[Proj] Datum shift for Dutch maps 1850-2000

[support.mn at elisanet.fi]

Hello,

north shifts can also be generated from transition between spherical
and elliptical earth... or between different ellipsoids. If it used it's
own ellipsoid (or sphere), I am rather sure that the shift comes from
there. Try an other ellipsoid or maybe even some sphere.. that might
explain the difference?

Regards: Janne.

-----------------------------------------------

Jan Hartmann [j.l.h.hartmann at uva.nl] kirjoitti: 
>   This is not a question about the program PROJ, but perhaps there are 
> people on this list that can help me. I am georeferencing the first 
> topographical map of the Netherlands from about 1850. That map is in a 
> Bonne projection centered at  51.5N / 4.8838828E,  and uses  its own 
> ellipsoid, computed by the Dutch engineer Ackermans (semi-major axis 
> 6376950.4m, inverse flattening 309.65m). Its PROJ-string is:
> 
> +proj=bonne +lat_1=51.5 +lon_0=4.8838828  +a=6376950.4 +rf=309.65 
> +units=m +no_defs
> 
> The present Dutch topograpical maps use a stereographic projection on 
> the Bessel ellipsoid, defined (without the WGS84 datum shift) as:
> 
> +proj=sterea +lat_0=52.15616055555555 +lon_0=5.38763888888889 
> +k=0.999908 +x_0=155000 +y_0=463000 +ellps=bessel +units=m +no_defs
> 
> 
> We have the complete lists of triangulation points used for both  maps, 
> 1000  in 1850 and  5000  in 2006.  Quite a few points are identical, 
> mostly church towers, so for these we have coordinates in both 
> projections. The  lists  also contain  the lat-lon values for these 
> points.  When I use  cs2cs to  reproject  the old points  into the 
> present system, I get a difference of about 650 meter, almost 
> exclusively in the Y-direction: the reprojected 1850 map lies about 650 
> meter to the north of the present map. This different is exactly the 
> same over the whole country, give or take a few meters that look random 
> to me (could be measurement errors, plus I am not sure they used the 
> same spot on the church towers in 1850 and the present).
> 
> This means I can get a reasonably accurate historical map by just doing 
> a linear shift of -10/-650 meters, but I am curious what happens here. 
> Of course the center of projection in 1850 could be actually 650m away 
> from its recorded position, but I am wondering if there are other 
> possibilities for such a shift. Am I doing something slightly wrong? Any 
> help would be appreciated.
> 
> Jan
> 
> Dr. J. Hartmann
> Department of Geography
> University of Amsterdam
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