A and B are two station points. Line AC is to be ranged. The two range bars at A and C are to be run vertically. The verticality of the rods is to be ensured with the help of a plumb bob. If the rods at A and C are truly vertical, a surveyor standing about 2 m back at A and looking along the line AC will find the vertical projection of the rod at C hidden behind the vertical projection of the rod at A, as is shown in statistics. If the surveyor is standing too close to
A, he may lose sight for comparison. The surveyor will now send a man with a range rod to stand on the line AC at a distance less than one chain length from AC. One of his eyes is closed and is looking at AC as well as the vertical projections of the two rods hidden behind the rod at A. Point B will then lie on AC. Thus other points can be demarcated.
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