Geodetic Data Collection Techniques - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 29

radio waves rangefinder
(microwave distance
measuring instrument)

geodimeter (light source:
incandescent lamp or
mercury lamp)

EDM instrument

visible light
distance
measuring
instrument

laser distance measuring
instrument (light source:
laser)
laser infrared EDM
instrument

light wave
rangefinder
(geodimeter)

infrared
EDM
instrument

light infrared EDM
instrument

2.1.3 Astronomical Measurement

Astronomical observation is a technique utilized to determine the position of a point
on the Earth's surface (astronomical longitude and astronomical latitude) and the
astronomical azimuth by observing celestial bodies (especially stars). Astronomical
observation is an ancient technology dating back to the era when human culture first
took shape. To figure out direction and determine time and season, the sundial and
gnomon were invented successively and Polaris was used for determining North.

Definition of Astronomical Coordinate System

In astronomical observation, the astronomical longitude, latitude, and azimuth of a
surface point obtained with reference to its plumb line and the geoid are the
representations of the surface point in the astronomical coordinate system.
As shown in Fig. 2.6 , NS is the Earth's axis; the two points N and S, where the
Earth's axis intersects the Earth, are the North Pole and South Pole, respectively;
O is the geocenter; the plane OWG 0 P 0 E through the geocenter perpendicular to the
spin axis is the Earth's equatorial plane; P is a point on the Earth's surface; PK 0 is
the plumb line direction of point P; the plane that contains the plumb line of point
P is called the vertical plane of point P, in which the vertical plane N 0 PP 0 S 0 K 0
parallel to the Earth's axis is called the astronomical meridian plane; NGG 0 S is the
initial astronomical meridian plane, also referred to as the astronomical meridian
plane (see e.g., Xia and Huang 1995).

Astronomical Longitude

The astronomical longitude of a point P on the Earth's surface is the angle between
the initial astronomical meridian plane and the local astronomical meridian plane of