Geoscience Reference
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Fig. 2.4 Method of angle
measurement in all
combinations
all combinations, for example, if four directions need to be observed at the station,
six single angles can be formed: (1.2), (1.3), (1.4), (2.3), (2.4) and (3.4) (cf. Fig. 2.4 ).
If the number of directions at the station is n, then the number of angles in all
combinations is given by:
1
2 nn
K
¼
ð
1
Þ:
ð
2
:
2
Þ
For each set of observations, only a single angle is observed and the observation
set for each combination angle is the same. The characteristic for such observation
is that the alidade is rotated in the same direction for both direct-mode and reverse-
mode reading. This is intended to better eliminate the errors due to backlash as the
alidade rotates. However, for the entire observation and each set of observations for
every single angle, the alidade should be rotated clockwise in a half set and
counterclockwise in the other half so as to reduce other errors better. During each
time period of observation this is achieved by either changing the rotational
direction of the alidade when half of the set is completed or changing the rotational
direction of the alidade in between the sets of observation alternately.
The above covers the direction method and angle measurement in all combina-
tions. The key advantage of the direction method lies in its simple observation
procedures and in the fact that less effort is required for operation. However, when
there are multiple angles at the station, it will be hard to obtain clear target images in
all directions. On top of that, the time consumed in a full set of observations is longer,
which may make it more likely to be affected by external conditions, so difficulties
may arise in achieving results with high accuracy. These are the disadvantages of the
direction method. The method of angle measurement in all combinations has certain
advantages, i.e., each single angle can be observed with a flexibly selected clear target
for each set of observations, the time for observation is quite short, and the impact of
external conditions on the observations is relatively small. However, the procedures
for observation are relatively complicated; the combinations of single angles increase
rapidly as the directions at the station increase and thus measurements require much
more effort to perform. These are the drawbacks of such a method. Generally, the
direction method is more applicable to angle observation with lower accuracy,
whereas the method of angle measurement in all combinations is quite suitable for
angle observation with higher accuracy.
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