Geoscience Reference
In-Depth Information
Fig. 2.9 Principle of
trigonometric leveling
h
12
¼
H
2
H
1
¼
S
0
tan
ʱ
12
þ
i
1
a
2
:
ð
2
:
8
Þ
If the measured slope distance is d, the height difference is:
h
12
¼
d sin
ʱ
12
þ
i
1
a
2
,
ð
2
:
9
Þ
which is the basic formula for computing the height difference using trigonometric
leveling. Given the height of point A, that of point B becomes:
H
2
¼
H
1
þ
h
12
:
ð
2
:
10
Þ
EDM Height Traversing
Electromagnetic distance measurement (EDM) height traversing is also called
precise trigonometric leveling. With the development of the electronic tachymeter,
accuracies of angle and distance measurements have been greatly improved. The
accuracy of distance measurement reaches over 1/100,000 and that of angle mea-
surement can amount to 0. 5
00
, which provides favorable conditions for precise
trigonometric leveling. At present, third- and fourth-order leveling can be
completely replaced by EDM height traversing and, accordingly, in China specifi-
cations have been made by the departments concerned. Replacing leveling with
EDM height traversing has proved to be notably economical in mountainous and
hilly regions.
The methods of height traversing include reciprocal, leap-frog, and unidirec-
tional. For the reciprocal method, the instrument is set up at each station to conduct
reciprocal trigonometric leveling. The leap-frog method involves setting up an
instrument midway between two targets. The targets remain at a particular change
point. Observations are carried out in a pointwise manner. The targets should be
set alternately and an even number of setups is used. This method is similar to
leveling, except for using an oblique instead of a horizontal line of sight. The
unidirectional method is based on the first and second methods, which is to observe
twice with different heights of instrument at one station or to observe twice the two
targets at each station. Tailor-made sighting vanes are used as the targets for all
three methods described above.
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