Impact of continental drift on GPS accuracy

Discussion in 'General GPS' started by David, Nov 2, 2004.

  1. David

    David Guest

    We all heard of the continental drift phenomenon. I understand the 5
    continents are moving a few meters/feet per year. Does the GPS
    satelite system have a correction algorithm ? If not, as years go by I
    would think that the same GPS coordinate does not locate the same
    place on earth.E.g : if my house doorstep is N 50°23.000 E 4°50.000,
    after a few years it could have become N 50°23.002 E 4°50.003, because
    of continental drift ?

    Thanks for your comments.
    David.
     
    David, Nov 2, 2004
    #1
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  2. I guess that's a mapping issue rather than a GPS issue. The satellites still
    report the correct coordinates, it just the land mass has moved.

    I think....

    cheers

    simon
     
    Simon Coupland, Nov 2, 2004
    #2
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  3. Your thought is right. GPS always gives absolute position, while maps
    get outdated :)


    Wastl
     
    Sebastian 'Wastl' Koopmann, Nov 2, 2004
    #3
  4. David

    BillC Guest

    As the continents drift, they drag the coordinate grid(s) along with them,
    wrinkling the datums. One of the reasons for researching the driving
    mechanism of plate tectonix, is to shut the goddamn thing down so we don't
    have to waste money on datum corrections every 50 years or so, and
    protecting land values.
     
    BillC, Nov 2, 2004
    #4
  5. David

    Stichting ST Guest

    As far as is known (military you know) GPS-WGS is connected to the whole
    globe. Astronomically. So because of plate tectonics all landmasses move with
    respect to this coordinate system. Movements are mostly a few mm.to cm. a
    year.
    Should your handheld GPSr be able to get absolute positions within a meter
    then, yes in about 50 years from now the difference would become measurable.
    Now the first handheld GPSr's become available (if you have a lot of money to
    spend) that manage to give absolute position within a few dm.
    But then you really get into trouble. Because of tidal movements you will see
    your GPSr going up and down. The crust also moves up and down just like the
    water. And atomic clocks are so precise and stable that you also get an
    east-west displacement because the earth does not turn every day the same
    amount slower.

    That surveyers with GPS manage to get coordinates up to a cm good is because
    they measure relative. They get bearing and distance to a reference station.

    And what they do with the continental drift?
    Here in Europe we made ETRF/S (European Terrestrial Reference Frame/System).
    Those coordinates are fixed (with help of many continuous GPS reference
    Stations) onto the European Plate.
    And we measure the movement of this frame against the ITRF (International
    etc.) every year. So within Europe we have no problem; surveyers keep stable
    coordinates. And beside: official recognized surveyer work depends on law and
    in most countries they have to work in the local grid.

    Piet
     
    Stichting ST, Nov 2, 2004
    #5
  6. David

    derob Guest

    Quite a good question. Basically the GPS network has its own reference
    system... the continents move slowly in that, as it is fixed to the average
    earth, so you are right your doorstep drifts... :) Hence, for high accuracy
    work, another reference system must be used that is fixed to the continent.
    This requires a correction which depends on time.

    I consider this http://www.gps.gov.uk/guidecontents.asp website to give an
    excellent explanation of this subject and on how coordinate systems work...
    in case you are really interested:) (if I remember well you need to sign
    in, i.e. set up a free profile)

    Greetz,

    Derob
     
    derob, Nov 2, 2004
    #6
  7. David

    tougharms Guest

    ROFLMAO!
     
    tougharms, Nov 2, 2004
    #7
  8. David

    Sam Wormley Guest

    Sam Wormley, Nov 3, 2004
    #8
  9. David

    Bad Idea Guest

    Real estate agents _hate_ this because all that land is changing hands
    and nobody is making a commission.
     
    Bad Idea, Nov 3, 2004
    #9
  10. So within Europe we have no problem; surveyers keep stable
    Except of course that here in the UK we like to be part of Europe but at the
    same time slightly different. The British mainland is steaming away from
    continental Europe at a few centimetres per decade (I forget the precise
    figure).

    Currently the British National Grid is nailed to trig points on the ground
    so it's moving with us. If I understand correctly, there are currently
    moves by the Ordnance Survey to adopt continental standards which means our
    grid will start to move with respect to our land mass.

    OK, the WGS coordinate system does not respect land mass movement but I
    presume it has to be fixed to _some_ reference point on the ground. As one
    of the last hangouts (presumably) of the empire, we Brits still claim to
    "own" the prime meridian. Is zero degrees still that line engraved on the
    floor of the Royal Observatory at Greenwich or is it some other, more
    absolute, standard? If the former, then we must be dragging WGS84 with us
    as we sail off into the Atlantic.

    A few years back, they excavated under the Beachy Head Lighthouse, put it on
    rollers, and wheeled it a few metres away from the crumbling cliff edge.
    Perhaps we should be considering doing something similar with the Royal
    Observatory so that it can stay still whilst the land beneath it moves
    around.

    Keith
     
    Keith Sheppard, Nov 3, 2004
    #10
  11. David

    Karl Pollak Guest

    x-no-archive: yes
    You may want to re-read the history a bit more carefully. The Prime
    Meridian was never "our own" and had nothing to do with the Empire.

    It was agreed to by an international conference.

    The fact that the prototype of the metre was stored in some basement in
    Paris did not make it a French measure, either.
    Don't quote me on this one, but according to some accounts I've read, the
    Royal Observatory is not exactly on the Prime Meridian, either. The claim
    was that the zero meridian was improperly fixed due to errors and primitive
    instruments.

