Spectra Precision Survey Pro v5.4 User Manual

Page 80

1. The instrument is set up somewhere close to the maximum allowable sight

distance (typically 30 meters) away from the opening CBM. A backsight
observation is taken to the opening CBM.

2. A foresight observation is made to the first turning point. This turning point

should be chosen in such a way that the instrument is located midway
between the opening CBM and that turning point.

Steps 1 and 2 are called the first turn, and are indicated as BS and FS in
the above diagram.

NOTE: A usually observed tolerance is that the foresight and backsight distances for
each turn should not be different from each other by more than ± 5 meters, or some
similar value. Refer to Level Settings on page 35 to set this tolerance.

3. The instrument is set up midway between the first turning point (TP1) and

the second turning point (TP2). A backsight observation is made to TP1
and a foresight observation is made to TP2. These observations constitute
the second turn.

4. The instrument is set up midway between the second turning point (TP2)

and the first point of interest (BM1). A backsight observation is made to
TP2 and a foresight observation is made to BM1. These observations
constitute the third level turn.

5. Make the same types of observations for the next level turns (TP3-BM1,

TP4-TP3, BM2-TP4, TP5-BM2, TP6-TP5, BM3-TP6, TP7-BM3).

6. In the last level turn, a backsight observation is made to TP7 and a

foresight observation is made to the closing CBM, which here is the same
point as the opening CBM (it could be a different one as long as it has
accurate elevation).

There may be variants to this simple level loop. You can for example shoot
sideshot points or stakeout points as you go along the level loop. This is
explained in Collecting Elevations for Sideshot and Stakeout Points on
page 73.