[Proj] puzzled with +towgs84=... being ignored

[Andre Joost]

Am 10.01.2014 01:28, schrieb J. Luis:
>
> I'm very lost with this results where setting the +towgs84 is ignored
> unless I set "+datum=WGS84"
> Why so? and why is the source "+ellps=WGS84" ignored as well?
>

This is the usual behaviour: +towgs84 is only used when a datum shift is 
defined on both sides (from and to), either by specifying

+datum=...
+nadgrids=...
+towgs84=...

HTH,
Andre Joost