Impact of continental drift on GPS accuracy

After some intelligent thinking surveyers and geodetics came up with a
solution.
Make a European network of continuous GPS reference stations. Keep measuring
the distances between these stations. Most of these stations then have very
litte movement between each other. Sometimes after an earthquake in Greece a
station there shifts compared to all other European stations.
We here are all on a continental plate that moves as a whole compared to the
international worldwide valid ITRF coordinates. So we take our positions in
ETRF, the european terrestrial reference frame.
Every year the movement of ETRS to ITRF is measured. So knowing the position
in ETRF in a certain year makes it possible to convert those to ITRF.
And remember: GPS-surveyers measure very precise (<25mm) relative positions.
Not absolute positions. They do not use the WGS84 coordinates and mapdatum.

Piet

 

10MM? "I gotta get me one of those."

 

Well, I know of some second hand equipment. See
http://www.lnrglobalcom.nl/site/products/second_hand.html
The cheapest are 4000 euro.
But because you measure 'between' stations you need 2 of these.

Piet

 

Dene,

not quite. Aren't those systems DGPS?

If so, then they are only that accurate relative to a reference
station. The reference station, however, would drift with the
continent.

Hans-Georg

 

Kind of.... RTK actually. I guess the previous poster who said it was
irrelevant was actually kind of right though in that most people using
RTK set it up for a day or so and then take it away with them to
another location. I don't think there'd be many RTK systems left in
place for long enough to be influenced by continentat drift. Could be
wrong though.

There's also a range of DGPS systems that get their correction signal
through companies like Thales and Omnistar and actually recieve their
correction signals from satelites. No ground refference station at all
and not effected by continental drift. Accurate to 300mm.

 

x-no-archive: yes

Hans-Georg,
you are probably right, but I have a bit different question:

Why should the continental drift matter to anybody?
And I really mean anybody, including the scientists. Why should we give a
damn whether the drift is 5mm a year or 25m a week?

If California is to one day bump into Australia, does anyone present here
today really give a fast fiddler's fart about it? Is there any chance that
even if you got so close that you could see Melbourne from Fresno if you
stand on your tippy toes, that you would not have enough time to pack up
and move to Colorado before the crunch?

Or are we simply counting the angels dancing on the tiop of the needle?

 

As someone living fairly close to the boundary between the North
American and Pacific plates, it matters a great deal to me
since it dramatically changes the probability of a devastating
earthquake. Measurements of plate movements have improved our
understanding of the mechanism of earthquakes and will hopefully
one day allow us to accurately predict them.

 

Karl,

at 25 m a week, if I try to find my garage door in fog, using my
GPS, I might end up in my neighbor's garage. -)

Maybe WAAS would compensate for that.

Hans-Georg

 

With a continental drift of 25m a week, your garage would be
destroyed by the daily massive earthquakes anyway ;-)

Juergen Nieveler

 

x-no-archive: yes

Come on, you know for dead cert that the Big One will come. The only thing
you don;t know is when. I happen to live on the outskirts of the shaky
zone, on a silt based island.

I assure you than nobody here gives a damn, no matter how many times you
tell them the while city will sink. "Isn't that something?" is just about
the most invoved response.

OK, if your city is about to sink, do I really care if you tell me about it
24 hours in advance? It will still sink, especially if you happen to be
right.

 

x-no-archive: yes

Is she good looking?

 

Karl,

I think she was once, judging from the last time she asked me to
adjust the clock in her central heating to winter time.

Women can find contact more easily than men. Imagine me asking
some imaginary, but attractive, female neighbor to set the clock
on my heating! -)

Trouble is, if the continent shifts the other way, I'd end up in
the neighbor's garage on the other side, and, believe me, that
would be no fun at all.

Hans-Georg

 

Leaving out any impacts caused by the movement of land based DGPS stations,
strictly speaking continental drift would hve no impact on GPS accuracy,
just on maps! A GPS would still say that some spot on the "spheroid" of the
world was at a specific set of co-ordinates, and they would be the same
tommorrow as today. However, the land masses would have slid sideways under
those co-ordinates. So a map showing the Washington monument to be at UTM X
and Y would be wrong. But a GPS that showed UTM X and Y to now be some
distance away from the monument would be correct. While to the casual
observer it would appear that the GPS was now reporting different
co-ordinates for the monument, and therefor the GPS was wrong, in fact the
monumnet had moved away from its original co-ordinates and the GPS was
correctly reporting the movement by reporting a "wrong" or different
position for the monument. The assumption that just because we stand on the
continent, it must be immovable is a common perception problem among you
eathlings. As a complete OT topic - its fun to learn about all the ways in
which the intellectual giants of the past tried to mathematically and
scientifically justify the Earth being the center of the universe. They had
real problems with the apparent retrograde motion of the planets - kind of a
parallel to assuming GPS must be wrong because the position of the monument
changed. Same kind of perspective issue. If we start with the assumption
that the continents are fixed (we are the center of the universe), GPS must
be wrong. The moral: you can't assume a critial variable is a constant and
arrive at a reasonable analysis.

To expand the question a bit more, if we then consider WAAS or DGPS, I think
the answer becomes more complex. As a layman who only has become as
immersed an an English lit degree will allow, I believe those systems assume
that the base station computing corrections "know" their exact position on
the face of the earth and therefor can compute error from the position as
reported by the GPS system. If the exact position of the base station(s)
change over time, I would guess that some correction of the computationally
"correct" position (the DGPS position relative to its spot on the land
surface - its spot on a map) must be resurveyed as well. This would be
relative to the resolution of the technology. If we have GPS technology
that was accurate to a few meters, and few mm might not be important (or
even noticed). However, when we have GPS systems that employ the technogy
to measure the movments of mountain peaks, then a minute amount of drift
would not only be noticed, but become very significant in employing the
technogy to it maximum ability.

 

Pieter,

now you explained in very many words what had already been said
in this thread.

Hans-Georg

 

I liked reading it though..