[Proj] Different results with +ellps and +datum

[Frank Warmerdam]

> When I enter:
> cs2cs  +datum=WGS84 +proj=longlat +to +ellps=bessel +proj=longlat 
> +towgs84=668,-205,442
> 15d29'1.2737 46d36'44.7813
> I got:
> 15d29'18.941"E  46d36'46.661"N -21.034
>  
> But if I change command line to:
> cs2cs  +ellps=WGS84 +proj=longlat +to +ellps=bessel +proj=longlat 
> +towgs84=668,-205,442
> 15d29'1.2737 46d36'44.7813
> I got:
> 15d29'1.274"E   46d36'42.631"N 704.788
>  
> I would expect the same result (the first one) in both cases.
>  
> Is this I bug or a feature?

Zarko,

This is a "feature".  Because the source coordinate system has no
datum defined, the code assumes no datum shift should be applied
during the translation.  Furthermore, because it never goes though
this step, the earth model change is not even applied (a bug). 
My intention is that in the first case the earth model and datum
shift should be applied.  In the second case only the earth model
change should be applied. 

I do anticipate fixing the lack of an earth model change in the near
future. 

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent