Topographical Surveys

TOPOGRAPHICAL SURVEYS The British empire affords examples of all possible methods of the survey for, and preparation of, topographical maps.

Great Britain itself is one of the few countries in which all mapping is combined under one department and in which the expense of duplicating surveys for different purposes is avoided. The six-inch plans are used in the field for a special revision, arranged to secure the correct information for small scale maps. The small scale maps are then drawn for reproduction by helio zincography, each scale serving as a basis for the next in order.

Canada supplies excellent examples of topographical mapping and particularly so in photographic method. The one inch maps of areas in the Rocky mountains are made by ground photographic methods. A control of triangulated points is first established. The photographic party is equipped with theodolites and cameras which fit upon the same stands. The cameras are so used that each portion of the ground to be mapped is photographed from two points of view on plates held in the vertical plane. The position of the camera is resected with the theodolite. The plotting is carried out during the winter recess, each photographic view being used as a record of angular measurement both horizontal and vertical. Positions are established by the intersections of rays.

The survey of the Orange Free State in the Dominion of South Africa is an example of a plane table survey in country admirably suited to its use. The total area of over 5o,000 square miles was mapped in 6 years by a Colonial Survey Section of 2 officers and 4 non-commissioned officers, R.E. There already existed, in parts of the area, chains of the Geodetic Survey of South Africa. Based on this triangulation, the officers of the party extended a ruling tri angulation of a secondary nature (mean triangular error of less than 3 seconds). The sides of this triangulation varied from r o to 45 miles. A tertiary triangulation, completed with a free use of intersection and interpolation, resulted in a fixed position and height every 4 or 5 miles. Plane tabling was carried out at the scale of each field sheet including 15 minutes of latitude and longitude. Progress averaged six or seven square miles per

man per diem.

An example of a different class of survey in much more diffi cult country is to be found in the forest regions of the Gold Coast and Sierra Leone. There is little triangulation available, nor would it be possible without heavy expense. The control, mainly astronomical and widely spaced, is provided by a special party equipped with theodolite and wireless receiving set. The detail party traverses along cleared paths with chain and compass between control points. The triangles are then cut up by a number of rope and sound traverses. Along the main traverses lines of rough levelling are run, and aneroid barometers are carried on the minor traverses. Field work is at the two-inch scale and each traverse is adjusted and compiled on a final compilation.

Plane Tabling.

The plane table is merely a flat board which can be attached to a tripod. The board varies in dimensions from 4o in. to 18 in. a side, a convenient size being 24 X18 inches. The board itself is generally made of wood, sometimes fitted with aluminium corners and fittings. Underneath, the board is strapped with metal, and has in the centre a ring with arrangements for attachment to the tripod. The tripod, of three girder pattern legs of light wood, can be clamped to the board or left friction tight to allow of rotation. Plane tables designed for large scale surveying often include a ball and socket joint for levelling, and an instrumental slow motion in azimuth. For topographical sur vey the board and legs are left as simple and light as possible. On the table is mounted drawing paper or some form of celluloid.

The principal accessory to the plane table is the sight vane or alidade, which is merely a ruler with sighting vanes which can be raised or lowered at will. For engineering surveys at a largish scale a telescope with stadia hairs and vertical circle is added to the alidade. A box compass and some form of clinometer for measuring slopes or vertical angles complete the outfit.