As a newcomer to the List, from the background of being a Geodesist, I've been wondering about the apparent precision of some List members' declared co-ordinates and to what geodetic framework they refer. Let's take three specific positions, in SouthEastern London, England, as a working example to illustrate what I'm trying to say: Position #1: 51° 28' 40.125" N 0° 00' 05.310" W Height 92.603m Position #2: 51° 28' 38.265" N 0° 00' 00.418" E Height 47.155m Position #3: 51° 28' 40.028" N 0° 00' 05.864" E Height 89.563m At first glance, it might appear that the three positions each describe three different places, with Pos#1 being 124.6m from Posn#2. The two places do not even appear to be in the same hemisphere! In fact, all three positions actually describe a single point in space, but only if I mention which spheroid and datum they refer to. Position #1 is expressed with reference to WGS84. Position #2 is expressed with reference to the Airy spheroid and Britain's national mapping datum OSGB36 in the horizontal plane and the height refers to Britain's Ordnance Survey datum which is actually referred to the mean sea level in 1921 at a small fishing town in the South West of England called Newlyn. Position #3 is with respect to WGS72. Note that the actual physical location of these three positions is in fact, by definition, exactly located on the 0° Prime Meridian at Greenwich. All three sets of co-ordinates describe the location of Airy's Transit, which was adopted by International Convention in 1884 to be the zero degree meridian for astronomical and navigational purposes. Without saying which horizontal and vertical datum(s) a particular reported position refers to, the stated position is quite useless for any but the most vague description of geographical location. Might I respectfully suggest to members of SeeSat-L that whenever they list their observational location co-ordinates, they also mention to which geodetic datum those co-ordinates refer? It may be impressive to list position to an apparent precision of five decimals of a degree or to a tenth of an arc-second of Lat/Long, but unless the geodetic datum to which those co-ords are referred is associated with such apparent precision, then any accuracy which might be associated with such co-ords is quite wasted. The same applies to vertical datums. Spheroidal height and ellipsoidal height are the same thing, but they are quite different to Geoidal height and are not necessarily co-incident with "sea level". In any case, it is essential to indicate exactly what a vertical "height" refers to. That might be "sea level" or it might be an ellipsoidal height. Some GPS units, for example, output height in one form, others in another. There are several very high quality software packages out there which appear to be able to list the orbital height of any satellite (given good enough elsets) to an apparent precision of a metre. Dr TS Kelso's TrakStar, for example, uses WGS72 rather than WGS84 as a geoidal model and geodetic framework, apparently because the TLEs which most users have access to are still based upon that otherwise obsolete reference frame. Of course for most casual observers of satellites, using binoculars and a stopwatch, a positional discrepancy in an observational location of a hundred metres or so is of no practical consequence whatsoever. It is only when high accuracy astrometry is used, such as is obtainable by CCD cameras on good telecopes, that the datum becomes a significant factor. Nevertheless, I would suggest that anyone who posts their geographical position of observation to high levels of precision (i.e. 10 to 20 metres or so) might usefully mention which spheroid and datum those co-ordinates refer to. Cheers, Chris Olsson 57° 02' 30.9" N 3° 10' 25.9" W Ht=314m wrt WGS84! ----------------------------------------------------------------- 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/seesat/seesatindex.html
This archive was generated by hypermail 2b29 : Thu Aug 16 2001 - 11:19:11 PDT