[Proj] Lat-Lon values under different ellipsoides

[Frank Warmerdam]

Jan Hartmann wrote:
> 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?

Jan,

As of PROJ 4.6.0, the policy is to not attempt any conversion of lat/long
values between coordinate systems where only an ellipsoid is given.  So, to
get a datum shift it is now necessary to provide some sort of datum
shift information for both the source and destination coordinate systems.

This is a deliberate change of policy to avoid lots of other complaints in
the past.

I would suggest using something appropriate like +datum=potsdam for your
Bessel data, and +datum=WGS84 instead of +ellps=WGS84.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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