SURVEYING is the art of determining the form and dimensions of tracts of ground, the plans of towns and single houses, the courses of roads and rivers, with the boundaries of estates, fields, &c. A survey is accompanied by a representation on paper of all the above mentioned objects, and frequently by a delineation of the slopes of the hills, as the whole would appear if 1projected on a horizontal plane.
When canals or railways are to be executed, a survey of the ground is combined with the operations of levelling, in order to obtain, besides a horizontal plan, the forms of vertical sections of the ground along the proposed lines, and thus to ascertain the quantities of earth to be removed.
In maritime surveying, the forms of coasts and harbours, the entrances of rivers, with the positions of islands, rocks, and shoals, are to be determined ; also the soundings or depths of water iu as many different places as possible.
Military surveying consists chiefly in representing on paper the features of a country, such as the roads, rivers, hills, and marshes, in order to ascertain the positions which may be occupied as fields of battle or as quarters; and the facilities which the country may afford for the march of troops or the passage of artillery stores.
For trigonometrical surveying, see TRIGONOMETRY.
Since the mea.suremeut of the distance between two objects means of a rod or chain is very laborious and inaccurate when that dis tance is considerable, particularly if the ground should have many inequalities of level, and be much intersected by walls, hedges, and streams of water, it will seldom be possible to execute even an ordinary survey by such means alone, and instruments for taking angles must be employed, together with the chain, in every operation of importance.
If within the tract to be surveyed there should be a road about half a mile in extent, and nearly straight and level, so that a line may be accurately measured upon it by the chain, and that from its extre mities several remarkable objects, as churches or mills, may be seen, it will be convenient to use such measured line as a base, and with a theodolite to observe the angles contained between the base and the lines joining the extremities to the different objects. The three angles
of each triangle formed by such lines should if possible be observed, in order that by the agreement of their sum with 180' the accuracy of the angular measurements may be tested ; and then the lengths of the sides of the triangles may be determined by the rules of plane trigonometry.
Let A 13 represent a base so measured in a road ; and let c, n, E, a' be four remarkable objects within or near the boundaries of the tract to be surveyed ; the distances A C, A LI, &c., B D, B C, &e., will be those which should be determined by computation. These lines may then nerve as bases, and if from their extremities be taken the angles con tained between them and lines supposed to connect them with any other objects, as houses or remarkable trees, the positions and distances of these objects may be determined by computation as before. Thus B C or B is will serve as a base by which the position of ci may be computed.
It will obviously be advantageous if the lines supposed to connect the objects lie nearly parallel to the directions of roads, lanes, streams, or hedges, on account of the facility which will thereby be afforded for laying down such roads, &c., on the plan. In order that it may be possible to place the theodolite at the angular points of the triangles, those points should not be precisely in the churches, mills, or other objects whose positions are to be determined, but should be indicated by poke set up near those objects, on spots of ground in such situa tions that each may be visible from the two others which with it con stitute the intended triangle. The place of the building may be ascertained hy its bearing and distance from the pole in its vicinity.