<HTML dir=ltr><HEAD><TITLE>[Proj] Datum shift for Dutch maps 1850-2000</TITLE>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=2>For that difference in dates, a mostly northerly shift is entirely due to something else other than different datums. Timekeeping back then was far from today's accuracy, and therefore the <U>major</U> shift is ALWAYS East-West with some difference in Northings. Furthermore, church towers are intersection stations and were not occupied. Those points normally have the least accuracy even though they generally are the longest-lasting.</FONT></DIV>
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<DIV dir=ltr><FONT size=2>I have no idea what you have there, but it is certainly not a classical datum shift.</FONT></DIV>
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<DIV dir=ltr><FONT size=2>Check out my February 2003 column on the Netherlands at <A href="http://www.ASPRS.org/resources/grids">www.ASPRS.org/resources/grids</A> .</FONT></DIV>
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<DIV dir=ltr><FONT size=2>C. Mugnier</FONT></DIV>
<DIV dir=ltr><FONT size=2>Louisiana State University</FONT></DIV>
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<FONT face=Tahoma size=2><B>From:</B> proj-bounces@lists.maptools.org on behalf of Jan Hartmann<BR><B>Sent:</B> Tue 25-Nov-08 10:42<BR><B>To:</B> PROJ.4 and general Projections Discussions<BR><B>Subject:</B> [Proj] Datum shift for Dutch maps 1850-2000<BR></FONT><BR></DIV>
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<P><FONT size=2> This is not a question about the program PROJ, but perhaps there are<BR>people on this list that can help me. I am georeferencing the first<BR>topographical map of the Netherlands from about 1850. That map is in a<BR>Bonne projection centered at 51.5N / 4.8838828E, and uses its own<BR>ellipsoid, computed by the Dutch engineer Ackermans (semi-major axis<BR>6376950.4m, inverse flattening 309.65m). Its PROJ-string is:<BR><BR>+proj=bonne +lat_1=51.5 +lon_0=4.8838828 +a=6376950.4 +rf=309.65<BR>+units=m +no_defs<BR><BR>The present Dutch topograpical maps use a stereographic projection on<BR>the Bessel ellipsoid, defined (without the WGS84 datum shift) as:<BR><BR>+proj=sterea +lat_0=52.15616055555555 +lon_0=5.38763888888889<BR>+k=0.999908 +x_0=155000 +y_0=463000 +ellps=bessel +units=m +no_defs<BR><BR><BR>We have the complete lists of triangulation points used for both maps,<BR>1000 in 1850 and 5000 in 2006. Quite a few points are identical,<BR>mostly church towers, so for these we have coordinates in both<BR>projections. The lists also contain the lat-lon values for these<BR>points. When I use cs2cs to reproject the old points into the<BR>present system, I get a difference of about 650 meter, almost<BR>exclusively in the Y-direction: the reprojected 1850 map lies about 650<BR>meter to the north of the present map. This different is exactly the<BR>same over the whole country, give or take a few meters that look random<BR>to me (could be measurement errors, plus I am not sure they used the<BR>same spot on the church towers in 1850 and the present).<BR><BR>This means I can get a reasonably accurate historical map by just doing<BR>a linear shift of -10/-650 meters, but I am curious what happens here.<BR>Of course the center of projection in 1850 could be actually 650m away<BR>from its recorded position, but I am wondering if there are other<BR>possibilities for such a shift. Am I doing something slightly wrong? Any<BR>help would be appreciated.<BR><BR>Jan<BR><BR>Dr. J. Hartmann<BR>Department of Geography<BR>University of Amsterdam<BR>_______________________________________________<BR>Proj mailing list<BR>Proj@lists.maptools.org<BR><A href="http://lists.maptools.org/mailman/listinfo/proj">http://lists.maptools.org/mailman/listinfo/proj</A><BR></FONT></P></DIV></BODY></HTML>