[Proj] Lat-Lon values under different ellipsoides

[Jan Hartmann]

Hi,

This is something I have long been banging my head against. I think it 
is a bug, but I am not sure. If I take a lat-lon value, computed on a 
particular ellipsoid, and convert it to the lat-lon value on another 
ellipsoid, I should get a different value, right? (e.g. cs2cs 
+proj=longlat +ellps=bessel +to +proj=longlat +ellps=WGS84). PROJ4 
always gives the same value, but I have an extensive list of coordinates 
of church towers in the Netherlands with their lat-lon values in 1850, 
based an a slightly smaller ellipsoid than we use nowadays, and the 
lat-lon of the same towers derived from our modern RD-system, based on 
the Bessel-ellipsoid, and without the WGS84 correction. There is a 
systematic difference of about 50 meters. If I do the same computation 
with the projected coordinates, I get the correct answer. Moreover, in 
that case the transformation changes when I change the 
ellipse-parameter, something that does not happen with lat-lon coordinates.

So, is this a bug in PROJ? If so, can someone with geodetic experience 
here explain to me how people can get different lat-lon values for the 
same point, based on astronomical measurements?

Thanks,

Jan