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.