The following material is copyright 1998 by Peter H. Dana.
From: Peter H. Dana <pdana at mail dot utexas dot edu>
To: Lloyd Wood
Subject: RE: idle geodesy note
Date: Fri, 23 Oct 1998 14:46:22 -0500
Thank you for your interest in the Geographer's Craft GPS Overview.
Those WWW pieces [that L.Wood referenced, including the previous issue of TBTF] are a bit naive I'm afraid. There are many dozens of defined poles with respect to different terrestrial reference systems. Most terrestrial reference frames either change constantly or are specified with respect to a moment in time. Poles are defined for reference systems used in geography, geodesy, astronomy, geomagnetics, and navigation (See National Oceanic and Atmospheric Administration. 1986. Geodetic Glossary. Rockville, MD: National Geodetic Information Center). WGS-84, the system on which GPS is based, has a three dimensional coordinate system that uses a Z-Axis defined as the direction of the IERS Reference Pole (IRP). This is the International Earth Rotation Service pole. This Z-Axis corresponds to the direction of the BIH Conventional Terrestrial Pole (CTP) epoch 1984.0 with an uncertainty of 0.005 arc seconds. This pole is the Bureau International de l' Heure pole for a specific moment in time. (See National Imagery and Mapping Agency. 1997. Department of Defense World Geodetic System 1984: Its Definition and Relationship with Local Geodetic Systems. NIMA TR8350.2 Third Edition. 4 July 1997. Bethesda, MD: National Imagery and Mapping Agency. Available on line from National Imagery and Mapping Agency (1145K PDF).
There is the Mean Conventional International Origin (CIO) Pole of 1903.0 (see Torge, Wolfgang. 1991. Geodesy, 2nd Edition, New York: deGruyter), and many more. Every one of the 20 or so commonly used reference ellipsoids (of hundreds) defines a center of the earth and a polar radius (semi-minor-axis) that puts the location of the Poles along the Z-Axis at a different place in space. These polar radii (and therefore the defined position of the Poles) range over many kilometers. The physical meaning of the Poles changes with difference contexts. Just the topographic surface of the earth at the north or south Poles can change with snow and ice cover, redefining the position of any Pole defined with respect to the surface of the earth. Polar motion, defined in those WWW sources as Chandler motion is also constantly affected by tidal forces and polar wander. It is amusing to note that the BIH CTP is not within the region defined by the extent of polar motion over the last few years; that is that Pole is not where the rotation axis of the earth now points (see Leick, Alfred. 1995. GPS Satellite Surveying. 2nd. ed. New York: John Wiley & Sons. page 25). So while it is amusing to talk of three or five or ten Poles, it kind of demeans the science of geodesy (and I'm not part of that group of folks so I'm not just protesting too much) to think that there is any specific "number of Poles." And certainly we would not want to get our concepts of geodesy from the Web or the Boston Globe.
Peter H. Dana - Department of Geography - University of Texas at Austin Austin, Texas 78712-1098