CO885-(21-23) — Page 423

PUBLIC RECORD OFFICE

Reference :-

C.O. 885

22 PUBLIC RECORD OFFICE, LONDON

ALLY WITHOUT PERMISSION OF THE BE REPRODUCED PHOTOGRAPHIC- COPYRIGHT PHOTOGRAPH-NOT TO

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With regard to intersected points, i.e., points for the use of topographers fixed by observations to them, but without being visited, it was decided that a departure from previous practice should be taken by fixing a very much larger number of points than had been the custom hitherto. Various considerations affect the number of intersected points that can be fixed. On a Boundary Commission it is often a matter of time or personnel available; and it was the custom on the Boundary Commissions of which the officers on the Topographical Survey had had experience to select such natural features or conspicuous objects as lent themselves to the purpose, and fix these for the use of the topographers; but to erect very few artificial marks. But experience showed that it would be much better if possible to considerably increase the number of these points, and to distribute them as regularly as possible over the area to be surveyed, for two reasons: firstly, it was anticipated that it would make the work of the topographers easier, and ease implied greater speed and greater accuracy; and secondly, it was found that in the dense fog which accompanies the dry weather it was often extremely difficult for the topographers to get on, unless there were plenty of marks easily visible.

It was, therefore, decided, on the Topographical Survey, to select natural marks or erect artificial ones, so that, as far as possible, the intervals between any trigono- metrically-fixed point and its immediate neighbours should not exceed five miles, on an average. This entailed a considerable amount of extra work in the erection of beacons, and in observing and computing; but the results, it was held, more than compensated for this.

Where possible conspicuous natural features were used for the intersected points, but as a rule it was found as quick and better to erect a tripod beacon.

Beacons and Station Marks.---On previous Boundary Commissions practically all the beacons erected were of tripod pattern, made of three poles 25 to 35 feet long, lashed together near the tips, and thatched to within 6 feet of the ground. This makes a useful form of beacon, being a good object to observe to, and forming a shelter for the observer. It was found, though, that the tendency was to build them too narrow and high, which made them not only less easy to observe, but, owing to their comparatively narrow base, more liable to warp or be blown or otherwise move from the true vertical position. It was, therefore, decided to adopt regular dimen- sions for the beacons, with the object of presenting a more solid and symmetrical mark for observing, while giving an equally good shelter for the observer.

The dimensions decided on for tripod beacons were as follows:- Thirty feet poles, lashed 7 feet from the tips, sunk for 2 feet of their length in the ground, at the angles of an equilateral triangle of 24 feet side; the whole thatched to within 6 feet of the ground.

This makes an excellent beacon, and when viewed" square on," i.e., from a point such that the line of sight is at right angles to one of the sides, the angle of inter- section of the poles is about 73°, which makes the beacon an easy object to intersect with the cross wires.

A tripod beacon, however, has the disadvantage that it only appears symmetrical when viewed" square on." When seen from other points it appears to lean over, and this, especially in an unfavourable state of the atmosphere, may tend to bad observing. A four-pole beacon, on the other hand, looks symmetrical from any point of view, and it was decided later to adopt these for all main stations; the dimensions found suitable being 30 feet poles, lashed 7 feet from tips, buried 2 feet of their length in ground at angles of a square of 20 feet side-thatched down to 6 feet above ground. The angle of intersection of such a beacon varies from 70° to 90°, according as it is viewed square or diagonally.

In many parts of the country surveyed it was impossible to obtain 30 feet poles. In Nsara, for instance, almost the only trees found are acacia thorn, from which one can rarely get anything longer than 20 feet, and more rarely still a straight pole of that length. In these parts we built beacons in the following way :-

Four stout poles or tree trunks, with a fork or Y at the top (known to the Swahili as nguzos) were set vertically in the ground at the angles of a 12 feet square, so that the bottom of the fork was 6 feet above ground. It was found that if these uprights were sunk 2-24 feet in the ground and well rammed, they were quite strong enough. On these were laid and firmly lashed 4 stout horizontal spars (of about 14 feet length). On to these again was hoisted a four-pole beacon, made of 19 feet noles lashed 7 feet from the tips: their butts being cut out to a Y shape and wedged firmly into the angle between the horizontal spars. This beacon was strongly lashed down to the uprights, and then cross-sparred and thatched down to the horizontal spars.

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The appearance from a distance was exactly the same as that of a four-pole beacon of 30 feet poles.

The dimensions given above are, of course, approximate only. If all four poles were longer than 30 feet (a rare occurrence), the dimensions generally would be increased in proportion. Usually it was found necessary to lash a short spar on to one or more of the poles to bring them up to about the 30 feet length: and in Nsura spars had often to be lashed together to make even a 19 feet length.

The large part of the beacon above the intersection of the poles is an important point, as this makes the intersection (which is arranged to be immediately above the station mark) easy to see in the telescope, and helps to accurate observation for both horizontal and vertical angles.

For the secondary triangles we made 4-pole beacons occasionally, but more often tripods, the latter involving, of course, less time and labour. For the intersected points a tripod was always sufficient, or, in fact, any mark that was conspicuous and unmistakable. Beacons of other patterns were tried for intersected points, such as "basket" or round beacons, or cutting away bush and leaving a conspicuous clump or tree; but as a rule, if a really good natural object did not exist it was found better to make a beacon; and the tripod beacon was found to be as quick and easy to make as any other, and more serviceable.

All main and secondary stations were permanently marked, usually by a screw or other piece of metal leaded into a stone and sunk underground.

Methods, &c.-The instruments and material used on the Boundary Commission were employed on the Survey, i.e., 5-inch Troughton and Simms's micrometer theodo- lites, Royal Engineer plane tables, Indian clinometers, &c. Ordinary alidades were used for plane-tabling, and were considered to be quicker, more handy, and easier to see with in foggy weather than the telescopic pattern, besides being more than sufficiently accurate for the work. Of the two theodolites in use, one began to give unsatisfactory results, and had to be abandoned. It had been used on a previous Commission, and was probably worn. A new one was cabled for, but when, after long delay, it arrived, it was found to be unusable, the vertical circle being quite out of truth. Thereafter, observations at main stations were taken with the one good theodolite (which was also an old one), while the other old instrument was used for secondary and intersected work.

Horizontal angles were measured on three arcs as a minimum on the main chain; often on four. The intention was to visit every secondary station, but in a few cases this was not done. Angles were measured at them on two arcs as a rule to other secondary stations, and on one arc to the intersected points (two measures, F.R. and F.L., being of course made on each arc in every case).

The main chain quadrilaterals were adjusted by the method of successive approximations, a quick and convenient process for this class of work.

The topography was carried out on the scale of 1/200,000, or 3·16 miles to the

inch. The total area mapped was 3,446 square miles.

The time occupied was as follows :-

A. Mapping in the field

B. Do.

C. Do.

including inking in

including all time from start to finish of work, inking in, travelling from one allot- ment to another, sickness, &c.

603 days. 712

The average rate of progress of the topographers was, therefore-

A. Mapping in field

B. Do. including inking in

C. Do. all time included

***

779

Sq. miles per man

per day,

5.7

4.8

4'4

In no case did the rate of progress of any topographer, all time included, fall below 4 square miles per diem. The quickest work was done by Sergeant Thomas, who did one area of 520 square miles in 72 days, field time only (72 square miles a day). This was a remarkably fine performance, the quality of the work being first class. Sergeant Archer's average of 41 square miles per day all time included is also good, as it includes 14 days' sickness. A table is appended giving the topographer's progress in detail.

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