[PATCH] Support for all Ozi datums
|Reported by:||jcrepetto||Owned by:||Even Rouault|
The current GDAL version supports only 3 datums in Oziexplorer .map files, while OziExplorer supports 123 datums.
The attached patch adds support for all OziExplorer datums. It uses two new files in the GDAL data directory :
- ozi_ellips.csv contains the ellipsoid definitions
- ozi_datum.csv contains the datum definitions
The structure of ozi_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 ozi_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 :
and GDAL will use 653,-212,449 instead of 682,-203,480 .
Provenance of the data files
I have created both files, they have not been taken from any other software.
The datums names are exactly the sames as those displayed by OziExplorer (Menu : Display, Datum Parameters). This is necessary for the string comparisons to succeed when parsing .map files. The 3-parameters transformation value are those displayed by OziExplorer (Menu : Display, Datum Parameters). They have been copied manually to the ozi_datums.csv file. Most of these parameters (and the datum names) are the same as in the datum list from the NGA, this is probably where the OziExplorer author found them.
I have put the area names in comments, they come from the EPSG database.
Change History (8)
comment:3 by , 11 years ago
|Component:||GDAL_Raster → OGR_SRS|
|Milestone:||1.8.1 → 2.0.0|