As Bruno points out, >SGP4/SDP4 use the equinox of date which for the next >few years is close enough to J2000. RA/Dec can, in the absence of contrary information, reasonably be presumed to refer to the 2000 epoch, despite the fact that there are still some star atlases and other datasets which refer to 1950. Times, unless stated otherwise, can reasonably be presumed to be UTC, despite the fact that there are several other time reference frames -- such as Terrestrial Time, GPS time, etc. For the IAU/Cospar convention of four decimals of degrees, one might presume that there is a common datum for all listed co-ordinates, but that would be wrong. The level of precision which is shown by many astronomers, amateurs as well as those who ought to know better, is hugely finer than the disparity between the numerous spheroids and geodetic datums. Although there may be one or two people who would prefer to have us dumb down positional accuracy and render geodetic positions meaningless by removing their underlying basis, no good can be served by reducing rather than improving the quality of people's observations and their stated positions. Several people have written asking about the most common datum in the (lower 48) States and wondering what the most likely spheroid and datum is when none is specifically mentioned. There are two commonly used systems in the contiguous States of the US. NAD83 is functionally identical to WGS84 for most practical purposes. It is now the datum which is preferred by the USGS and is the one which you see used on such online services as terraserver.com and one or two others. The other system in the lower 48 is its predecessor which predates satellite positioning by several decades. That is NAD27. It is based upon a quite different spheroid and uses the Meade's Ranch datum. For astronomical purposes, it is quite sufficient to simply label NAD27 co-ords as such. NAD27 is neither more nor less "correct" than NAD83, merely different. Last week Japan formally abandoned its own national mapping datum in favour of WGS84 and promulgated that fact widely among its user base. The difference is about 400 metres. For many years to come, there will still be plenty of "legacy" datasets which are based on the "old" system, just as there are in Hawaii, for example. That is not a problem because the responsible course of action is to label any geodetic co-ordinates with their geodetic basis to make them useable. If visual observations are made to a precision of of an arc-second of Declination and someone wants to match that observation to another made elsewhere on another datum, then a conversion to a common geodetic framework can easily be computed to get the best out of any observational accuracy. Such commonality is only possible if the geodetic basis of a position is explicitly stated. Those who are familiar with the SeeSat/VSO FAQ will be aware that the custom is to measure time with reference to UTC. It is also requested that the "observer's exact location" should be measured by means of "geodetic maps". A geodetic map which omits the geodetic basis of its plotted data is not a geodetic map at all. In order for such co-ordinates to make any sense, they must refer to a recognisable spheroid and geodetic datum. Otherwise the apparent precision, which may or may not be commensurate with its actual accuracy, is rendered effectively meaningless. For all practical purposes, NAD83 may be regarded as synonymous with WGS84, as may Euref89, Sweref99 and all of the ITRFs of all epochs. Other datums should not be presumed to be similar to eachother or to WGS84. Differences of 400 metres or more are not at all uncommon and any such differences might become additive if two or more different observation locations are used in a single computation. The efforts of those who strive for single arc-second (30 metres) precision in their reported observations are blown away by an immediate error of a couple of stations each having 400 metres disparity, summing to a systematic error which is something like half a mile. Most observers who take the time and trouble to measure their observation locations to a precision of a tenth of a mile or better do so because they care about standards and about doing the best they can to meet the IAU/Cospar conventional standards. It seems a pity to make a mockery of their efforts by omitting the fundamental basis of those measurements and leaving the reader to flounder in guesswork when trying to make sense of a declared position which has an apparent precision of 11 metres or less. I, for one, am glad that the IAU and Cospar standards are not being eroded by those who would try to dumb down accuracy and precision and drag us all down to the level of the lowest common denominator. I applaud those who take the time and trouble to make sense of otherwise insensible terrestrial co-ordinates by the simple expedient of openly declaring the basis of stated Lat/Longs. They have nothing to hide and nothing to gain from hiding the basis of their published co-ordinates. Cheers, Chris Olsson All generalisations are false ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www2.satellite.eu.org/sat/seesat/seesatindex.html
This archive was generated by hypermail 2b29 : Thu Sep 20 2001 - 17:55:52 EDT