#Adirondacks Hot Spot
26 messages in this thread
The following info comes from an AP article dated yesterday:
A team of geologists and graduate students are conducting a study in the
Adirondack Mountains in upstate New York which they hope will show the
existence of a 'hot spot' in the Earth's mantle deep below the area.
Mantle hot spots generate pockets of geothermal and volcanic activity in the
crust above them. The Hawaiian Islands were formed from a hot spot below the
Pacific crustal plate, and the northwestward arc of the island chain shows how
the plate has moved across the hot spot over the last few million years.
Another hot spot is thought to be responsible for the geothermal activity in
Yellowstone National Park.
Now geologists Steve Roecker and Steve Tice, of the Rensselaer Polytechnic
Institute in Troy, NY, are investigating the possibility of a hot spot beneath
the Adirondacks. According to Yngvar Isachsen, chief geologist at the New York
State Geological Survey, the Adirondacks are a circular uplift, unlike the
Appalachians and most other mountain ranges which are the results of folding
and faulting. Isachsen believes the circular uplift is the result of a thermal
plume from the Earth's mantle. He also thinks the mountains are still growing,
possibly by as much as a millimeter a year.
The survey currently underway requires that Roecker, Tice, and their students
locate a number of markers left by geologist Verplanck Colven in his detailed
survey of the area 130 years ago. They will then use satellite transmitters
and the Global Positioning System to obtain extremely accurate data for the
current positions of those markers. The GPS position data will be compared to
Colven's data and also to data from more recent surveys. If the markers are
found to be moving apart, that will be excellent evidence for a "doming" type
uplift and hence for a hot spot below the Adirondacks.
"Even though the old surveys were far less accurate than what we'll get with
GPS, if the Adirondacks are moving up at a high rate, there should be enough
displacement over the 30, 60 or, in Colvin's case, 130 years, for us to see
it," Roecker says.
Jon W.
Jon,
Thanks for the info about the Adirondacks. It raised one question at the end:
>> "Even though the old surveys were far less accurate than what we'll get
>> with GPS,
I wonder how they're getting that kind of accuracy with GPS? Maybe they have
access to the military version? Or are they just interpolating from known
benchmarks.
Some of the "old surveys" actually set survey markers within inches of true
position, where the best you can get with normal GPS is 10 meters or so, as I
understand it.
Any expert input?
Al
,<<I wonder how they're getting that kind of accuracy with GPS? Maybe they have
access to the military version? Or are they just interpolating from known
benchmarks.>>
In static differential mode, with 12 channel receivers, relative height
difference between 2 points should be computable to around 0.5 mm plus 1ppm of
the baseline length at the 2drms confidence level. No "military version"
required. I've been getting closures with conventional control to better than
1cm with a few seconds observation on each site using "on the fly" techniques
which are much cruder than the static obs proposed.
You do have to remember, though, that GPS heights are relative to the
ellipsoid, and not to the geoid, so the observers will need a *very* good model
when reducing the obs.
BTW: are you the Alan Balmer who used to work for Geco?
Rob
Rob,
>> relative height difference between 2 points should be computable to
>> around 0.5 mm plus 1ppm of the baseline length at the 2drms
>> confidence level
I think I'm feeling symptoms of future shock <g>. I once read a 1966 revision
of "Surveying: Theory and Practice", Davis, Foote and Kelly. Seems we've come a
way since then. Surveying is an interesting subject, and serves as a great
example of what careful measurements, intelligently processed, can accomplish.
Do you know of anything on modern surveying methods which is written for the
intelligent layman?
Geco – Sorry, not me. I've worked as an electronic engineer and a software
engineer for several companies, and am now independent.
Al
>>> I once read a 1966 revision of "Surveying: Theory and Practice", Davis,
Foote and Kelly. <<<<
Unfortunitly later edtions became less readable. I was teach an apprentice on
the island Yap surveying using a 70's edition. He read it and understood. I
also had the latest edition and the language lost him. Same material just
rewriten for less clarity. The real pity in updating textbooks is the need to
write out the meaning to rewrite something.
I work as a surveyor for the Navy (Seabee). Mostly construction surveying but
am interested in the others areas also.
Craig
>> The real pity in updating textbooks is the need to write out the
>> meaning to rewrite something.
I've noticed that in other cases. I think it gets worse when someone other than
the original author does the revisions. I suppose the revisor has to change
things just to prove he's earning his money.
One interesting naval application of surveying is ship building. Thousandths of
an inch over *big* machines. I suppose that's all done by laser beams and
computers now.
Al
<<Some of the "old surveys" actually set survey markers within inches of true
position, where the best you can get with normal GPS is 10 meters or so, as I
understand it.>>
What do you mean by this, Al? "within inches of the true position" has very
limited meaning to a surveyor. Your coordinates depend on the datum used, as
well as the projection. Your position on the topography is related to the
geoid, whch in turn is related to any arbitrary datum ( and hence coordinate
set) you wish to define. A point can have literally millions of latitudes and
longitudes, all perfectly "true" within their own reference systems.
