Re: Geodetic precision

From: Jonathan T Wojack (tlj18@juno.com)
Date: Tue Aug 21 2001 - 14:19:10 PDT

  • Next message: Robert Oler: "Re: Progress undocking/docking"

    I'm not sure why I'm replying to this, but I guess it's just my nature. 
    What ever I miss has been gloriously picked up and said in the wondrous
    e-mail by Mike McCants.
    
    > Leaving aside the semantic issue of whether the differences between 
    > national
    > datums constitutes an "error", 
    
    Call it a "discrepancy" if you wish, then.
    
    >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.
    > 
    > Note too that these differences can become cumulative when two or 
    > more datums
    > are involved in measurement of the same celestial object from 
    > different places
    > on Earth.
    
    The exact numbers are irrelevant.  The point is - the discrepancies will
    hurt none of us if they are ignored.  Do you really think that moving 50
    to 100 meters is going to make a noticable impact on a pass prediction? 
    I haven't noticed a difference in making pass predictions for a point
    10,000 meters away from my observing location.
     
    > 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.  
    > 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?  Will you tell, them?
    
    First, do you honestly believe that TLE's can provide that level of
    accuracy?
    
    Second, Why would anyone care about that level of accuracy?  Ignoring
    your altitude above sea level will not affect your predictions, except
    maybe, just maybe, in extreme situations.  What good is it for me to know
    exactly how many meters a satellite is above the Earth's surface, when my
    40m elevation is basically irrelevant?
     
    > Are you suggesting that SpaceCom somehow deliberately degrades the 
    > orbital
    > parameters of satellites such as NavStar and Transit to no better 
    > than 500
    > metres?  If so, it seems a rather pointless degradation as it is so 
    > easily
    > corrected by the user community.  The IGEX programme for example, 
    > tracks
    > Glonass and NavStar (and occasionally a few other) satellites to an 
    > accuracy of
    > well under five metres and cumulatively to sub-metric precision, 
    > albeit in
    > relation to the rather wacky PZ-90 datum.
    
    I'm saying that TLE's are only so accurate.
    
    > >Clock tolerance level:   10 to 30 seconds<
    > 
    > Take a look at the SabObs on this list and you will see observation 
    > times
    > listed to a precision of a hundredth of a second and declinations 
    > listed to an
    > arc-second or even to a hundredth of an arc-minute.  
    
    I rarely do sat-timings.  All I care about is seeing a satellite. 
    Knowing the exact hundreth of second a satellite will come to maximum
    illumination does me little good.  Just so I have the correct time to
    within 10 to 30 seconds or so, and I'm good.  It's normal procedure to
    look a minute or so earlier and later than the predicted pass time,
    anyway.
    
    >Note that there 
    > are many
    > different time datums, such as terrestrial Time, GPS time, GMT, UT, 
    > UT1, and
    > UTC, but by common consent, most people have either adopted UTC or 
    > some
    > specifically declared variant, such as EDT, which is easily 
    > understood by other
    > observers.
    
    So?  What does that have to do with this?
     
    > As for datum differences (or "errors", if you insist) being less 
    > than 50
    > metres:  you are simply wrong.
    
    40 meters, 75 meters; what's the difference?
     
    > The local datum co-ords (AS58 on the International spheroid) of the 
    > USAF Space
    > Command tracking station on Ascension Island are known to great 
    > precision. 
    > That position is 68 metres from the same location when expressed on 
    > WGS84. 
    > That is clearly more than 50 metres, but there are much greater 
    > differences
    > elsewhere.
    
    Ignore them, lose sleep at your own risk.
     
    > For example; the USAF GPS tracking station on Diego Garcia in the 
    > local datum
    > (ISTS072 on the International spheroid) is also known to great 
    > accuracy.  That
    > position is more than 413 metres from the WGS84 figure for the same 
    > location.
    
    That is probably an extreme example, but has no bearing on pass
    calculations.
     
    > 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.
    
    You can also look for dust on your computer monitor.
     
    > The Cospar standard of precision for observational location happens 
    > to be four
    > places of decimal degrees.  I have no opinion whether that should be 
    > improved
    > or not. 
    
    Improved?!  4 decimal degrees is an accuracy of 11.1 meters.  Take a few
    steps and you're out of your prediction radius!
     
    > Yes, there are those who will exhort us to dumb down our co-ords to 
    > two places
    > of decimal degrees (or worse) 
    
    Like me?  That's still an accuracy of 1.1 kilometers.  I can get
    predictions from 10kms away, and it is still perfectly accurate.
    
    > I think it was Albert Einstein who once said that:
    > 
    > The Truth ought to be expressed as simply as possible, but never 
    > more so.
    
    Napolean:  You can lie to people through statistics, and they will think
    that you are telling precise truths.
    
    Now, that's enough of geodetic precision for a long time.
    
    ------------------------------
    Jonathan T. Wojack                 tlj18@juno.com
    39.706d N   75.683d W           
    http://www.angelfire.com/stars2/projectorion
    4 hours behind UT (-4)
    
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