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MapTack
Posts: 8
Joined: 10/11/2012
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Finding North America & Ferdinand Hassler's Chain |
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Re: Richard Stachurski - <Longitude by Wire>
Part 1.....
Having only recently obtained this book I came across in Chapter
3/Station Head & Horns - "Constant pulling on the chain to
straighten it lengthened the rings that connected the links.The links
could be bent & shortened<<by being drawn over fences, rocks & other unyielding obstructions>>"
"The crude tool produced crude results, a discrepancy of about
1:38000." The Common Land Chain probably gave a precision of 1:500 to
1:1000 for good work. The accuracy was even in doubt by surveyors
particularly in the 16th & 17th C.
From research, the 20 metre chain with 1m links was made for Hassler by
Edward Troughton and was of the same basic design as that used by
William Bald in his baseline measurements for the triangulation of Co.
Mayo in Ireland (giving up a precision if not accuracy of 1:127,000).
This design by Troughton was to supply a chain capable of somewhat
similar accuracy to the Ramsden Geodetic Chain made for Wm. Roy but
considerably cheaper. This chain would not have been subject to the same
drawbacks as the Common Land Chain. I presume that the Fire Island
Baseline, since it was particularly selected as a site by Hassler, did
not suffer from being drawn over fences, rocks &c and this reference
is to the general failings of the Common Chain in such particular
circumstances. It is interesting that the Troughton 20m chain did not
number in the inventory of Hasslers' purchases when in Europe. I
strongly suspect that Hassler may have purchased this at his own expense
and did not have confidence in it as a tool for his purpose and thereby
not declaring it in the inventory of purchases. Hassler's technical
ambitions & exactitude are very similar to that of Thomas Colby of
the Board of Ordnance in his compensating bars which were used to
measure the Lough Foyle Baseline in N. Ireland. Colby had used the
Ramsden Chain previously but went on to develop his baseline contrivance
somewhat similar to that which Hassler eventually used. Adequate was
alien to both proponents.
Hassler's "lack of confidence" would account for the earlier
introduction of a secondary baseline. William Bald's response to not
using trestled coffers but measuring along the ground with his great
chain was to introduce more frequent baselines. The 1:38,000 is not a
bad result particularly since Hassler may not have invested the effort
in something in which he had little confidence. Although low by geodetic
standards it would have been sufficient for small scale mapping
particularly with additional baselines.
MapTack
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Saturday, October 13, 2012 at 8:10:47 AM |
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MapTack
Posts: 8
Joined: 10/11/2012
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Re: Finding North America & Ferdinand Hassler's Chain |
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Re: Richard Stachurski - <Longitude by Wire>
Part 2.....
Colby could have sufficed with the scaled distances brought across the
Irish Sea by Triangulation and perhaps made do with the Ramsden Chain
for a check base in Ireland. Hassler too had used the Ramsden Chain in
Switzerland prior to his emigration to North America. Hassler &
Colby acting no doubt in the spirit of the Enlightenment were not
prepared for anything but scientific perfection. I was unable to track
down the A.M. Harrison report on the Plane-Table but presumably the
chaining in this report refers to the common chain & not
specifically to Troughton's chain for Hassler.
The Troughton design persisted into the 20th Century however as it found
its specialist niche in acting as a Standard Chain and was known as
Troughton's Standard Chain. This particular chain was used to
standardise common working chains particularly within the Ordnance
Survey but also with the British Inspectors of Weights & Measures.
The chain in this form was made by Doyle & Son, London and by
C.F.Casella, London no doubt among others. The Troughton pattern chain
was also used in South Africa when neither the Ramsden chain nor the
Colby Compensating Bars were available but the chain could be compared
against previous measurements through triangulation taken with the
foregoing devices. Froma general interest, both the Ramsden Chain &
latterly Colby's Bars were used extensively in the GTS (Great Trig.
Survey) of India - the Ramsden being also used a a Standard Chain prior
to the availability of Colby's Bars.
MapTack
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