Geodetic Data Collection Techniques - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 26

The measurement of vertical angles requires little for observation and accuracy.
What needs to be done is to observe the target using the crosshairs of the telescope
during the direct-and-reverse modes of readings, which constitutes a complete set
of observations. The vertical angle can be computed accordingly.

2.1.2 Distance Measurement

For hundreds of years, graduated tapes (measuring ropes, tape measures, and steel
tapes) have been used to measure distance by means of direct comparison. The
major flaw of such a method, however, is that it is easily subjected to the influence
of topographic conditions along the survey lines. To obtain distance measurement
with higher accuracy, one has to invest large amounts of human and material
resources to choose and arrange the survey routes, which can be complicated and
costly. Moreover, such distance measurement cannot be carried out if there are
rivers, lakes, hillocks, or ravines along the survey lines.
With the progress of science and technology, in the 1940s a new type of distance
measuring instrument came into being—the optical-electro distance measuring
instrument, which was also the earliest type of electromagnetic distance measuring
(EDM) device. Later, microwave, laser, and infrared EDM instruments emerged
one after another. Even today, the total station electronic tacheometer that inte-
grates angle and distance measurements is still available. It has created a new era of
using EDM to replace the direct comparison method using graduated tapes and the
indirect method using the optical tacheometer.

Principles of Electromagnetic Distance Measuring

In Fig. 2.5 an electromagnetic wave transmitted from a rangefinder placed at point
A travels to the reflector at point B and back to point A, received by the rangefinder.
Thus, the round-trip travel time t 2D of the electromagnetic wave between points
A and B can be measured by the rangefinder. The distance (D) can be calculated
according to the following formula:

1
2 Vt 2D ,

D ¼ ð 2 : 3 Þ

where V is the velocity of propagation of the electromagnetic wave in the atmo-
sphere, c is the velocity of the electromagnetic wave in vacuum, and n is the index
of refraction (refractive index) for the electromagnetic wave:

c
n :

V ¼ ð2:4Þ