Re: Geodetic precision

From: Joerg Kampmann (
Date: Sat Aug 18 2001 - 23:43:46 PDT

  • Next message: Anthony Ayiomamitis: "ISS-STS105 pass late tonight"

    Hi, yes I agree - can you tell me how I find out the datum which
    Mapblast uses here in Europe??
    On the other hand - as I stated before: to what accuracy do we
    aficionados need the precision differences ??
    Chris Olsson schrieb:
    > Bruno Tilger wrote:
    > >I am surprised at the large difference (between Posns 1&3) in longitude of
    > >11.174".
    > The longitudinal difference between WGS72 and WGS84 at Greenwich is 0.554",
    > Bruno.
    > My typo in which I inadvertently typed "E" instead of W in Pos#3 is to blame
    > and I apologise to the List if any members were genuinely confused by my typing
    > error.
    > Of course, both the WGS72 and WGS84 positions were in the same hemisphere.
    > WGS84's zero degree meridian (ignoring a little bit of tectonic plate movement
    > since 1989) lies 102.478 metres East of the Prime Meridian at Greenwich.
    > The difference between WGS72 and WGS84, although quite comprehensive in view of
    > the fact that the size and shape of the two spheroids are rather different and
    > the fact that the two datums have a rotational difference, as well as markedly
    > different geoids, is a mere handful of metres in most places around the work.
    > The greatest 3-D difference is little more than half a dozen metres or so.
    > Converting between WGS 72&84 is very easy, so there is little scope for
    > confusion
    > The big geodetic difference is when co-ordinates are misunderstood as to their
    > geodetic basis.  For example, if a Cospar site in Scotland lists its position
    > to a precision of a tenth of an arc-second of Lat/Long and another Cospar site
    > in Sweden lists its position to four places of decimal degrees, then there is
    > an easy trap for the unwary who might make the error of presuming that both
    > co-ordinates are on some kind of common ground, such as WGS84 or WGS72.
    > Note that neither the Scottish nor the Swedish position is wrong in any way.
    > In fact they are quite accurate and the basis from which they were derived is
    > fundamentally sound.  The potential error is in the presumption that they have
    > a common basis.  In fact: the Scottish point is listed with reference to the
    > Airy spheroid and the OSGB36 + OD(Newlyn) datums; and the Swedish position is
    > listed -- equally legitimately -- with reference to its national mapping datum
    > of Sveriges Lantmäteriverket RT90 datum på Bessel's 1841 spheroid in the
    > horizontal axis and the local RH70 datum in the vertical axis.
    > Each has a sound provenance and each is capable of being used with suitable
    > precision in 3D calcs upon either WGS72 or WGS84, but the important thing to
    > recognise is which geodetic basis is associated with any stated position.
    > An analogy might be the European price of oil.  If I simply state that the
    > current (epoch: close of business on Friday evening) Rotterdam market spot
    > price of Brent Crude oil is 25.12345, then my stated price is capable of
    > misinterpretation unless I declare whether I am referring to Dollars or Euros
    > and whether or not I confirm that I'm referring to a barrel of crude or a tonne
    > or a ton or a tun or a firkin or whatever.
    > Note too that the stated Edinburgh and Malma locations are not in any way wrong
    > and that they are just as legitimate as a position which is expressed in one of
    > the international standards such as ED50 or WGS84.
    > It is reasonable to presume that the Swede is using Degrees, not Gon, as his
    > angular measure, just as it is reasonable to presume that the Scotsman is using
    > arc-seconds rather than Groats or Drams as his.
    > It might be reasonable to presume that both of those well-qualified observation
    > locations are using a common spatial datum.  Such a presumption would be quite
    > wrong.
    > My thesis is simple.  I am suggesting that we either adopt a single common
    > standard or else -- perhaps more practically -- declare what geodetic standard
    > we are referring to when we list apparently precise geographical (or geodetic)
    > co-ordinates.  The differences between the actual locations of stated positions
    > may be small or great.  We can only know what those discrepancies (they are
    > *not* errors) are if we have some indication as to what they refer to.
    > It is sufficient to simply say that a particular position is with reference to
    > WGS84 or a particular national mapping datum or whatever the basis of the
    > stated position is.
    > By stating the geodetic basis of stated co-ordinates, we give a positional
    > indication which is almost as important as stating the epoch of an elset or a
    > SatObs.  It gives a pre- and post- calculable peg upon which other spatial
    > calcs can be attached and assessed.
    > Cheers,       Chris Olsson
    > (In the Northern and Western hemispheres, well above sea level, wrt WGS84!)
    > -----------------------------------------------------------------
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    Dr. Jörg Kampmann - IBK-Consult for Real-Time and Embedded Systems
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