Basic Technical Concepts - Practical Boundary Surveying - page 24

Sometimes on a very steep hill it is not possible to hold the tape level. Even if
the uphill person holds the end of the tape on the ground the hill may be so steep
that the downhill person may not be tall enough to raise the far end of the tape to
a level position. In such cases the tape can be held at some convenient angle, per-
haps parallel to the slope of the hill. The angle of the tape from horizontal must
be accurately measured and recorded along with the distance. The angle is called
a Vertical Angle , because it is measured in the vertical plane and the distance is
called a Slope Distance to differentiate it from a horizontal distance. Once the
vertical angle and slope distance are known, the horizontal distance can be accu-
rately calculated. It can be seen from Fig. 4.2 that the horizontal distance is always
shorter than the slope distance.
When measuring up or down a steep hill, if it is not possible or conveni-
ent to measure the vertical angle of the tape, it will be necessary to Break Tape .
Breaking tape means that instead of using the full length of the tape, 100 feet for
example, a shorter distance is measured. In the previous example, 50 feet could be
measured twice, as in Fig. 4.3 . Using a shorter tape length decreases the vertical
distance making it easier to complete the measurement.
Surveyors carry a handheld sight level when taping to ensure that the tape is held
level. In an emergency, if the level were to become lost or broken for example, it is
possible to tell if the tape is level by the measurement itself. For example, when meas-
uring between fixed points, such as two stone bounds, a series of measurements can
be taken with the one end of the tape held at varying heights. When the tape gives the
shortest distance between the fixed points the tape will be level and the distance can
be recorded. When setting out a point just the opposite procedure is used. One end of
the tape is raised and lowered until the distance between the points is the greatest.
Most steel tapes in common use for land surveying in the U.S. are 100 feet in
length. In order to make an accurate measurement the tape must be pulled tight with
a certain amount of tension. The tension reduces the amount of sag in the tape. The
amount of tension depends on the particular tape, but for a 100 foot tape, the tension
Fig. 4.2 Measuring a slope distance with a tape