Re: Geodetic precision

From: Chris Olsson (olsson@globalnet.co.uk)
Date: Tue Aug 21 2001 - 17:55:23 PDT

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    Mike McCants misquotes what I actually said, and replied:
    >>As for the error range of the TLEs being 500 metres, I'm surprised if it is
    >>true that precise elsets are no better than half a kilometer.
    
    >I did not say that. <
    
    
    
    Mike, I never ever said that you had said any such thing.
    
    If you read what I actually wrote, you will see that I was making an attributed
    quote of something which was said by Jonathan Wojack.  It was he, not you, who
    had wrongly declared that geodetic datum differences amount to no more than 50
    metres, and quoted the figure of 500 metres for elsets.  I mis-typed his
    surname, for which I apologise, but even the most hypertensive of imaginings
    could not reasonably construe my quote of his words, as in any way implying
    that you were the author of them, Mike.  I'm astonished by the hostility of
    your tone and by the difference between your attribution of my words and what I
    actually said.
    
    
    >>Perhaps Dr TS
    >>Kelso and others are wasting their time when they produce excellent
    >>computational software which outputs to an arc-second of declination and to a
    >>metre of orbital height?
    
    >They wasted their time "once" - when they created their programs.<
    
    
    That is a matter for you to take up with Dr TS Kelso et al, not me, Mike. 
    Please
    address your poison-pen emails elsewhere, not at me.  TS Kelso makes a
    well-argued case for benchmarking of observational predictions.
    http://celestrak.com/columns/v03n02/
    Res ipso loquitor
    
    Please direct your bilious comments about the value of his work in bringing
    elsets and resultant computations into the public domain to him, not to me. He
    expresses his opinion in a well-argued manner, without bile or venom, Mike.
    Your rancour is in stark contrast to the style of Messrs Kelso, Ransom, and
    Traufler.  They do their best to raise standards, and they do so elegantly and
    without personal rancour.  If you wish to sneer at their work, please do not do
    so in a missive which in any way connects my name with your opinion.  If you
    wish to confront them with your negativity, please do so in a manner which
    excludes me.
    
    It appears to me that those three gentlemen, inter alia, are doing their best
    to raise and maintain quantitative standards, rather than dumb down astrometric
    and geometric quality of observations and of resultant computations.  If you
    wish to denigrate their efforts, then please do not involve me in your
    campaign.
    
    
    >someone who advocates 5 decimal places in lat/long positions
    >obviously has no experience in this field either.
    
    Who, exactly, has advocated such precision? Certainly not I.
    
    Read what I have actually written on the subject:
    
    "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."
    
    Note that I do not in any way "advocate" such precision.  Indeed, I point out
    its meaninglessness.
    
    
    >I have convinced one [SeeSat-L member] that it was silly [to include the basis of a stated position] and he >has now removed "WGS84" from his signature.
    
    That, I suppose, is some kind of victory for those who would dumb down spatial
    data.   Removing the label of a measurement has the 'advantage' of placing the
    expressed numerical data into the domain of the "need to know" classification. 
    That is one approach to Science, but it is one which has many opponents.  It
    has little to commend it.  You can (and have done, many times, in the past!) do
    better than that, Mike.
    
    
    >I request that anyone sending you a non-hostile communication send me a copy so I can explain to them how the >real world really works.
    
    A programme of hostility is easy for you to perpetuate, Mike.  I hope that you
    will reconsider.  By relaxing a little and lightening up, you will not only
    ease your psychosomatic hypertension, but also allow yourself to feel at ease
    with the concept of benign accuracy.
    
    I have merely suggested that those who list a precise geographical position
    also mention the geodetic basis of that position.  I'm astonished by the
    aggressive hostility shown by some intemperate remarks which that innocuous
    suggestion has aroused.  
    
    If I asked any scientist to mention what scale a stated temperature referred
    to, most reasonable people would understand that 123.45, for example,  needs
    to have a datum to have any meaning.
    
    A response to such a reasonable request might be: "Degrees Kelvin"; or perhaps
    "Celsius"; or perhaps "Fahrenheit".
    
    An unreasonable response would be: "Hey, its a hundred degrees. Shutup".
    
    
    >5 significant figures is quite enough.<
    
    Yes, but only if we are using a common (or stated and mathematically
    inter-convertible) datum for that measurement.  Mentioning what that datum is
    will not undermine the measurement, only strengthen it, Mike.
    
    
    Dumbing down to a lower precision is a lousy alternative to stating the datum
    of a measurement.
    
    
    
    Cheers,       Chris Olsson
    
    
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