.opengeospatial.org/files/?artifact_id=40537 . The issue is that 
the matching between those GEOTRANS bases datum codes and their EPSG 
equivalents must be manually done... So they tend to be added on a case by 
case basis. In that instance OHA-M stands for 'OLD HAWAIIAN (CC), Mean' 
according to the annex.

In gdal/gt_datum.csv (derived from GeoTrans):

OGRFeature(gt_datum):153
  CODE (String) = OHA-M
  NAME (String) = OLD HAWAIIAN (CC), Mean
  ELLIPSOID (String) = CC
  DELTAX (String) = 61
  SIGMAX (String) = 25
  DELTAY (String) = -285
  SIGMAY (String) = 20
  DELTAZ (String) = -181
  SIGMAZ (String) = 20
  NORTH (String) = 17
  SOUTH (String) = 24
  WEST (String) = -164
  EAST (String) = -153
  ROTX (String) = (null)
  ROTY (String) = (null)
  ROTZ (String) = (null)
  SCALE (String) = (null)

But there are other variations of OHA- (A, B, C and D)

In the EPSG world, there's just one as far as I can see. And by comparing the 
DELTAX, DELTAY and DELTAZ parameters of the above definition with the TOWGS84 
of the below definition, OHA-M seems to match the 'Old_Hawaiian' datum from 
EPSG:6135

$ gdalsrsinfo EPSG:4135

PROJ.4 : '+proj=longlat +ellps=clrk66 +towgs84=61,-285,-181,0,0,0,0 +no_defs '

OGC WKT :
GEOGCS["Old Hawaiian",
    DATUM["Old_Hawaiian",
        SPHEROID["Clarke 1866",6378206.4,294.9786982138982,
            AUTHORITY["EPSG","7008"]],
        TOWGS84[61,-285,-181,0,0,0,0],
        AUTHORITY["EPSG","6135"]],
    PRIMEM["Greenwich",0,
        AUTHORITY["EPSG","8901"]],
    UNIT["degree",0.0174532925199433,
        AUTHORITY["EPSG","9122"]],
    AUTHORITY["EPSG","4135"]]

So you could likely replace the DATUM section of the WKT definition returned by 
the PDF driver with the DATUM section of the above and put that in a foo.wkt 
file, and then use -a_srs foo.wkt / -t_srs foo.wkt depending on the utility.

I've just implemented in trunk the interpretation of OHA-M based on above 
assumptions.

Even

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