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Des Barres' Survey Methods

On the 27th May 1776 Des Barres forwarded a copy of his 'Map of that Part of the Coast of Nova Scotia' to Real Admiral Alexander Colvill, Commander-in-Chief of the North American Station.  Along with the map Des Barres included an account of his surveying methods as follows:

I measured a Base of 350 Fathoms along a Plain on the western Side of Exeter Harbour, and from its Extremities, having, with a Theodolite, taken the angles of Visual Rays to Objects placed on the opposite Shore, which being calculated trigonometrically and protracted in their proper Bearings on Paper fixed upon a Plain Table, I then repeated with the Plain Table, the same Operations over again, and intersected the same Objects from the Same Extreemities of the Base Line, by which and other Iritersections, or Series of Triangles, I had the Distance between an Object placed on Point Bulkeley and another on Newton Head; from whence, by farther intersections performed in the Same Manner, I determined the true Emplacement of Winter's, Roger's and Barron's Islands, and of all the Ledges; thence, repeating the former Operations from all these Islands, I found all the Angles, and Distances to agree with what I had layd down, from the above mentioned Observations, before. From Points as were most commodiously Situated on those Islands, and Head Lands, I observed the Distant Head Lands, Bays, Islands, points, and other remarkable Objects, as far as they could be distinguished.


Next I went along shore, and reexamined the Accuracy of every Intersected Object, delineated the true Shape of every Head Land, Island, Point, Bay, Rock above water, etc and every Winding and Irregularity of the Coast; and, with Boats sent around the Shoals, Rocks and Breakers, determined, from Observations on Shore, their Position and Extent, as perfectly as I could. When the Map of any Part of the Coast was compleated in this Manner, I provided immediately each Craft with Copys of it: The Sloop was employed in beating off and on, upon the Coast, to the Distance often and twelve miles in the Offing, laying down the Soundings in their proper Bearings and Distance, remarking every where the Quality of the Bottom. The Shallop was, in the meantime, kept bussy in Sounding, and remarking around the Headlands, Islands, and Rocks in the Offing; and the Boats within the Indraught, Upwards, to the Heads of Bays Harb" etc. Soon after, coming to reflect on the extreemely irregular, winding and broken Shape of this Coast, -- which is mostly covered with Spruce and thick Underwood, and thence perceiving the difficulties I had to encounter with, in finding convenient Places of Sufficient levell and extent for to measure Base Lines upon, I tryed the following Experiment. I soaked a Dipsy Line in Salt water, till it was fully imbibed, and then Stretched and rubbed it tought, and with an Iron Chain, measured 100 Fathoms of it, with Marks every 10 Fathoms.


Just before the Change of the Tide, on a calm Day, I fixed the One End of this 100 Fathom Line to a Station on Point Bulkeley and, with the other End, rowed right out for another station on Newton Head; (whose Distance I already knew) when I got the Line tought, I made its End fast to the Grapnell, and let it run to the Ground; After this, I caused another Boat to take the first End (which was fixed to the Station on Shore) and hawl in the whole Line till it came to be perpendicular with the Grapnell let down by the first Boat and thence to proceed rowing out again for the said Station on Newton Head, till the Line got to be tought, made fast to the Grapnell, and let down as before; And so continued: By which Measurement, I found the Distance to be 510 Fathoms; longer by 5 feet than I had found by Intersections.


This same method I repeated in the Mensuration of a Line from the same Station on Point Bulkeley to an object on Roger's Island, and found 1586 Fathoms to be the Distance; longer, by 19 feet than I had found it to be, by the Method of Triangles. Many subsequent Tryalls and Examinations of this new Method have convinced me how Surprisingly it coincides with Mensurations performed, by the means of an Iron Chain, on Shore, And, from these Considerations and other Cogent Reasons, I have been induced to apply it very advantageously during the Course of my Survey. The irregular and Hilly nature of the Lands along Shore was, everywhere, sketched off, upon the Draught, on the Spot. The Interior Parts ofthe Country were layd down from the Accounts of Bearings, Distances and Descriptions, observed, with a common Pocket Compass, by Arcadians: Here the Lands have a better Aspect, the Hills being but low, of gradual Ascent,-- fiatt at their Tops, and generally covered with Beach, Maple, Oak, Pine and other large Wood and the Lakes mostly bordered with Wild Meddows and rich Interval Lands.


At the first Glanze of this Draught, it will, I am sensible, appear inconvenient by its great Size; but when Your Lordship comes to reflect on the Nature of the Coast, Surrounded with so many Bays, Harbours, Inlets, etc and Small Islands, Rocks, Shoals, etc between most of which there are Passages for Ships and Vessells, You'll obviously conceive that the confused indistinctness unavoidably resulting from a Contraction of the Scale would have rendered it of little use or no Service upon approaching the Land, where the Necessity is greatest. I did, at first, propose to make a Reduction of it on a Scale of Six Miles to an Inch, but the Short Winter's Days, notwithstanding the closest Assiduity, scarcely afforded me Sufficient Time for compleating One Single Original Draught from my Plain Table Sheets, I was forced, for the present, to give up the Thought of it. The Time and Opportunities I have as yet had in examining the Course of Currents and settings of the Tide, are insufficient to enable me to represent their divers Directions and the Velocities with which they run, in the accurate Manner they necessarily ought to be described in a Sea Chart; as the least Error or Omission of them, is often the Cause of Perplexities to Navigators, and sometimes of Disasters.


This present Summer, will, I hope, prove greatly instructive in this Article.