[Proj] Datum shift for Dutch maps 1850-2000

[Clifford J Mugnier]

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 360º.  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.
 
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.
 
C. Mugnier

________________________________

From: proj-bounces at lists.maptools.org on behalf of Duncan Agnew
Sent: Tue 25-Nov-08 12:12
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] Datum shift for Dutch maps 1850-2000



I suspect that in the Netherlands in the 1850's* church towers *were*
stations--where else could you see
from? 650 m is about 20 arcsec, which is, I agree, too large for
observational error (or for deflection of
the vertical).

Duncan Agnew

*and before--going back to Snel.

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