NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Cook's surveys. was: Re: Circle charts
From: George Huxtable
Date: 2009 Feb 1, 20:50 -0000
From: George Huxtable
Date: 2009 Feb 1, 20:50 -0000
Nicolas de Hilster wrote- "For what I understood Cook charted New Zealand using a plane table from a ship and still did quite a proper job. He must have used similar techniques to get his positions right." I've no great knowledge of surveying, and I don't think Cook ever described in detail how he went about it. But I doubt if Cook ever tried to use a plane table from a ship, though he certainly would do so from on land. Cook learned about the techniques of military survey of land from Samuel Holland in Eastern Canada in 1758, during the war with the French. This acquainted him with the use of the plane-table, a simple analogue device for mapmaking. But that was a device that needed to be firmly planted on solid ground, so that the plotted azimuth of each landmark stayed constant, while bearings of other landmarks were added. He was to put it to good use in his surveys using land-stations, both in Canada and later in New Zealand. Cook's great advances in maritime survey mere made by combining such observations, from on-land viewpoints, with other observations taken from on board a ship by compass and horizontal sextant angle, both at anchor and under way. But a plane table couldn't be used at sea because its orientation was unstable, as the ship's heading would keep changing as the vessel yawed under way, or swung at anchor. Instead, accurately timed observations of horizontal angles between landmarks, compass bearings, steering compass readings, soundings, and Sun altitudes would be recorded. These would then be combined in a large chart, perhaps in real-time, down in the captain's cabin while the survey was progressing, but more likely in harbour later. The important thing was the careful noting of all the necessary observations, leaving nothing out. Cook's survey of Newfoundland was even able to align the whole series of charts to a precise Greenwich longitude, because he had been lucky enough to observe, and time, an eclipse of the Sun. Cook wasn't then confident enough to analyse the results on the spot, but he knew what he had to measure, and took his results back to Greenwich next winter for the gurus there to work out his longitude, a result that stood the test of time. You will find Eclipse Island, where the observation was made, on the Western coast of Newfoundland. As Nicolas says, Cook certainly did "quite a proper job" in surveying New Zealand. He wasn't provided with a chronometer (to test out) until his second circumnavigation, so the New Zealand survey was done without one. And the effects are noticeable, here and there, particularly around Coromandel peninsula in North Island, which ended up in Cook's chart quite a lot slimmer than later mapping showed. This was the problem. Endeavour had sailed up the East coast of that peninsula, including a stop, to observe a transit of Mercury, in Mercury Bay, which would help to fix longitude. That survey corresponds remarkably well with modern mapping. Then, after rounding the North end, he could make an equally accurate survey of the West Coast of the peninsula, down to the Firth of Thames. But the trouble was in rounding that North tip, where Endeavour met contrary winds that called for much beating about, enough to confuse any attempt to make a proper running-survey around that point. And so the only way those two coastlines could be connected was by comparing their longitudes, which relied on the accuracy of an ordinary pocket-watch (not a chronometer) over an interval of several days. As a result, Cook's chart shows the Coromandel a lot narrower than it really is. What was then called for was a shore-party to ascend a central peak from which both coasts could be seen, but that couldn't happen until much later. ============================ Richard Pisko in [7213] mentioned a recent Canadian TV programme describing Cook's surveys of Newfoundland, adding "I wonder if he had a way to keep the sextant level, so as to avoid taking the "slanted" angle, while still being able to align his chosen reference stations." My guess is that slanted angles would be avoided, where possible, by choosing landmarks at the shoreline, to keep everything in the same plane. I doubt if the height of the poop above water would have a significant effect on such horizontal angles. Where landmarks at an altitude were called for, I imagine that together with the slant angle, an angular height would be required. I expect that Richard understands such matters rather better than I do. George. contact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---