Civil Engineering Reference
In-Depth Information
averaged) angle to the foresight. The instrument is then moved to the foresight and
the previously occupied point becomes the backsight. So when traversing, the data
collector increments the point numbers of the backsight, occupied point and fore-
sight. When taking shots with a data collector, the user is given the choice whether
to take a sideshot or a traverse shot.
5.14 Improving Accuracy When Measuring Angles
Before we discuss methods for measuring angles with a total station or theodolite,
it is important to have an appreciation of just how precise surveying instruments
are, and how little movement it takes to dilute this accuracy. We noted earlier that
one of the great challenges in boundary surveying is making extremely precise
measurements under difficult environmental conditions. The survey crew therefore
needs to be aware of factors which can reduce measurement accuracy.
We already discussed the fact that a 1 second total station is able to resolve
about 1/16 of an inch at a distance of 1,000 feet. With this in mind, it should be
obvious that the slightest disturbance of the total station will cause the crosshairs
to move off the target. When measuring angles it is crucial that the techniques
being used do not reduce the accuracy of the measurements. Experienced instru-
ment persons understand this and they are sensitive to the factors which have the
potential to affect the accuracy of the angles being measured. It is important that
other members of the crew also have this awareness, particularly when they are
working in the vicinity of the instrument.
One of the most important factors which has the potential to affect the accuracy
of an angle is the firmness of the ground upon which the instrument is placed. The
softer the ground is, the more likely the instrument will move. Movement of the
instrument can be caused by the weight of the instrument allowing it to slowly
settle into the ground. In other cases, the ground may be stable and firm enough
to support the instrument without its moving, but the weight of a person walk-
ing around the instrument will cause the ground to move enough so that the angle
accuracy is affected. People weigh several times more than an instrument and tri-
pod so there is great potential for body weight to affect measurements, particularly
when set up on soft ground.
We have discussed the effects of thawing ground and softening pavement due
to heating by the sun as conditions which can cause the tripod legs to settle into
the ground. If the tripod were to move, ever so slightly, between sighting the back-
sight and foresight, the angle being measured will not be correct. Although not
commonly seen these days, some survey crews have used tripod “shoes”. These
were simply thick plywood or boards, 6 or 8 inches square, which had an inden-
tion for the tripod leg points. They could be nailed into the pavement to prevent
them from slipping. The large surface area distributed the weight of the tripod and
instrument over the pavement, reducing surface loading and tripod movement.
