Murdoch Mackenzie Snr, The Orcadian surveyor
Murdoch Mackenzie (1712-1797) was to become perhaps the principal figure in the development of accurate coastal charting, or hydrography. Today his name is to be found in very few record books or treatises, yet his influence and the influence of members of his family, was considerable. In a highly critical commentary on coastal maps that had been produced prior to his own work Mackenzie noted:
“In these Charts, as all the Distances are either taken by the Eye, the Log-line, or the Ship’s Traverse, without any allowance for the Influence of Tides and Currents; and the Courses, for the most part, deduced by the Reckoning, or sometimes taken at Sea by a Compass; great Exactness is not to be expected: and as the Scale is very small, no striking Resemblance of any Part of the Coast can be expressed in the Draught, to point out with certainty where Shoals or Harbours lie.”
The maps are located in the Orkney Archives which together with his “Notes for surveying an Island” in hand-written script in almost certainly formed two key parts of the prospectus material that Mackenzie used in promoting his venture.
A copy of the original prospectus text, entitled “Proposals for surveying and navigating by subscription the Orkney Islands”, has recently been located in the Bodleian”, Oxford. The text in the top right hand corner of the map reads:
“How very serviceable it would be to a great Part of the trading Nations in Europe to have the Orkneys rightly navigate, will be obvious, from their Situation, to all acquainted with mercantile Business: and will also appear from the vast number of Shipwrecks that happen there. On this small Island of N. Ronaldsay alone, about twenty British and Dutch vessels have been lost within the last 30 or 40 years many of them with very Valuable Cargoes, besides a much greater number on other Parts of that Coast; most of which might have been prevented by such a Chart of these Islands as is here proposed.”
Mackenzie ‘s Orkney maps were published as an atlas of 8 charts (5 of the Orkneys and 3 of Lewis in the Hebrides) in May 1750, with Kirkwall in the Orkneys taken as the meridian rather than Greenwich. For the main island of the group where he began his survey he erected beacons on the hilltops for his triangulation stations. He then used his theodolite to measure horizontal angles between the various stations.
In order to provide a scale for the Orcadian survey Mackenzie had to measure a base line and then connect it to the triangulation survey. The site he chose was on the North West branch of the Loch of Steakhouse (Stannous, very close to Mackenzie’s own home): “... catching the Opportunity of a very hard Frost … Poles were fixed in the Ice in a straight Line, extending from … the parish boundary [of].. Sand wick and Habra, to Bragger near the north-most Bridge, and Lines or Ropes stretched on the Ice from Pole to Pole, along which was carefully measured, with an Iron-Chain, the Distance of three Miles and three Quarter.”
By this method Mackenzie obtained a long and accurate baseline from which he could extend the coast and land detail. This he achieved by chaining for measurement of selected point-to-point distances, by angular measurements using his theodolite and when offshore, employing his quadrant, by sketching, and finally by using lead lines for depth measurement. Most depth measurements he took were derived from one or two passes by boat, although occasionally in very complex or rocky areas more detailed soundings were taken.
The quality of his Orkney maps were also greatly aided by his personal local knowledge of the islands and the fact that he had both the funding and the time to carry out the task with great care.
Following the Navy’s embarrassing failures in 1745/6, and impressed by his Orkney charts, Mackenzie was asked by the Admiralty to make an accurate series of maps of the coastal waters of the Hebrides and Western Scotland (commissioned in 1751, completed in 1757).
He was subsequently commissioned to map much of the west coast of England and Wales as far as the Bristol Channel, together with the coast of Ireland. The main charts, which are in a very large format, were published privately in 1775/6.
His maps focus on the coastlines, charting depths in fathoms and including notes on unusual tides and currents. His primary objective was to determine safe passages and guidance for sailors.
However, he does not appear to have applied as rigourous standards and procedures for the Hebrides and subsequent surveys as he did for the Orkneys work – his maps of the latter are remarkably close to those of the present day, whilst the former show many disparities, especially with respect to the overall shape and size of some of the islands.
Mackenzie was elected as a Fellow of the Royal Society in 1774, with his sponsors including Sir Joseph Banks. Following the publication of his charts and books (1774-1776) he moved from London to Minehead in Somerset, to live with his nephew (also called Murdoch Mackenzie). His legacy was then continued under the guidance of his nephew, and subsequently their cousin, Graeme Spence. He died in 1797 at the age of 85 and is buried in Minehead.