Establishment of Geodetic Coordinate Systems - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 349

Fig. 7.6 Polar coordinate
system
(IPMS), and the Bureau International De L'Heure (BIH). Since the 1970s, with the
births of new technologies like Satellite Laser Ranging (SLR), Lunar Laser Rang-
ing (LLR), Very Long Baseline Interferometry (VLBI), and the Global Positioning
System (GPS), the traditional method of determining polar motion, optical astrom-
etry, has been replaced by the state-of-the-art method of space geodesy. Hence,
international organizations have decided to adopt the technologically advanced
International Earth Rotation and Reference Systems Service (IERS) instead of
organizations like IPMS and BIH.
For several decades, various polar systems have been created with gradually
more in-depth studies on polar motion. At the 32nd symposium of the IAU in
collaboration with IUGG, held in Stresa, Italy, 1968, the 1900-1905 mean latitude
of the five International Latitude System stations was recommended to be the
reference to define the position of the mean pole. This position of the mean pole
relative to the 1900-1905 mean pole of the epoch (1903.0) is called the Conven-
tional International Origin (CIO). ILS, IPMS, BIH, and other such organizations
have successively used different optical instruments and mathematical processing
approaches in an attempt to maintain this polar origin, so there are different CIO
systems. The BIH systems to which CIO is subordinated include BIH 1968.0 ,
BIH 1979.0 , and BIH 1984.0 , etc.
At present, the CIO system is maintained by the IERS. It provides users with data
on the instantaneous pole through solution and regular publications based on the
data obtained from the global observation stations; Fig. 7.7 illustrates the motion
path of the instantaneous pole relative to the CIO from 1 July 2010 to 1 July 2013.
Unlike the geodetic latitude, where the equator is a natural origin, there is no
natural starting point for the geodetic longitude. The standard reference meridian
adopted to determine the geodetic longitude and universal time is called the initial
meridian. The origin of longitude is defined by where the initial meridian and the
equator intersect.
In 1884, astronomical delegates met at the international conference held in
Washington to decide the meridian (line of longitude) that runs through the
Observatory at Greenwich in southeast London, England as the initial meridian.