[gdal-dev] Improved support of OziExplorer datums

[Jean-Claude REPETTO]

Hello,

Now that GDAL supports the OZF2 and OZFx3 formats, I think it is time to 
improve the processing of the .map files.
The current implementation supports only 3 datums, while OziExplorer 
supports 123 datums.

I have added the support for all OziExplorer datums, but before sending 
the patch, Even Rouault suggested me to post this message, so that 
anybody can send comments.

I have created two files in the GDAL data directory :
- gdal_ellips.csv contains the ellipsoid definitions
- gdal_datum.csv contains the datum definitions


The structure of gdal_ellips.csv is straightforward :

   ELLIPSOID_CODE,NAME,A,INVF
   0,Airy 1830,6377563.396,299.3249646
   1,Modified Airy,6377340.189,299.3249646
   ...


The structure of gdal_datum.csv deserves some explanations :

   NAME,EPSG_DATUM_CODE,ELLIPSOID_CODE,DELTAX,DELTAY,DELTAZ
   Adindan,4201,5,-162,-12,206	# Africa - Eritrea, Ethiopia and Sudan
   ...
   DOS 1968,,14,230,-199,-752              # Gizo Island
   Easter Island 1967,4719,14,211,147,111
   Egypt,,14,-130,-117,-151                # EPSG 4199
   ...
   Hermannskogel,3906,3,653,-212,449       # GDAL uses 682,-203,480
   ...
   NAD27 Alaska,,4,-5,135,172    # EPSG:1176 Coordinate Transformation
   NAD27 Bahamas,,4,-4,154,178   # EPSG:1177 Coordinate Transformation
   ...
   South Asia,,8,7,-10,-26       # Singapore (unknown EPSG code)

The second column contains the EPSG code, but the following columns also 
contain the ellipsoid code and the 3-parameters transformation values.
This is necessary, because for some datums used in OziExplorer :
- either there is no equivalent EPSG code : South Asia (Singapore)
- either GDAL doesn't know the 3 parameters : Egypt (EPSG:4199)
- either there are several transformation parameters, and GDAL and 
OziExplorer are not using the same ones : DOS 1968 (Solomon Islands), 
where OziExplorer uses the transformation for the Gizo island 
(EPSG:15805), while GDAL uses the transformation for the Guadalcanal 
island (EPSG:15807)
- or there are several transformations for the same datum, and it is not 
possible for GDAL to know from the EPSG code which transformation must 
be used : NAD27, Narwhan, ED50, etc...

Storing the datums in data files instead of source code has two advantages :
- users can add custom datums and ellipsoids (they can do it in OziExplorer)
- users can get exactly the same results with GDAL than with 
OziExplorer, if they wish. For example, for the Hermannskogel datum, the 
transformation parameters differ. If a user wants GDAL to use the 
OziExplorer parameters, all he has to do is to remove the EPSG code from 
the ozi_datum.csv file :
    Hermannskogel,,3,653,-212,449
and GDAL will use 653,-212,449 instead of 682,-203,480 .

Best regards,

Jean-Claude