Re: Geodetic precision

From: Ed Davies (edavies@nildram.co.uk)
Date: Fri Aug 17 2001 - 09:11:23 PDT

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    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.
    
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