Re: Geodetic precision

From: Michael McCants (
Date: Tue Aug 21 2001 - 09:16:15 PDT

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    Chris Olsson wrote:
    >Leaving aside the semantic issue of whether the differences between national
    >datums constitutes an "error", I think it is worth noting that such differences
    >tend to be more than 50 metres rather than less.  WGS84 itself is more than a
    >hundred metres of Longitude from the Prime Meridian at Greenwich and many
    >national datums are also more than 50 metres either from the old Prime Meridian
    >system or from WGS84.
    Irrelevant to visual satellite observing.
    >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.  You have distorted my words.  I said that I was
    happy if I could derive an elset for a classified object based on visual
    observations that was accurate to 500 meters.  Since you have never tried
    to do anything like that, it would seem that my sentence didn't make any
    sense to you.
    >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.
    They are wasting our time many times - if we are dumb enough to actually
    print out such predictions.
    >Will you tell, them?
    Why bother?  They don't really care.  Most of them do not make visual
    satellite observations.  None of them make "positional observations".
    None of them use positional observations to derive element sets for
    classified objects.  They do not have any experience in this field.
    And someone who advocates 5 decimal places in lat/long positions
    obviously has no experience in this field either.
    >Are you suggesting that SpaceCom somehow deliberately degrades the orbital
    Your "straw men" arguments are silly and way, way "off topic".
    This SeeSat-L mailing list is for visual satellite observations.
    Your posts on "geodetic precision" are no longer interesting and
    are "off topic".
    >There are even greater differences elsewhere in the world, if you care to look
    >for them.  That is why it is important to state the geodetic basis of
    >apparently precise co-ordinates.
    When you have actual observers in those parts of the world and they
    submit accurate observations that appear to have unexplained systematic
    residuals greater than 500 meters, then I will be interested.  I do
    not hope to live that long considering how your posts have raised
    my blood pressure the last few days.
    >You could, I suppose, replace the ellipse of rotation with a simple sphere, but
    Your "straw man" arguments are really, really silly.
    >All I have suggested is that we should have some common Lingua Franca when
    >expressing that position.
    I am expressing my opinion that you have no experience in what a very
    few of us are doing.  My opinion is that your suggestion is completely
    >Yes, there are those who will exhort us to dumb down our co-ords to two places
    Your "straw man" arguments are really, really silly.
    >I suggest that those who care about precision; and those who understand the
    >difference between accuracy and precision: give a thought to what those
    >concepts actually mean and present suitable labels upon their stated co-ords. 
    I suggest that they not concern themselves with the subject.
    >Since I have broached the subject on the List. I have noticed that several
    >List-members have admirably done so
    I have convinced one that it was silly and he has now removed "WGS84" from
    his signature.
    >and I have also received a remarkable
    >number of non-hostile private communications from others who are also
    >interested in making good use of the data which they have at their disposal.
    I am not one of those and 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.
    >I think it was Albert Einstein who once said that:
    >The Truth ought to be expressed as simply as possible, but never more so.
    Sophocles: "The truth is always the strongest argument".
    We have been making obs and generating elsets.  You showed up a week
    ago for the first time.
    Eistein: "As far as the laws of mathematics refer to reality,
              they are not certain, and as far as they
              are certain, they do not refer to reality."
    SGP4 may be mathematically precise, but the reality of human visual
    satellite observation means 5 significant figures is quite enough.
    Mike McCants
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