In a brief conversation I had with Chris Peat of Heavens Above fame a while back Chris wrote: > Hi Ed, > Thanks for the message. We use WGS-84 as the reference geoid. I will add a > statement to the FAQ. > > Chris > > > -----Original Message----- > > From: Ed Davies [mailto:edavies@nildram.co.uk] > > Sent: Sunday, November 05, 2000 12:33 > > To: Chris.Peat@heavens-above.com > > Subject: Geodetic datum for positions > > > > > > Many thanks for your interesting and impressive site. > > > > For most satellite observations the geodetic datum for the coordinates > > used is not important as the accuracy is not required. However, for > > Iridium flares I notice that you give relative distances which can be > > of the order of 100 metres at which point the geodetic datum is just > > about on the edge of being important. > > > > I guess that WGS-84 is used but wonder if perhaps some simple spherical > > Earth model might be used. > > > > Perhaps a note on your FAQ list under the "setting up your observer > > location" section would be helpful, particularly to people who have > > GPS receivers. > > > > Regards, > > > > Ed Davies. > > My own experience of dealing with geodetic datums for aviation applications is that the differences between lat/longs relative to different datums can be of the order of a few hundred metres. The largest I've heard of was, I seem to remember, a 600m discrepency on a South Pacific island which caused an instrument approach procedure to be designed much too close to a mountain. Luckily, in this case the procedure was flight checked in visual conditions by the operator prior to use in instrument conditions and the problem was picked up. I think that for most SeeSat applications a few hundred metres doesn't really matter so it is not too critical to state the datum. On the other hand, it is also looks a bit daft to specify coordinates with a precision of better than a few hundred metres without stating the datum. I'd strongly urge the use of WGS-84 whereever high precision is required. Apart from it being the basis of GPS and pretty much the world standard it also has the advantage that the least badly defined transformations between datums exist with respect to it. Most transformations between other datums go via WGS-84 anyway. For example, Ordnance Survey (OS) grids used in Great Britain are now based on the WGS-84 ellipsoid (well, strictly speaking, the ITRF - International Terrestrial Reference Frame - which forms the approximate basis of WGS-84) via a long chain of transformations which try to take out the effects of continental drift, the differences between the original Airy Ellipsoid used in the British Isles and WGS-84, and the survey errors made in the original triangulations of Britain. If you only want to read one thing about geodesy I'd like to recommend the PDF file which used to be linked at: http://www.o-s.co.uk/services/gps-co/geo6.htm ...but now the OS seems to have "dumbed down" their web site and I can no longer find it there, but a Google search found it at: http://www.badc.rl.ac.uk/coordinates/ordnance_survey/OSGB.pdf The title is "A guide to coordinate systems in Great Britain" but don't be put off - there's lots of general stuff worth reading and much of the GB specific stuff is probably analogous to what is happening or will happen in other countries. Ed. ----------------------------------------------------------------- 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 : Fri Aug 17 2001 - 09:13:52 PDT