Geodetic Data Collection Techniques - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 24

Fig. 2.3 The direction
method
method. The purpose is for an additional check to see if any variations in the
tribrach of the instrument have appeared during the direct or reverse mode of
observations. The direction method and closing the horizon method are basically
alike, and can be collectively called the direction method. When the number of the
observed directions is equal to or less than three, the time consumed by one set of
angle observations is quite short, and the direction method can be adopted
(non-back-to-zero or not being zeroed); when the number of directions is greater
than three (multiple angles), the closing the horizon method should be employed.
As to closing the horizon, whether the selection of zero direction is appropriate
or not will exert some influence on both the accuracy and time of observation at the
station. Therefore, the zero direction should be the one with appropriate length,
good intervisibility, and clear target image.
Currently, the direction method is primarily used in angle measurement where
lower precision is acceptable. Using the direction method, one obtains the values of
all directions at a station. The value of the chosen zero direction is equal to zero.
The angle between directions is the difference between the two directions.
Method of Angle Measurement in All Combinations (Schreiber's Method
of Observation)
The direction method is simple and requires less effort in observation. However, the
sides of the national higher-level control networks are of greater length, and the
different target images cannot all maintain good quality at the same time. More-
over, the time consumed in one set of observations is rather long. It is therefore hard
to achieve results with significantly greater accuracy. To overcome these deficien-
cies, the method of angle measurement in all combinations can be used. The major
characteristic of this method is that it only measures the angle between two
directions each time. In so doing, it is possible to overcome the difficulty in
maintaining the clarity and stability of various target images simultaneously.
Meanwhile, it also helps shorten the time used in one set of observations and
makes it possible to achieve awesome results with higher accuracy, making it the
preferred method for accurately measuring horizontal angles.
Each time, two directions are selected out of all the directions to be observed at
the station and these are combined to form single angles; this is called the angle in