    Same as the US-Canadian border on the West Coast is not exactly on the 49th
    parallel as fixed by a treaty. The error there is about 200m and can't be
    blamed on any continental drift.
     
    Karl Pollak, Nov 3, 2004
    #11
  12. David

    Stichting ST Guest

    In the document History of the Prime Meridian -Past and Present by Jeremy Paul
    http://gpsinformation.net/main/greenwich.htm
    you can read (and I quote):
    "Even today, it can be confusing as there are four Meridians all passing
    through the Old Royal Observatory.
    The earliest is Flamsteed's, named after the first Astronomer Royal, which
    was established in 1675. In 1725, Edmund Halley, the second Astronomer Royal
    established a second Meridian.
    The third was defined by another Astronomer Royal, James Bradley, in the mid
    18th century, and is still used as the basis for map-making in Britain today.
    The fourth Meridian was established in 1851 by yet another Astronomer Royal,
    Sir George Airy, who set up new measuring equipment in a room alongside
    Bradley's original equipment. It is the positioning of this neighbouring
    equipment, just 5.79 metres (19ft) away, which eventually became the basis for
    international time. "

    Piet
     
    Stichting ST, Nov 3, 2004
    #12
  13. David

    David Lee Guest

    Keith Sheppard wrote inter alia...
    No - it's not fixed to any single reference point. The International
    Reference Meridian and Pole and, hence, the WGS84 (ie GPS) datum is defined
    to be stationary with resect to the average of all global motions. This
    means that the WGS84 latitudes and longitudes will drift to a greater or
    lesser extent at any given location. For example the drift is about 2.5cm
    per year in a north-easterly direction in the UK. Elsewhere on the globe
    drift can be very much greater eg Australia and Hawaii are moving at up to
    10 cm per year.

    I'm not totally sure of the rest of my facts, but I believe that the
    practical realization of the WGS84 datum is based upon the International
    Terrestrial Reference Frame as defined in the year 2000. (The definition of
    ITRF2000 contains positions, velocities and estimated positional errors of
    all the reference stations so that the reference frame can be corrected to
    any other time.)

    David
     
    David Lee, Nov 3, 2004
    #13
  14. You may want to re-read the history a bit more carefully. The Prime
    There was a certain amount of tongue in cheek throughout my posting, but I
    take your point :)

    I would point out, though, that whether or not it was agreed by an
    international converence _what_ they agreed upon was still Greenwich. I
    presume the reason for this was in order not to change the "legacy systems".
    In other words, existing maps of the day presumably measured longitude from
    Greenwich. Whether or not there were other systems, the one based upon a
    reference point in Britain was deemed to be the most appropriate choice.

    Maybe it's OTT to say we "own" the prime meridian but I would wager that had
    there not been a history of British empire and exploration leading to some
    sort of de-facto standard, the modern prime meridian would not run through
    Greenwich. I'd guess it would probably be somewhere in Portugal or Spain.

    Keith
     
    Keith Sheppard, Nov 4, 2004
    #14
  15. David

    Q&A Guest

    Not a valid question considering his GPS is at best 3m accuracy and
    continental drift is 0.03m per year at best.

    You might detect that your house has moved relative to the satellite
    coordinates after a 100 years or so...if the same satellites are still up
    there that is...
     
    Q&A, Nov 4, 2004
    #15
  16. David

    Karl Pollak Guest

    x-no-archive: yes
    They had to agree on something, and Greenwich happened to be "it".
    Wrong. English maps were referenced to the Greenwich meridian. Scottish
    and Welsh weren't. Neither were anybody else's. That was the entire point
    of the various scientific international societies getting to gehter to
    agree on a common meridian.
    Actually it is bordering on being American.
    It almost smacks of the Soviet claim to have the tallest midgets in the
    world.

    It's time for the British to realize that the sun has already set ...
     
    Karl Pollak, Nov 4, 2004
    #16
  17. David

    BillC Guest

    If you're refering to WGS 84, that's a datum, not a coord system. Unlike
    other datums, WGS has its reference point within the earth, versus being
    some point on the surface.

    As one
     
    BillC, Nov 5, 2004
    #17
  18. It's time for the British to realize that the sun has already set ...

    There are two national pastimes here - talking about the weather and talking
    about some mythical olden days when Britain was "Great Britain". But you're
    talking to the wrong person here. I consider myself a European and my
    references to the "grand old days of empire" should, at best, be considered
    poking fun.

    I was particularly amused by Ricky Gervais' comment to an American audience
    at a recent awards ceremony:

    "I'm from Britain. You know. We used to run the world before you guys took
    over."

    Regards
    Keith
     
    Keith Sheppard, Nov 5, 2004
    #18
  19. If you're refering to WGS 84, that's a datum, not a coord system.
    Understood and apologies for being a bit sloppy in my terminology.

    What I actually meant was degrees of latitude/longitude, in the WGS84 datum,
    measured from the equator/Greenwich respectively - but that's a bit of a
    mouthfull. By the same token, my "Hoover" is actually made by Electrolux ;)

    Keith
     
    Keith Sheppard, Nov 5, 2004
    #19
  20. David

    Joe Fabeitz Guest

    SHIT! One more thing to worry about ;-)
     
    Joe Fabeitz, Nov 5, 2004
    #20
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