Surveying.

Surveying. Land-surveying may be considered the earliest practical application of the art of geometry or earth measurement, and must have been in some more or less rude form coeval with agriculture and the division or appropriation of the soil. In Rome surveying was considered one of the liberal arts, and the measurement of lands was entrusted to public officers who enjoyed certain privileges; and it is probable that the system of measurement practised by them was very similar to our plain surveying with the chain and cross-staff of the present day, and has been handed down to us through the feudal period. An examination of ancient records and title-deeds will show that both areas and boundary lines of the different enclosures forming fields, hundreds, town-lands, &c. are often laid down with much accuracy.

Land-surveying may be considered under the following heads: (a) Plain surveying with the chain, and without the aid of angular instruments, except the cross-staff or fixed angle of 90°; (b) modern engineering surveying, in which angular instruments are used; (c) coast and military surveying; (d) trigonometrical surveying.

The fundamental rule of every description of land-surveying, from the humblest attempt to lay down an irregular garden-plot to the trigonometrical survey of a large extent of the earth's surface, when the aid of the most refined improvements of modern science is indispensable, is simply to determine three elements of a triangle, and thence to calculate its area.

In plain surveying with the chain the three sides of the triangle, ABC, are supposed to be accessible, and are carefully measured on the ground, and then laid down or platted to scale on paper, when an accurate figure of the triangle will be obtained, on which the length of the sides can be marked. To get the area, however, it will be necessary to determine the length of the perpendicular line AD, and this is usually done (when possible) on the ground by means of a simple instrument called a cross, which consists of two sights or fine grooves at right angles to each other; which being placed on the line BC (keeping B and C visible in one of the sights), nearly opposite the angle A, is moved gradually till the angle A is intersected by the other sight. The line AD can also be laid down on the drawing, and its length found by scale and afterwards verified on the ground, or it may be at once laid down on

matter what the form of the surface to be surveyed may be—polygon, trapezium, or trapezoid—it may thus be determined by a judicious subdivision into triangles; and when the survey is not of a very extended nature or character, and when no serious obstructions exist, chain surveying is both accurate and expeditious, especially if proof or tie lines are properly introduced for the purpose of testing the accuracy of the work.

In every description of surveying it is best to make the original triangle as large as possible, and to work from a whole downwards rather than build up a large triangle by the addition of several small ones. It would be impossible here to lay down rules to meet the many difficulties which arise in the practice of surveying; indeed the best test of a good surveyor is the ease with which he will overcome local obstructions which appear almost insurmountable to a novice, or even to a theoretical surveyor with little field practice. Where buildings or other impediments are found in the measurement of a straight line, they are generally passed by the erection of short perpendiculars sufficient to clear the obstacles, and a line parallel to the original measured as far as they exist, when the original line can be again resumed. Differences of level occurring in measuring a line where no instruments are used are generally compensated or allowed for by the judgment of the surveyor.

In registering the dimensions taken on the ground, such as sides of triangles, offsets, intersections of roads, fences, &c., and everything necessary to make a perfect delineation or plan of the surface, surveyors use what is called a field-book, the mode of keeping which varies very much with individual practice. Some surveyors use hand sketches or rough outlines of the form of the ground, and mark the dimensions on them, while others use the ordinary form of field-book, or a combination of the two methods, which perhaps is the best when any difficult complications happen on the ground, such as the frequent occurrence of buildings, enclosures, water, &c. along the line. In the ordinary field-book the centre column, commencing from the bottom, represents the length of any line or side of a triangle, and the figures in the column the distance at which the offsets to the right or left are taken, or where roads, streams, fences, &c. cross the line, or buildings adjoin the same. We give herewith the field-book of the assumed survey of the triangle ABC, with the different offsets and insets on its sides, and where roads, fences, streams, &c. cross them, the detail of which can be obtained by subdividing the triangle into smaller internal ones. The figure can thus be laid down from the book, and its area calculated by the formula $\backslash frac\{AB\; \backslash times\; CD\}\{2\}$ and the offsets and insets calculated, and added or deducted.

Ponds, plantations, and enclosures of different kinds may be surveyed with a chain, especially if their form be such that they can be conveniently included in the area of a triangle, the correctness of which being proved by proper tie-lines, the form, area, &c. may be ascertained by offsets, or rather insets from the sides.

See ORDNANCE SURVEY, LEVELLING, MENSURATION, THEODOLITE, CONTOUR, &c.; and for the United States Coast Survey, see CHART.

The line ab may be similarly booked and platted. the ground by the use of the chain alone. An improved reflecting instrument, called an optical square, is also often used for this purpose. Any boundaries along the lines or sides of the triangle, ABC, can be determined by the use of offsets or insets, as they occur on right or left of line. No triangle, the correctness of which being proved by proper tie-lines, the form, area, &c. may be ascertained by offsets, or rather insets from the sides.

See ORDNANCE SURVEY, LEVELLING, MENSURATION, THEODOLITE, CONTOUR, &c.; and for the United States Coast Survey, see CHART.