After the main triangulation for a bridge is finished, the next step is to compute the angles to the various points on the piers that will be needed during the sinking. For a single cylinder pier it will suffice to triangulate to the centre only, and for a pier composed of two cylinders a triangulation to the centre of each cylinder will be enough; but for a rectangular pier it will be necessary to locate not only the centre, but also another point near the periphery, in order to prevent the pier from being rotated about its vertical axis in going down. After the calculations are completed a triangulation-sheet should be prepared, on which should be shown all of the triangulation with the various distances on all lines and the exact angles for all deflections.

Foresights should next be located for the bridge tangent and for all pier points, so that the transitman shall never be under the necessity of turning off an angle when locating a pier. The position for any foresight is generally determined by convenience, but it should be chosen so as to avoid any probability of disturbance. Each foresight, which consists of a substantial wooden target, is located by turning off the proper angle from the base-line, and is then fixed immovably in position, after which a series of from ten to thirty readings of the angle is made, the corresponding centre lines being marked on the target. The average of all of these centre lines is then determined, and is assumed to be the true centre, which is marked conspicuously on the target. Each target is to be marked also with its characteristic letter or number, so that its individuality may be recognized by the transitman from the most distant point of observation. The angles for determining the correct centre of any target should be laid off continuously on the limb of the transit. All foresights should be inspected occasionally so as to see that they have not been disturbed, although any disturbance will be discovered, the first time that the foresight is used, by the three lines failing to intersect in a point.

When piers are to be built in open coffer-dams, the work of locating  them  is comparatively simple, for when they are