<HTML dir=ltr><HEAD><TITLE>Re: [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>Church towers were extremely rare as triangulation observation stations - that would require scaffolding symmetrical about the spire - an impractical enterprise. When observing directions with a Direction Theodolite, you hit everything sticking up above the horizon for 360<SPAN style="FONT-SIZE: 12pt; FONT-FAMILY: 'Times New Roman','serif'; mso-bidi-font-size: 10.0pt; mso-fareast-font-family: 'Times New Roman'; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA">º</SPAN>. Back then it was Church towers, castles, some prominent buildings, and not much else other than the heliographs in the day time and the lantern/beacons at night. In the 20th century, technology added radio towers, microwave towers, and water tanks - all quite useful for doing a three-point resection to determine your own position once the intersection station coordinates are known. I used them for many years in the South Louisiana marshes BEFORE the advent of GPS.</FONT></DIV>
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<DIV dir=ltr><FONT size=2>Where else could you see from? Hills, mountains, and double scaffolds made from scantling. Check out the old manuals of triangulation. That's why they used Strength of Figure computations to decide WHICH hill to use for observations or if they had to build a scaffolding tower themselves.</FONT></DIV>
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<DIV dir=ltr><FONT size=2>C. Mugnier</FONT></DIV></DIV>
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<FONT face=Tahoma size=2><B>From:</B> proj-bounces@lists.maptools.org on behalf of Duncan Agnew<BR><B>Sent:</B> Tue 25-Nov-08 12:12<BR><B>To:</B> PROJ.4 and general Projections Discussions<BR><B>Subject:</B> Re: [Proj] Datum shift for Dutch maps 1850-2000<BR></FONT><BR></DIV>
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<P><FONT size=2>I suspect that in the Netherlands in the 1850's* church towers *were*<BR>stations--where else could you see<BR>from? 650 m is about 20 arcsec, which is, I agree, too large for<BR>observational error (or for deflection of<BR>the vertical).<BR><BR>Duncan Agnew<BR><BR>*and before--going back to Snel.<BR><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>