[gdal-dev] OGR projections - towgs84 issue

[Even Rouault]

Steve,

when you specify a proj.4 string to GDAL/OGR, it doesn't keep it verbatim but 
translates it into its internal SRS model (based on WKT format). When it must 
pass a SRS back to proj.4, it translates from this model.

During this translation, if it detects an explicit datum known by PROJ.4, such 
as NAD83, it just omits the towgs84 parameter.

+datum is a shortcut for specifying both +ellps and +towgs84. See this thread 
http://n2.nabble.com/How-to-use-towgs84-td2393158.html where this was 
debated.

So just remove the "+datum=NAD83" parameter. The +ellps and +towgs84 will then 
be passed to PROJ.4.

Best regards,

Even