All systems are relative in the end, so what we're really talking about here is
the accuracy or "closeness to truth" with which we can define the length and
orientation of a baseline within a reference datum. This is made up of the
*precision* of the measurements plus their *reliability*. The precision has to
have a confidence level associated with it ( ie, "within inches" should
encompass a precision measure: ie, 1mm at the 95% confidence level). The
reliability is quoted in terms of the probability of detecting an error
exceeding a particular size.
Both precision and reliability of DGPS measurements are far better than
anything achievable with conventional equipment. In 1986 I retriangulated part
of northern Brazil with the crude Block 1 satellites. We were getting results
better than 3 ppm of the baseline length at 95% confidence even then. We also
found a 56 METRE bust in the Brazilian PSAD 56 ( offshore) datum!!
Rob
>> within inches of the true position" has very limited meaning to a
>> surveyor
That's OK, 'cause I'm not a surveyor <g>. The info came from a USGS publication
intended for schoolkids, as I recall. I'm sure that it was intended to give a
feel for the type of precision involved, not to be taken as a springboard for
professional discussion. What I was actually interested in was the GPS methods
being used, and how they compared to the kind people install on their boats,
RVs, long-haul trucks, etc.
Al
Al,
According to the article, the GPS technology being used will yield an accuracy
of 1 millimeter in 60 miles.
Jon W.
>> According to the article, the GPS technology being used will yield an
accuracy of 1 millimeter in 60 miles. <<
60 miles from what?!?<g> That dosn't make any sense! If GPS uses sattelite
data to fix each point, then each point will be roughly the same distance from
all those sattelites wizzing overhead, wouldn't they? Or does this mean 60
miles from the sattelite???
Greg,
I _think_ (but am not at all sure) that they mean if you start at point A and
head for point B 96km away, and navigate only by a GPS system, you will wind up
at point B with a maximum error of 1mm plus or minus.
Jon W.
>> I _think_ (but am not at all sure) that they mean if you start at point A
and head for point B 96km away, and navigate only by a GPS system, you will
wind up at point B with a maximum error of 1mm plus or minus. <<
When I first read it, that's what it seemed like they meant to me as well. But
that is why I thought it didn't make sense. My understanding of GPS is that
you fix your points from a series of measurements taken from sattelites. Thus,
if point A and B somewhere on the earth, the distance between A and B are not
relevant to the accuracy of the measurements!
I have not used GPS, but I have colleagues who have and I'm starting to plan
two projects that will use them, and that's my understanding of how the system
works.
Whatever!?! Probably a quirk of the media, as usual!<g>
Cheers,
GTL
Greg,
>> But that is why I thought it didn't make sense. My understanding of GPS is
that you fix your points from a series of measurements taken from sattelites.
Thus, if point A and B somewhere on the earth, the distance between A and B are
not relevant to the accuracy of the measurements! <<
That's my understanding as well. The distance between A and B isn't a _factor_
in GPS measuring. It's a _result_ of it. Figure the position of A, figure the
position of B, then calculate the distance between 'em. Your calculated figure
will differ from the actual figure obtained by traveling from A to B by 1mm (or
less) per 96km of distance.
Jon W.
>> That's my understanding as well. The distance between A and B isn't a
_factor_ in GPS measuring. It's a _result_ of it. Figure the position of A,
figure the position of B, then calculate the distance between 'em. Your
calculated figure will differ from the actual figure obtained by traveling from
A to B by 1mm (or less) per 96km of distance. <<
Right, that makes sense, but I still don't see how it is distance dependant!
Each point will be off from some theoretical point to a certain degree, this
error does not depend on any other measurements you do or do not make, and
thus, any two points anywhere in the world will have the same degree of
relative inaccuracy!
(Or so I would have thought, but since I don't know what I am talking
about…..<g>)
GTL
Greg:
See my previous post ( and the ones of last week, if they haven't scrolled
yet). The "accuracy" with which you can measure a baseline is dependent on the
"precision" ( spread of a number of repeat readings, expressed in Standard
Deviations, or in error ellipse size/ orientations, or in error ellipsoids, or
as a drms or 2drms figure. Definitely NOT expressed as a CEP {circular error
probable}) and the "reliability" ( probability of detecting a measurement error
of a given size).
This accuracy itself relates to the measurement "frame of reference" which you
are working in, Within any particular reference frame, 2 points will have
"exact" mathematical separations in lat/long/ht, or in XYZ. There is an
infinite number of possible reference frames, though. These are usually based
on models simplifying the shape of the earth to an ellipsoid. The true "base"
shape is a geoid, which is mathematically irregular, On top of that is the
topography, which you rockhounds want to chip away until it actually confroms
to the shape of the geoid, as far as I can see <g>. One point to note is that a
pouint can have an *infinite* number of latitude/longitude coordinates… each
one based on a different reference frame. Eg, the GPS WGS 84 reference datum
places the Greenwhich meridian about 10 m west (? I think so… it might be
east, but I haven't got my calculator here) of the meridian located by the
OSGB (Ordnance Survey of Gt Britain) datum.
