Leveling operations require the teamwork of both the instrumentman and the rodman to achieve consistent results. The accuracy of the survey depends upon the refinement with which the line of sight can be made horizontal by the instrumentman, the ability of the rodman to hold the rod vertically, and the precision with which the rod reading is made. Some of the basic procedures and preparations applicable to direct leveling are presented below.

Terrain and atmospheric conditions are the main considerations affecting the selection of setup points. It is essential that you select a point from which you can best observe a rod reading on the BS and FS points. In the interest of balanced shots, a setup point should be about equidistant from both BS and FS. In addition, shorter setup distances will result in smaller instrument errors caused by the atmospheric refraction and curvature of the earth.

The average instrument height at any setup is about 5 ft (1.5 m). On even downhill slopes, the ground where the instrument is set up may not be more than 3 to 5 ft below the TP for a level BS. On the FS, the extended rod can be held on the ground about 8 ft (2.5 m) below the instrument ground level and still permit a reading to be taken. This means that the tendency will be to make FS distances longer going downhill and to make BSs longer going uphill.

Therefore, it is necessary to conduct a reconnaissance of the terrain before you start leveling. You should note probable locations of instrument setup and TPs. During the reconnaissance, you should estimate the line of sight by sighting through a hand level.

In setting up the tripod, you first hold two tripod legs with both hands and spread the tips of these legs a convenient distance apart. Then bring the third leg to a position that approximately levels the top of the protector cap when the tripod stands on all three legs. Then unscrew the protector cap.

Next, you lift the instrument out of the carrying case by the footplate or level bar, NOT by the telescope, and set it gently and squarely on the tripod head threads. Rotate the footplate counterclockwise one-fourth turn or until the instrument seats itself; then rotate it clockwise to engage the head nut threads to the tripod head threads. If the threads do not engage smoothly, they are cross-threaded. Do not force the head if you encounter resistance, but back it off, square up the instrument, and try again gently to engage the threads. When they engage, screw the head nut up firmly but not too tightly. Setting up the instrument too tightly causes eventual wearing of the threads, making unthreading difficult.

After you have attached the instrument, if you are set up on stable soil, thrust the tripod legs tips into the ground far enough to be sure of a stable support, taking care to keep the footplate approximately level. Some tripods have legs equipped with short metal stirrups. These stirrups

If you are set upon a hardened surface, such as concrete, make sure the tripod legs do not accidentally spread, causing the tripod to collapse. In figure 14-6, view A, the legs tips are inserted in cracks in a concrete pavement. In figure 14-6, view B, they are held by an equilateral wooden triangle called a floor triangle.