Civil Engineering Reference
In-Depth Information
Setting up on soft ground is sometimes unavoidable. It may be necessary to
traverse through a swamp for example. In such cases it is often possible to find
small islands, or raised areas, formed by tree roots and vegetation upon which
the instrument may be set. These areas may still be soft but they may be the best
choice available. If the raised area is next to a living tree and it is windy, there is
a good chance that the movement of the tree in the wind will be enough to affect
the angle. The author recalls such a setup. When looking through the instrument
telescope the crosshairs could be seen moving from one side of the target to the
other in concert with the tree swaying in the wind. Although the upper branches of
the tree could be seen waving in the wind, there was no perceptible movement of
the large trunk. Nevertheless, the movement was enough to make it impossible to
accurately measure the angle.
When set up on soft ground, the instrument will move when the instrument
person walks around the tripod. If the angle to be measured is close to 180° the
instrument person must walk around the tripod in order to sight the target. If the
ground is soft, the change in pressure on the ground from the instrument person's
body weight will move the instrument. Sometimes the angle is small enough that
the instrument person can stand in an intermediate position where both targets can
be sighted without moving. In some cases, merely shifting body weight from one
leg to another will be enough to cause errors in the angle. Where the angle is near
180°, one solution is to have two people measure the angle. Each person takes a
position on opposite sides of the tripod. One person can then sight the backsight.
The second person will then sight the foresight. Each person must be careful not to
move while the angle is being measured.
When set up on soft ground, the instrument will move when the instrument
person walks around the tripod.
Even when the ground is relatively firm, some general rules apply to working with
an instrument. The instrument person should never place any weight on the tripod.
When sighting angles, the tripod should not be grasped with one hand in order to
steady the body. When it is necessary to walk around the tripod in order to turn an
angle, it is best to walk in a large circle around the instrument rather than stepping
in the vicinity of the tripod legs. When measuring angles, keep feet away from tri-
pod legs and try to place them between the legs. In fact, never step on the ground
next to a tripod leg. Everyone on the survey crew, except for the person running
the instrument, should stay a reasonable distance away from the instrument. There
is rarely a need for a crew member, other than the instrument person to be standing
near the instrument.
One example where there is an understandable tendency to touch the tripod is
when the ground is soft. The instrument person may try to stand a little further
away from the instrument in order to avoid disturbing the ground. She then must
