In fact the situation is further complicated by polar motion. The
earth's axis of rotation differs from its axis of figure (the maximum
moment of inertia). The rotation axis moves slowly around the axis of
figure in a quasi-circular path. The maximum amplitude of the polar
motion is typically about 0.3 arc seconds (about 9 meters on the
surface of the earth). This motion has principal periods of 365 and
428 days. Finally, the motion is affected by unpredictable
geophysical forces and is determined from observations.
I consulted "The Astronomical Almanac, 1999," page B 60 for most of
the above information. "The Explanatory Supplement to the
Astronomical Almanac" contains further information about polar motion,
including a graph of about fifteen years worth of such motion.
The closer one looks at astronomical reference frames, the more
complexity is revealed.
Jim
------------------- ---------------------- --------------------
| Jim Cobb | 540 Arapeen Dr. #100 | [EMAIL PROTECTED] |
| Parametric | Salt Lake City, UT | (801)-588-4632 |
| Technology Corp. | 84108-1202 | Fax (801)-588-4650 |
------------------- ---------------------- --------------------
There is nothing worse than a sharp image of a fuzzy concept.
-- Ansel Adams
> There's a difference between astronomical latitude and longitude and geodetic
> latitude and longitude. Prof. Charles Merry at the University of Cape Town
> should be able to help you out with the specifics of geodetic datums used in
> South Africa: [EMAIL PROTECTED]
> -- Richard Langley
> Professor of Geodesy and Precision Navigation
>
> On Wed, 13 Jan 1999, Anton Reynecke wrote:
>
> >Apologies for this non-sundial question, but I do hope someone can help.
> >
> >I've always been under the impression that Latitude/Longitude is a universal
> >and unambigious method of indicating a position on the earth but now I am
> >not so sure....
> >
> >In South-Africa, the National survey system is based on a Gauss conform
> >system with the Clarke 1880 (Modified) Ellipsoid.
> >
> >It is fundamentaly the same as the wordwide UTM system, with a slightly
> >different scale factor, and the width of a system is only two degrees in
> >longitude, one on either side of a central meridian, whereas UTM covers six
> >degrees.
> >
> >Now the National system is based on the the same projection but we are using
> >the WGS 84 ellipsoid.
> >
> >That caused the Latitude of a fixed point to increase South by 2,04 arc
> >seconds, and Longitude West by 1,06 arc seconds (approximately), and the
> >projected co-ordinates changed by approx. 296 metres South and 27 metres
> >West (Differences calculated by comparing coordinates from the old system
> >with the new system, around Pretoria).
> >
> >I am under the impession that Lat/Long is astronomically fixed from distant
> >objects, with the origin being the rotation axis of the earth, so how can
> >the values be influenced by adopting a new ellipsoid ?
> >
> >What fundamentals am I missing?
> >
> >
> >D. Anton Reynecke
> >
> >
> >
> >
> >
> >
> >
>
>
> =============================================================================
==
> Richard B. Langley E-mail: [EMAIL PROTECTED]
> Geodetic Research Laboratory Web: http://www.unb.ca/GGE/
> Dept. of Geodesy and Geomatics Engineering Phone: +1 506 453-5142
> University of New Brunswick Fax: +1 506 453-4943
> Fredericton, N.B., Canada E3B 5A3
> Fredericton? Where's that? See: http://www.city.fredericton.nb.ca/
> =============================================================================
==
>
