Ticket #122 (closed defect: fixed)
removal of +datum names from nad/epsg in r1874 has bad repercussions
|Reported by:||hamish||Owned by:||warmerdam|
While debugging grass ticket # 1452 I tracked the problem down to a broken epsg file. OSGeo4w seems to be shipping the unreleased 4.7.1 version of proj4 which includes that file.
The problem is that for any given epsg code (AKAIK) EPSG does not define which transform terms should be used, and it seems that proj4 is here silently picking one on our behalf. And the one it picks is not necessarily the correct one for the end-user to use!
Also, with the removal of the +datum term there is no way to undo or work around this (even if there were it would still require an unpleasant hack). Consider a parallel epsg.proj4 file if the standard epsg data is going to be modified/subsetted. (hmm, I'm not even sure if their data license allows to continue using their name if it has be modified..)
Especially 7-term params will be very localized, while the projection terms themselves may be more regional -- this is a problem for anyone with multiple +towgs84 and/or grid files to chose from within a particular epsg definition.
In my case it is a problem for epsg:27200 (NZ Map Grid + NZGD49), whereas before I could pick between 3-term, 7-term, or a NTv2 grid file to go with the epsg code, but now I get the 7-term imposed upon me with no way out.
please don't take our datum names away. :-(