CO885-(18-19) — Page 325

PUBLIC RECORD OFFICE

Reference :-

C.O.8

885

18 PUBLIC RECORD OFFICE, LONDON

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

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The survey of Mamponsu Timber Concession (about 300 square miles) neces- sitated the running of about 200 miles of primary compass work.

Lance-Corporal Mathieson was detached from the main party in January for the survey (including the necessary astronomical observations) of Imbatiem, in Ashanti, and the boundary between the stool-lands of Bekwai and Adansi.

On the conclusion of the concession work around Tarkwa and Mamponsu in March, the remainder of the non-commissioned officers moved across country in a north-easterly direction, surveying with compass and chain a belt of land about 1,800 miles in area.

REMARKS ON THE FIELD PARTY'S WORK.

3. The field party as constituted this season was an experiment. Instead of having two parties, each consisting of two skilled civilian surveyors and 44 carriers as in the previous season, the field work was carried out by the one party mentioned in the preceding paragraph.

Captain Symons generally had two or three concessions in hand at one time, and, in the intervals of his own work, travelled from one to another of his detach- ments to check their surveys.

The non-commissioned officers were capable of preserving their true bearings while cutting the concessions, of accurate chaining, of the computation of co- ordinates, and of plan making. With the exception of Lance-Corporal Mathieson, they could not take astronomical observations.

the old

The results of the new arrangement were highly satisfactory, both in the quantity and quality of the work, and very much more economical in money than

way, carriers'

and high salaries for skilled men both being saved. pay The advantages, however, would be lost if the Chief Surveyor's health broke down or if the concessions were too far apart. The system, too, requires good organisation in arranging for food and money supplies, rendezvous for detached sketching parties, and many other details incidental to work in the dense tropical forests of the Gold Coast. Captain Symons's arrangements were in all respects of the best, and the success of the work is entirely due to his organising power, indefatigable energy, and good health.

TOPOGRAPHICAL, WORK.

4. Besides the topographical surveys executed by Captain Symons's party, a large amount of very valuable work was carried out by the native assistant sur- veyors from headquarters.

About 3,000 square miles of country between 0° 0′ and 1° 15′ W. and 5o 30' and 6o 30' N. were blocked in by secondary compass traverses between the months of July, 1906, and January, 1907, by Assistant Surveyors Smith and Vaughan.

A third party under another native surveyor had time to complete about 350 square miles in the Anum District before breaking up from unavoidable

causes.

In January Assistant Surveyors Esuman-Gwira and Smith started the survey of secondary compass traverses radiating from Coomassie to the west, north-west, north, and north-east.

These traverses connected the rigid survey of the Colony to Captain des Voeux's rigid traverse from Salaga to Sikasiko, which received its true meridian values by the rigid Volta River traverse from Kpong to Yeji in the season 1905-6.

These compass traverses will be referred to again in paragraph 9.

SYSTEM OF TOPOGRAPHICAL WORK ADOPTED IN GOLD COAST.

5. As plane-table work is prohibited by the dense forest covering the Gold Coast, the system has been adopted of filling in the topographical details of the country by means of compass traverses.

These traverses start from, and in nearly all cases end at, some fixed point on the rigid theodolite traverses.

They are executed on four methods according to the importance of the survey, As the rays of legs are very short, it has been found not only more accurate and convenient but just as quick in plotting to take out the co-ordinates of these traverses from tables.

The four methods are as follows:-

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COMPASS TRAVERSES. Definitions.

Primary Traverse.-A 4-inch compass or over on tripod. Forward and reverse bearings read to the nearest minute. Mean bearings calculated and variation of compass subtracted. Legs measured to the nearest inch. Co-ordinates taken out of Gurdens's traverse tables to the nearest minute and decimal of a foot. Co- ordinates checked by Chambers's tables to the nearest foot and degree for gross

errors.

Secondary Traverse.-A 4-inch compass, or a military prismatic compass, in the hand, or on tripod, forward and reverse bearings to the nearest quarter degree. Mean bearing calculated and variation of compass subtracted. Legs measured to the nearest foot. Co-ordinates taken out of Chambers's tables to the nearest degree and foot.

Tertiary Traverse. Any compass, forward (or reverse) bearing only, read to the nearest degree. Legs measured to the nearest foot. Magnetic variation sub- tracted and map plotted by protractor on true north.

Quaternary, Traverse.-Legs measured by a rope about 100 yards long. Bearing by the surveyor at the back end of the rope to the sound of a cooey from the leader, who marks his position with an arrow, and then advances, dragging the rope. On reaching the arrow, the surveyor pulls the rope, the leader stops, cooeys, puts in another arrow and again advances, &c. An allowance of from 2 to 5 per cent. has to be made for the winding of the path when plotting the legs. This can only be fixed by experience.

N.B. As this system is inaccurate and only suitable for joining two fixed points, the map may be plotted without any allowance being made for wind of path, and the sketch afterwards reduced automatically.

Pacing can be substituted for the fixed length of rope in making short surveys

of under two miles.

REMARKS ON NATIVE SURVEY PARTIES FOR TOPOGRAPHICAL WORK.

6. Each party consists of a second-class assistant surveyor (£10 per month) and eight carriers (about 1s. 1d. per diem per head).

In surveying, two carriers work the 500-foot steel tape, one man ranges with the banderoles, and four men cut the lines, the eighth man being cook's mate.

A minimum of clearing is done, merely cutting projecting branches, tall grass, &c. In the dense forest legs of 180 to 200 feet are generally obtainable with a little clearing along the bush paths, and a progress of 2 to 3 miles a day is good work.

In more open country no clearing is required, but the tall grass limits the rays. The paths are, however, a good deal straighter than in the forest and legs of from 300 to 1,000 feet are often easily obtainable. Five miles a day is good sustained work.

With quaternary traverses, 10 miles a day is a fair average day's work with plotting in the evening, 15 miles a day cannot be maintained for more than three or four days running, and 20 miles is very exceptional.

The magnetic variation of the compass is obtained whenever possible from the survey beacons left in pairs on the theodolite traverses over the Colony.

The results of the compass work are in the majority of cases wonderfully

accurate.

In several cases inexplicable errors of unadjustable proportions point to large and very diverse local attractions. This merely emphasises what we already know, namely, the unreliability of compass work when the ends of the survey are not tied to fixed points.

In connection with this subject it is interesting to note the comparative results of compass and theodolite work. The survey from Cape Coast to Prahsu, a dis- tance of 70 miles, executed with theodolite, azimuths every 6 miles, and a Gunters's 100-foot chain in 1902, was believed to contain a gross error and was checked during the season by a primary compass survey modified by reading bearings to quarter degrees. Measurements with an fth-inch steel tape, 300 feet long.

The co-ordinates of the theodolite traverse were computed from Shortrede's, and those of the compass traverse were taken out of Chambers's tables to the nearest degree and foot. The difference between the two traverses was 1,800 feet, which