[Proj] Datum shift for Dutch maps 1850-2000

[Frank Warmerdam]

Jan Hartmann wrote:
> Well, changing the ellipsoid seems to help. The ellipsoid that is 
> officially documented for this map, and the one I have been using has 
> +a=6376950.4 +rf=309.65.
> This gives the 650m Nord shift. Replacing this ellipsoid by the Bessel 
> ellipsoid (a=6377397.155, rf=299.1528128) gives a 50m South shift. 
> Again, I am reprojecting to the present projection which has a Bessel 
> ellipsoid. I can see that the first ellipsoid has a significantly 
> smaller semi-major axis than the second one. Does this explain the  Nord 
> shift?

Jan,

A large north-south shift would be consistent with proj.4 attempting an
automatic datum change via the ellipsoid (converting from latlong on one
ellipsoid to geocentric, and then to lat/long on the other ellipsoid).

Best regards,
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