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4. The horizontal axis must be perpendicular to the vertical circle and pass through
its center. When the vertical axis is perpendicular and the horizontal axis is
horizontal, the vertical circle is parallel to the vertical plane of sighting passing
through the point of observation and the angle measured in such cases will be the
true vertical angle.
5. Once the vertical circle index bubble is centered, the index for reading the vertical
circle must be horizontal or vertical. Thus, the angle between the index of the
reading scale and the collimation axis of the telescope will be the vertical angle.
The major parts of a theodolite are structured according to the relationships
mentioned above. It is generally required that the relationship between three axes
(the vertical and horizontal axes and the collimation axis) and between two circles
(horizontal and vertical circles) be free of errors, which is crucially important.
Unlike the optical theodolite, the electronic theodolite provides a visual digital
display of the circle readings instead of having to view through a reading eyepiece.
It is therefore also called an electronic digital theodolite. The electronic theodolite
is composed of optical mechanical devices, electronic sensor, microprocessor, etc.
The configuration of its shafting, telescope, and clamp (tangent) screw are identical
to that of the optical theodolite. The difference is that the electronic sensor is used to
replace the index of the reading scale in the ordinary theodolite. Following the
method of analog-to-digital conversion, it first receives electrical signals from the
circle and then converts these electrical signals into angles and displays them on the
monitor. Generally, there are kinematic and static angle measurements according to
the rotation of the circle, and there are circular encoders and circular raster scales
for angle measurements according to the different ways of circle graduating.
Detailed principles are not discussed further here.
Methods for Observing Horizontal Angles
The Direction Method and the Closing the Horizon Method
For each set of observations, all directions at the station should be observed to get
the angles. As shown in Fig.
2.3
, the directions that need to be observed at station
O are OA, OB,
and ON. Let OA be the starting direction (also referred to as zero
direction). First, point A is sighted facing left and the reading is recorded. Then the
alidade is rotated clockwise and the points observed in order from A to N and the
readings noted, as one half of the full set of observations. Then the telescope is
turned to face right, the alidade rotated anticlockwise, and the observations repeated
facing right. The horizontal angles are then recorded again in reverse order from
N to A as the other half of the full set. At this stage, one set of angles has been
completed. Such a method is known as the direction method.
While using the direction method, before completing the direct-mode or reverse-
mode readings, zero direction A will be measured again (called back-to-zero or
being zeroed). Given the fact that each half of the whole set will combine and close
up to the starting direction as a full circle, it is referred to as closing the horizon
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