Rob
Hi
I did read your posts, and I think I'm getting most of your "points"<g>,
especially this bit about the earth not being round (but not flat, either, I
hasten to add!). Anyway, this still does not explain what appears to be
wierdness in the original statement. It seems to me that the two points that
are actually being used for any GPS measurement is some theoretical point in
the sky determined by sattelites, and a particular, and singular, point on the
ground. As you point out, and I'm sure you are completely correct, the
accuracty of that point depends on various stuff that I'd rather let you
explain than to reiterate. But when you are comparing two points on the eath,
each independantly and separately measured by the GPS technique, the relative
accuracy of those two points shouldn't be dependant in any way on the distance
between them.
I think.
Cheers,
GTL
Greg Laden
Dept. of Anthropology
Harvard University
Cambridge MA 02138
>>>I _think_ (but am not at all sure) that they mean if you start at point A
and head for point B 96km away, and navigate only by a GPS system, you will
wind up at point B with a maximum error of 1mm plus or minus.<<<
I suspect what they are actually saying is that if you make simultaneous
measurements from A and B against the same satellites, you can determine the
distance from A to B very precisely. There's a subtle difference between
saying that and saying the you can navigate to the same precision. Simultaneous
measurements against three or more satellites will allow you to measure
distances between base stations knowing just the distances from the satellites
to each other. To navigate over time, you need to know the satellite position
very accurately at different times. While the arithmetic to predict satellite
motion is routine rocket science, actually predicting satellite motion with
very great accuracy isn't all that easy.
This is correct, except that you need *four* satellites. Your receiver clocks
are not perfectly synchronised with the satellite clocks, so you have four
unknowns, XYZ and T (time). With 4 observations, you can solve for the user
receiver clock offset to GPS time. With more satellites in view, you can
compute some statistics to give you a handle on the accuracy ( precision plus
reliability) being achieved.
Rob
<<I _think_ (but am not at all sure) that they mean if you start at point A and
head for point B 96km away, and navigate only by a GPS system, you will wind up
at point B with a maximum error of 1mm plus or minus.>>
This is not the case ( my previous posts on Differential GPS must have
scrolled). In differential mode, you occupy each end of a baseline ( points A
and B ) *simultaneously* with 2 GPS receivers. Over baselines of " a few
hundred" km, if you are tracking the same satellite constellation from each rx,
effects of timing, orbit, ionospheric etc errors will largely be modelled out,
allowing you to resolve the length of your baseline to around 1mm , PLUS 1-2
parts per million of baseline length ( at the 2drms confidence level) with a
reliability of, say, 80%. (NB: quoting an accuracy number *without* giving a
confidence level and reliability is meaningless to a surveyor).
Navigating in single receiver mode, you'll do well to locate yourself within 30
m.
Rob
Ah, since you put it that way, I think I get it. More or less.
I just hope my new GPS machine comes with a good reference manual!
(well, I only need to be accurate to a meter or two in any direction
anyway….)
Cheers,
GTL
Greg Laden
Dept. of Anthropology
Harvard University
Cambridge MA 02138
Meters? Miles? Huh?
Please be careful about your frame of reference, don't mix units. My
speedometer is calibrated in furlongs per fortnight and I have a stack of
tickets 2.75 cubits thick!
Daniel,
That figure came straight from the article I have. Don't blame me, blame the
fool reporter. <G>
Jon W.
Daniel –
PMJI. And I thought the use of "furlongs per fortnight" was dead. Back in the
early 70's I helped program an instrument that measured the rate of nylon fiber
production in a manufacturing facility in North Carolina to read in that unit.
We thought it had much more class than "meters/minute". The Area Supervisor –
as I recall – was not amused. So much for creative thought!
– Paul
I don't think they are going to have any problem proving that the Adirondacks
are rising (in fact, I think that's known data). The problem is going to be
separating rising due to "hot spot" thermal pluming from glacial rebound,
that's known from Holocene marine fossils in the Champlain Valley to be 150
meters or more and is probably still happening.
I wonder what the RPI folks really tried to tell the reporter?
Don,
>> The problem is going to be separating rising due to "hot spot" thermal
pluming from glacial rebound, that's known from Holocene marine fossils in the
Champlain Valley to be 150 meters or more and is probably still happening. <<
I think they're well aware of that. They're specifically looking not only for
vertical uplift but also for horizontal spreading. They hope to find a doming
pattern to the motion, which would indicate a thermal plume from below rather
than postglacial rebounding.
Jon W.
>>
I wonder what the RPI folks really tried to tell the reporter? <<
Very good question!