[Proj] Datum shift for Dutch maps 1850-2000
Jan Hartmann
j.l.h.hartmann at uva.nl
Wed Nov 26 12:06:05 EST 2008
This certainly makes a difference! Adding +nadgrids=@null to the
original definition gives a map with a 120m South shift, still too much
but nearing a solution. Isn't this a bug in PROJ? I did not request a
datum shift, and neither the from-projection, nor the to-projection had
a datum defined, only ellipsoids. I am using Proj 4.6.1.
I looked again at the 19th century documentation, and it says the map is
in the "modified Flamsteed or French" projection. All later
documentation refers to "Bonne" as the projection for this map and its
successors; they are actually still called "Bonne maps" here. The
Flamsteed projection is the classic sinusoidal projection, which is
certainly not the one for this map, and the "modified Flamsteed
projection" seems to be some sort of conic projection, but I haven't
found any information on this. It could be the same as the Bonne
projection, which is conic too. Does anyone have any information on this?
Jan
Frank Warmerdam wrote:
> 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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