Page 54 - Moving the Earth_ The Workbook of Excavation

SURVEYS AND MEASUREMENTS

2.4 THE WORK

FIGURE 2.4 Parts of level transit. (Courtesy of David White, LLC.)

Surveys are generally based on the true north, from which the compass north varies rather
widely. Part of this variation may be obtained approximately from the map, Fig. 2.5, or exactly
from local sources.
If you are in an area of west magnetic declination, the compass needle will point west of the
true north by the amount shown on the map.
Another source of error is the magnetic attraction of magnets, iron, and iron ore for the com-
pass needle. It is also affected by the time of day. No confidence should be placed in a compass
reading taken near machinery or electrical apparatus. Metal objects in the observer’s pockets may
cause errors.

Setting Up. The first step in using the instrument is to set up the tripod. The top should be as level
as possible, and the legs pushed into the ground firmly. On a slope, two legs should be downhill.
The protecting cap is removed and the instrument screwed on. The telescope frame should be
unlocked so that it is free to rotate. The telescope can then be held in one hand and the base
screwed on the tripod with the other.

Leveling. The instrument must now be leveled by means of the four screws. The telescope is turned
so that it is over two of them, and those screws are adjusted until the bubble in the level is exactly
in the center of the scale. The screws are turned at the same time in opposite directions, so that
one pushes the leveling head up while the other makes space for it to come down, as it pivots on
its center pin.
The bubble moves in the same direction as the left thumb, as indicated in Fig. 2.6A. If the two
screws are turned exactly the same amount, the tension on them will remain constant. If the screw
toward which the bubble is moving is turned farther, it will jam both screws. If the screw behind the
bubble is turned the most, the tension will be reduced and the screws may lose contact with
the turntable.