Surveying

Methods of Survey.

The simplest and most ancient method of making a plan or map is by means of direct measurement on the ground, with a rope, a chain, or a tape, of known length, suitably divided. As a simple illustration let us take the case of a level field of which a plan is wanted.

Suppose the field is as represented in fig. 1, the boundaries of the field being the irregular lines ABCD. Then, to survey this field with a chain or tape, put pickets at the points ABCD, some where near the corners of the field, these four points being inter visible and the lines between them being free from obstruction. Then measure, with the chain or tape, the direct straight-line distances, AB, BC, CA, AD, DC, BD. This will provide the necessary frame-work. It will be seen that for the plotting of the frame-work, only one diagonal, that is, either AC, or BD, is re quired. The measurement of both gives a check on the work and shows the surveyor how much reliance he can place on his measurements; it also enables gross errors to be detected. This principle of providing checks, and of never depending upon one measurement, is of great importance. To survey the irregular boundaries, all that is necessary is to measure "offsets" from the main chain lines, at known distances along the lines, and to note these in the field book. The offsets are measured with an offset rod or a tape at right-angles to the chain line. With all this in formation in the field book there is no difficulty in plotting the plan on paper to any scale that may be required. It is usual to lay down some limit for the off sets.

An elaboration of this simple method is the scheme which was adopted by the Ordnance Survey of Great Britain and Ireland, when large-scale surveys were first commenced officially in1825.

The country was covered with a triangulation, of which the sides averaged, for the original six-inch map, some 5 miles. Each of the sides of these triangles was chained, and notes were made in the field book of points where the chain lines crossed detail, such as edges of roads, banks of streams, hedges, walls, and so on. The great triangles were broken up by other chain lines into smaller triangles, and lines tying on to these at their extremities were run along the detail to be surveyed, offsets being measured as usually.

Modern Methods.

We may now consider the system which would be adopted in a perfectly up-to-date, modern state. In such a case, there would be a geodetic survey covering the whole country with a system of triangulation of the first order; that is to say, the country would be reconnoitred beforehand, and mutu ally intervisible stations would be selected, some ten to thirty or forty miles apart, covering the country with a net-work, or series of chains, of triangles. At the apices of these triangles horizontal and vertical angles would be observed with a theodolite ; one of the triangles would be connected with a measured side, called the base or base-line. From the known length of this side the lengths of all the other sides would be calculated. The latitude and longi tude of one point in the triangulation would be determined by as tronomical means, and also the true bearing of one line is obtained; it is then clearly possible to determine the latitude and longitude of every other point. The meaning of the term "first order," used above, is that the average triangular error of the net-work, or system of chains, of triangles, that is, the error of the sum of the 3 angles of a triangle, should not exceed one second.

Now we have to hang all other surveys upon this frame-work. A frame-work of the kind described has the fixed points too widely separated to be of much use for the detail survey, so the first thing to be done is to provide a closer frame-work; and this would usually be done by executing another triangulation, of less accuracy than the first, but depending upon it. There would be more points in this secondary triangulation, and the points would be distant from each other some 5 to Io miles. The triangular error would not exceed 5 seconds. If now it were required to make a map on a scale of, say, i :too,000 the frame-work would be sufficiently close, and the detail could, in suitable country, be carried out by plane-tabling which would be based upon the points so provided.