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Subject: Re: The Mapmakers--I need more!
From: Trevor J. Kenchington (Gadus@XXX.XXX)
Date: Sat Dec 07 2002 - 15:18:11 EST
Some time back, Rodney Myrvaagnes wrote:
> It would be interesting to see something about charting procedures in
> the 18th C as well. I remember going to the WInter Antiques Show at
> Park Ave Armory about 25 years ago and seeing an admiralty chart from
> about 1770 for Nantucket Sound. It was surprisingly modern in many
> respects. Somebody had done a lotr of soundings.
>
> One thing really shocked me. There were instructions for piloting from
> Nomans Land to the corner of Chappaquiddick by sighting on the corner
> of land itself, there being no light house there at the time. The tidal
> currents around there would make this exceedingly hazardous in an 18th
> C frigate or the like.
>
> I wanted that chart very badly, but not enough to part with the $4000
> he wanted for it. Maybe I was wrong.
That chart was likely from the "American Neptune", the result of
DesBarres' surveys. There are quite a number of bound copies of the
Neptune around, plus many loose charts. Although a lot of them are held
by public institutions, others are in private hands and appear on the
market from time to time. The great prize is often considered to be
Desbarres' chart of Sable Island, though it is some 8 feet long, which
makes it a bit hard to display!
A couple of years back, I ran into a history student who was writing a
PhD thesis on DesBarres and his survey methods. He claimed, though I do
not know how seriously, that DesBarres trained Cook in surveying, which
would have to be counted quite an achievement even if Cook went on to
exceed his mentor's capabilities.
DesBarres was a military engineer, of Swiss origins, who was trained in
mathematics and engineering at Woolwich in the early 1750s. His first
chart, based on French materials captured in Louisbourg, was prepared in
1759 and covered the St.Lawrence. Thus, along with Cook, he made
possible the conquest of Quebec and New France by providing the English
fleet with the information needed if it was to work up the river. Since
the great flowering of hydrographic surveying dates from about that
time, I have supposed (without much supporting evidence) that its
origins lay in the terrestrial survey methods which DesBarres and other
engineers took to sea. If so, those methods had been developed early in
the 17th century, when the increasing firepower of artillery drove
fortress builders to a new level of sophisticated, mathematical design
and their opponents to a matching level of sophistication in the layout
of siege works.
As to the methods actually used: DesBarres left one account of his own
methods, with comments on measuring out a baseline on shore, then using
theodolites and plane tables to map the coastline. Copies of that were
provided to a sloop (i.e. a small warship, not a sloop-rigged boat)
which beat on and off the shore to a distance of ten miles or so, taking
soundings, to a "shallop" which did the same around headlands and
islands, and to "boats" which worked the "indraught" to the heads of the
bays. He did not remark on the instruments used on the sloop, shallop
and boats but I'd guess a leadline and quadrants used for horizontal
angles. They apparently also laid a 100-fathom line taught across the
bottom to give themselves a measured baseline in the water.
Where his crews took the time to get it right, DesBarres surveys are
about as accurate (though not as detailed) as mid-20th century ones.
However, I know of points on the Nova Scotian coast where the complexity
of the rocky shoreline seems to have overwhelmed the survey parties,
resulting in substantial errors such as an entire headland being missed
and the coves on either side run together.
One further point: The Admiralty commissioned the surveys of the
American coastline, so far as I can tell, before much of England was
properly charted. Why? My guess is that they had traditionally-trained
pilots who knew the English coast without needing charts, whereas ships'
Masters needed to navigate along American coasts that they had never
seen before and without the aid of people who knew every rock and
channel by heart. Hence, modern concepts of paper-based navigation were
needed on the unknown coast, in place of the memory-based pilotage that
had served mankind for millenia.
Trevor Kenchington
-- Trevor J. Kenchington PhD Gadus@XXX.XXX Gadus Associates, Office(902) 889-9250 R.R.#1, Musquodoboit Harbour, Fax (902) 889-9251 Nova Scotia B0J 2L0, CANADA Home (902) 889-3555Science Serving the Fisheries http://home.istar.ca/~gadus
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