An Alidade, Consisting of Two Sights and a Straightedge, Takes the Place of the Two Needles
Two men are required for measuring, or "chaining," a head and a rear chainman. The rear chainman holds the 100-ft. end of the tape on the station point, while the head chainman takes his end forward toward the station to which they are measuring. When he has gone nearly the length of the tape, the rear chainman calls "halt." The head chainman stops and draws the tape up tight, while the rear chainman holds his division end on the starting point. Then the head chainman sticks a spike into the ground to mark the place where his division end comes, calls out "stuck," and starts on toward the object point.
Large spikes make good marking pins, especially if they have little red or white strips of cloth tied to them. Surveyors use 11 markers. One is stuck into the ground at the starting point and is carried forward by the rear chainman, who also picks up the markers at each 100-ft. point as soon as the head chainman calls "stuck." In this way, the number of markers which the rear chainman has in his hand is always the same as the number of hundreds of feet which the last set marker is from the starting point.
In measuring between two points, care must be taken to draw the tape out taut and straight, its two ends must be level with each other, and it must be exactly in line with the two points between which the measurement is being made. In measuring downhill, one end may have to be held up high, and the point on the ground where the end division would come, found by dropping a stone from the place where it is in the air and watching for the spot where the rock strikes the ground. A surer way to do this is to hold a plumb-bob string on the last division and carefully let the bob down until it touches the ground. A rod with a red or white flag on it ought to be placed at or just beyond the point to which the measurement is to be made so that the rear chainman can easily line in the head chainman. The latter, before he places his marker, looks back to the rear chainman to be told whether or not he is "on line" with the object point. If he is not, and ought to go to the rear chainman's right to get "on," the latter holds out his right arm and the head chainman moves accordingly. When he reaches the right point, the rear chainman signals "all right" by holding out both of his arms and then dropping them to his side; the marker is stuck, and both move up a hundred feet and repeat the process.
After all the points possible have been located from Sta. A, and the direction lines labeled lightly in pencil so that they can be distinguished when the board has been removed from the station, the plane table is picked up and carried to Sta. B. Here it is again set up, leveled, and oriented by making the direction of the line AB on the paper exactly the same as that of the line from Sta. A to Sta. B on the ground. This is done by placing needles at points A and B on the table and then turning the board until the two needles and Sta. A are in line. Sights are taken on the same objects which were "shot" at Sta. A, and to objects which were not visible from Sta. A. The intersection of the lines of sight toward a given object from A and from B marks the location on the paper of that object. If the two ends of a straight fence have been located in this way, a straight line joining the points will show the location of the fence on the map. By exactly similar methods, every other object is located on the paper.
In order to avoid errors, it is an excellent scheme to locate three stations near the outside edges of the area to be mapped, and locate all objects possible by sights from each of the three stations. If, instead of all three crossing each other at a point, the lines of sight from the three stations form a triangle, something is wrong. If the triangle is very small, it may be safe to use its center as the correct point; if not, the work must be repeated and checked. Locating even a few points by this method may prevent some bad blunders. The three stations ought to form as nearly as possible, an equilateral triangle; and the distances between all of them should be measured and laid out accurately on the plane table.
A Rigid Tripod is Made of Strips for Legs, Which are Fastened to a Large Top