PUBLIC RECORD OFFICE
Reference :-
C.O. 8
ALLY WITHOUT PERMISSION OF THE BE REPRODUCED PHOTOGRAPHIC- COPYRIGHT PHOTOGRAPH-NOT TO
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20 PUBLIC RECORD OFFICE, LONDON
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(B.) Reconnaissance and beaconing of the base-extension figure, consisting of
two quadrilaterals, and of one intersected point.
(C.) Observation of the inner quadrilateral with a mean triangular error (for
3 triangles) of 1.66".
(D.) One month's tidal observations for determination of a M.S.L. datum,
and of height of each base-terminal.
The advantage of carrying out a small complete piece of work has been that valuable experience has been gained as to equipment and methods required for each class of work in the field in Fiji,
(A.) BASE-MEASUREMENT.
The selection and clearing of the base line occupied a period of three weeks, and the actual measurement in two directions 17 working days-the whole of the field- work not being completed until the 18th of January.
The unusual time taken was due to difficulties of ground and to the method involved by the use of the Stanley apparatus.
Selection. The Navua River, on debouching from the hills (Long. 178° 8'), has formed an area of about 20 square miles of very flat, alluvial land through which it flows in two main streams. This land is closely grown with sugar cane, usually 7 feet high, fringed on the sea-shore with bush, dotted with numerous Indian houses, intersected with a network of tidal ditches, and is generally boggy in wet weather.
The three solitary mounds in the area and the low hills surrounding it were all too steep to be made use of.
To obtain a base of over 1 miles long, capable of extension, the choice lay between one involving either the steep slopes of hill-sides, a break of 150 yards at the river, or terminals requiring tall signals for mutual visibility.
The latter was preferred, but the clearing of cane and fitting in of a line avoiding various obstacles was a laborious undertaking.
A line running nearly magnetic north and south, and about 3 miles long was finally obtained in extension of a straight piece of tramway, 1,300 yards long, belonging to the Sugar Company, but the terminals required 20-ft. signals for visibility, and the soil at the south end was boggy.
Beyond the erection of the end signals, the construction of a framework on piles for the theodolite-stand at S. end, the accurate alignment by theodolite of three intermediate flag-poles, and the fixing of the terminal-points and check-marks at each end, no further preparation of site was required.
Terminals.-The south terminal, whose approximate position was enforced, was in unsatisfactory ground, water being found 1 foot 6 inches below the surface. It was impossible to put in a concrete or other heavy terminal at either end without danger of settlement taking place during measurement and theodolite observations. Temporary terminals, consisting of 4-inch x 4-inch hardwood piles about 6 feet long with zinc-covered tops, were driven in flush, and four similar piles, placed symmetri- cally 3 feet distant, were used for check-marks at each point. These terminals proved rigid, and no move has been detected. They will be replaced by concrete pillars as soon as the whole base figure has been observed.
Signals. The signals, only required for mutual visibility from terminals, were erected in the first instance about 15 yards in extension of the base line, the terminals of the latter being later determined. They were constructed of 4-inch x 2-inch squared poles 20 feet long (carrying a white vane 12 inches x 9 inches x inch with a centre dot), bolted and lashed to a 9-foot rail driven into the ground, guyed with wire-rope to rail-pickets and strutted at the base with 4-inch x 2-inch wood struts. They were accurately verticalised by theodolite, and remained rigid and vertical throughout.
As it was desirable to have no large residual, i.e., a whole number of tape lengths --the exact point at the N. end was determined by theodolite after the first measure- ment of the base had been completed, and the cross-line drawn.
Apparatus. The base-measuring apparatus made by Stanley & Co. had been purchased in 1907. It included:-
(1) Two optical transferring instruments on tripod stands, each consisting of a heavy plate on three foot screws, carrying a central transferring telescope, a small aligning telescope, a brass trough to carry the tape, and a microscope with vernier for reading the latter, the whole fitted with cross-levels, and capable of lateral adjustment within short limits.
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(2) Two straining tripods, each carrying a brass pulley on a heavy bracket. (3) Nine intermediate supporting tripods with cross-pieces for carrying the
tape.
(4) A 100-foot Invar tap, inch broad, with one inch at each end divided into 1/100ths, tested on the flat at Kew in August, 1907, under 20 lb. tension.
The microscope vernier was intended to read to .001 inch.
In addition to the above there had been brought out from England :-
(5) A 100-foot Invar tape inch broad, with one inch at each end divided to 1/25 inch, tested in catenary at the Ordnance Survey, Southampton, in August, 1908, under 20 lb. tension.
Method employed. Measurement with a 100-ft. tape supported by tripods at 10-ft. intervals, each of which requires alignment and adjustment to level, is an operation that, on soft ground intersected with ditches, is a most laborious and difficult one. Each of the ten sections of tape is, moreover, a catenary whose supporting points are places of considerable friction. In the present case, in addition, there was large inertia and friction in the heavy brass pulley.
It was decided to adapt the apparatus to the small Invar tape hung in catenary, and to use the -inch tape as a standard for comparison only. The trough-carriers were fitted with central pins to act as tape supports, whilst the brackets of the pulleys were inverted and arranged to allow of some lateral motion. The pulleys themselves were cleaned and oiled regularly during the course of the work.
Comparison of Tapes.-Reliable ground-points, owing to the boggy nature of the soil, could not be fixed without long delay, but sets of observations on the tapes without reference to the ground were made both before and after each of the two base-measurements. Probably owing to the large friction with the 4-inch tape, at pulleys and supports, differences as large as 1/45 inch between means of sets were found.
The clip handles of the Field tape had been fixed by the makers to the tape with two small screws. The tape, being of very narrow dimensions, was thus greatly weakened at these points, and during the course of the measurement broke, and the measurement had to be stopped. Though refixed with fresh screws it again broke. Fresh handles were made, rivetted and tinned to the tape, and proved quite satis- factory.
Some small kinks had developed, rendering comparison with a standard the more necessary. It was therefore decided, after the base-measurement was com- pleted, to send the tape to England to be re-standardised at the Ordnance Survey, Southampton.
System of Work in the Field.-The following system was employed: A total of three white men and 11 natives were distributed thus-
One observer and one native at each instrument;
One observer with the level;
Three natives in charge of tape;
Two natives with weights and straining tripods;
Four natives carrying instruments, tripods, boxes, tools, &c.
To avoid mistakes of reading, each instrument observer read at one and the same end of the tape throughout, but with alternate instruments.
The tape having been carried forward, the fore straining-tripod and pulley were first put up at the correct distance and aligned on the distant flag-pole by a field-glass from behind the back instrument, final adjustment being made by lateral movement of the bracket and pulley. The tape was then placed in position over the pulleys, and weights attached, the fore instrument put up at the 100-ft. mark and roughly levelled. The instrument was then adjusted to the tape by the plate-screws so that the microscope was exactly over the alignment and its scale parallel to the tape-scale. A set of simultaneous tape readings, a thermometer reading, and a reading of the level-scale were then booked by each observer.
The level-scales, which were hand-made, divided to .01 foot, and fitted with cross-pieces at their bases, were placed on the instrument-plate and read by a 10-inch level erected near the centre mark of the tape.
The tape-readings were compared before the next move was made.