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Topographical Surveys. The past history of the topographical map of Ceylon is a somewhat unfortunate one. When Sir T. Holdich visited the island he found that, despite the fact that a Survey Department had existed for many years, no attempt had been made to construct a general topographical map, and he strongly He further advised that this most necessary task should be initiated at once. advised that the map should be on the one inch to one mile scale, and that the Indian method of plane-tabling should be used. In order to train men of the Ceylon Survey Department in this, to them, new class of work he suggested that the Survey Depart- ment of India should be asked to lend two or three skilled plane-tablers, to act as instructors, for a period of two years.

The then Surveyor-Genera! of Ceylon accepted these suggestions, with the excep- tion that instead of asking for Indian surveyors to come to Ceylon, he proposed to send some of his men to India who would study the plane-table methods used there and would thus be able, in their turn, to impart instruction to the Ceylon staff. Having, however, formally expressed his acceptance of the recommendations, which, in fact, he stated were practically the same' as those he had himself already proposed, he took no steps whatever to carry them out. Instead of doing so he started the topographical work upon a cumbrous, inaccurate, and expensive method. the introduction of which has had the nett effect of postponing the issue of the contoured 1-inch of Ceylon by ten years.

map

The defects of the old system are well known to the present Surveyor-General, who has already taken steps to terminate it, and who lately secured the services of Mr. G. T. Hall, of the Survey of India, to teach a few members of the staff the use of the plane-table. Mr. Hall spent three months in the island, with the result that 12 men have been well grounded in this work. During the present recess these men are, in their turn, teaching others, so that at the conclusion of the monsoon season, say, in August next, there will be a body of about 24 competent plane-tablers ready to take the field.

Before discussing the precise methods of survey most suited to the capacities of the staff and to the requirements of the island it will be convenient to give a short description of the old, now discarded, method.

Upon this system the field work was done upon the scale of 5 inches to 1 mile, executed mostly by compass and tape, though in the case of the areas recently done the detail was put in with theodolites and stadia rods. No contours were surveyed at first, but in the last three years a certain amount of contouring has been accomplished with special reference to those parts of the country through which railways are likely to be required. Apart from the uncertainty of the datum level this contour work has been of a quite high order of accuracy, but is altogether too elaborate and costly for extension over the whole country.

The original 5-inch sheets were photographed down to the 2 inch to 1 mile scale and blue prints were taken. Tracings of these were made with the names typed in and were then further reduced by photography to the 1-inch scale and printed for issue. A map on the 4-inch scale, prepared by re-drawing the 1-inch sheets and photographing them down, is also in hand. Specimen sheets, coloured to show the different cultivations, have been prepared and, when finished, this will be a useful general map for many purposes. Single-sheet maps of the island, on scales of 8, 12, and 16 miles to the inch have been produced, mostly from very imperfect material, and can be revised and re-issued when the present 1-inch is finished. Of this survey about 17,000 square miles are now done, leaving a balance of 8,000 square miles to complete.

For the new system of survey, which will be executed with plane-table only. contours and detail being inserted simultaneously, it is recommended that the 2 inch to 1 mile scale be employed, with contours at 50 feet vertical intervals, the sheets being reduced to 1 inch to 1 mile for publication. It will be observed that this is in conformity with the recommendation of the Committee of 1904-5 on the Survey Department of India. It may, however, be added that the justification for doing the field work on twice the scale of publication is much stronger in the case of a country of such close cultivation and intricate features as Ceylon than it can be over large portions of India. The detail should be surveyed by ordinary plane- table methods and the contouring done with the Indian pattern clinometer. The ordinary Indian method of sketching in approximate contours is, however, hardly

sufficiently accurate for the requirements of Ceylon and should be modified in the following particulars :-

(a) By the determination of clinometric heights at a much larger number of

points than is customary in India.

(b) By the running of at least one line of spirit levels through every plane-

table sheet.

(e) In flat country; by discarding the method of observing slopes with the clinometer and substituting that of traversing and plotting each individual contour with clinometer, sight vane, plane-table and tape.

This extra precision will naturally somewhat increase the total cost, but it will be amply justified in view of great value that the map will have for irrigation purposes as well as for the preliminary planning of new railways.

The difficulty of obtaining the required datum levels has already been dealt with. It is hoped that a primary line of levelling can be run through the area now in front of the topographers in a few months. In the meantime the secondary check lines can be run at about 7-mile intervals, leaving the final adjustments of absolute heights to be made when the initial line has been computed. These check lines will naturally be run in circuits closing on primary bench marks.

The sheet lines of the existing 1-inch map are arranged upon a conventional system. The size of the individual sheets, 13 by 83 inches, is somewhat small for practical use. It is recommended that, for the 8,000 square miles now uncompleted, the 1-inch map be published in two forms :-

(a) An uncontoured series in continuation of the old series in the same style

and with the same sheet lines.

(b) A final contoured map in sheets 15 minutes square (174 inches) with sheet lines on geographical co-ordinates, i.e., at every quarter degree of latitude and longitude.

No alteration should be made

the present system of projection, which is that of rectangular coordinates. This is the same as that used for the 1-inch Ordnance Survey map of England, and is a perfectly suitable one for a country of the size of Ceylon. The minute curvature of the meridians and parallels will be entirely imperceptible in a length of 15 minutes of arc, hence the margins of the new sheets can be drawn as straight lines. The whole series will exactly fit together into one map so that the reductions to smaller scales, 1 mile to 8, 12, and 16 inches, can be made by direct photography. The detail on the new sheets will also exactly fit that on the old sheets without any distortion.

The new series should be printed in four colours: black for the detail, brown for contours, blue for water, and green for forest areas. The best method of reproduction would undoubtedly be that of engraving upon copper plates, work which it would not be practicable to carry out locally. No exact estimate of the cost can be given until specimen sheets are available, but it is not likely to exceed Rs. 7 per square mile.

On the completion of the present 1-inch series over the whole island there will remain for final survey the 17,000 square miles now done upon the old system. Owing to the defects of the method of survey employed, and the inefficient super- intendence of much of the early work, no reliance can be placed upon it. As, moreover, the ground would in any case have to be covered by the plane-tablers for the tracing of the contours, and as the extra work involved in the insertion of the detail is small, it will be better to discard entirely all the old detail and make a complete resurvey of the area. The question will also arise as to what use shall be made of the block surveys for topographical mapping, and whether it is better to take information from the large scale plans and place it on the plane-table sheets or to use such materials only for the purpose of checking the work of the topo- graphers. In general may be laid down as a principle that it is dangerous to give the plane-tabler any detail upon his sheet unless such detail is absolutely complete, and unquestionably accurate. Thus, for example, the block surveyor may traverse portion of a stream, and fix its position with precision. The upper part of the stream may, however, come out of a block of cultivation, when its precise course will present no further interest to him, and he will leave it undefined. Should such a stream be entered upon the blank plane-table sheets there is great danger that the plane-tabler may assume that it actually begins where shown on

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