[GRASS-user] gps import: definition of WGS84

Paul Kelly paul-grass at stjohnspoint.co.uk
Mon May 26 09:45:02 EDT 2008

On Sun, 25 May 2008, Hamish wrote:

>> But the way I read the above nabble thread (and especially
>> Frank's quote) the issue is that some projections are missing
>> in the proj and thus the QGIS database because of the multiple
>> transform possibilities
> strictly speaking the "projection" (lcc) is there and fine, but (AFAIU) the datum transform params for a given datum are left out if there are more than one option in the upstream EPSG database. But the +datum= name should still be given in that case.

There are actually only a few +datum= codes recognised by PROJ.4 - most of 
them are specific to GRASS. So it isn't meaningful to include a 
+datum=xxxx if the PROJ string is destined to be used by something other 
than GRASS. The OGR functions that GRASS uses to interpret EPSG codes 
somehow manage to return the full EPSG name for the datum, which is 
then compared to GRASS's datum.table (the second field in this table holds 
the full EPSG name). Combined with datumtransform.table that gives GRASS 
enough information to go on to present the user with a choice of datum 
transform paramteters. However the full EPSG name isn't included in the 
PROJ.4 string, and if that is all QGIS is using then maybe that's where 
its deficiency stems from. It is all very messy and complicated.


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