[Proj] WGS84 parameters in epsg:31370 and 28992 (Belgian and Dutch coordinate systems)

[Jan Hartmann]

Hi,

In the epsg-file in Proj-4.7.0, the Belgian coordinate system (31370) 
has a +towgs84 clause, and the Dutch one (28992) hasn't. Is there any 
reason for this? EPSG:28992 is much more usable *with* the +towgs84 
parameter, as conversions to WGS84 latlon (EPSG:4326)  won't be correct 
without it.  The same goes for conversions between Dutch and Belgian 
coordinates: they *both* need the  +towsg84 parameter:

$ cs2cs +init=epsg:28992 +to +init=epsg:31370 # (only one projection or 
none  with +towgs84, incorrect transformation)
300000 180000
368686.96       35802.01 0.00

$ cs2cs +init=epsg:28992 +to +init=epsg:31370 # (both projections with 
+towgs84, this one is correct)
300000 180000
368729.27       35841.07 -301.04

My impression is that ellipse conversions (bessel to intl in this case) 
are only done when both projections have a +towgs84 string. Is this 
correct? I know that this is the case for lat-lon values (see Frank's 
response at: 
http://lists.maptools.org/pipermail/proj/2009-January/004303.html), but 
is this also the case for projected coordinate systems? Doesn't look 
right to me.

Jan