How do you measure sea level? (1 Viewer)

[Lord of Planar]

We can observe that movement with GPS.

Seriously?

I doubt we can do that with any accuracy that surveying requires. Just how accurate is this GPS measurement suppose to be, and what reference do these satellites use them?

 

[MoSurveyor]

ARRGH!!!!!

That is a big part of my point.

1 The GPS will give you an answer to the mm.

2 The GPS will be calibrated from a "fixed point" such as a radio telescope somewhere which is not fixed.

3 The result will not be relevant to the volume of the ocean because it does not take into account changes in the shape of the ocean floor or the exact point of the gravitational center of the earth which is changed by lots of things such as earthquakes.

Overall the use of sea level as an indicator of ocean volume and thus ice melt is not useful if it's not understood how wooly the numbers are.

1. It takes a several hours to nail an elevation down to that small a position but, yes, it gives elevations to the mm (~3 mm, actually) and does use other fixed points in a given system, which is why the US generally uses NAD83 domestically. However, several points on the NAD83 network are constantly referenced to other points all over the world in the WGS84. See #2.



2. GPS satellites are "calibrated"** from many fixed points (not just one) as I noted in the previous post:

"If a given (earth surface) reference point moved it would be easy to spot because it's position relative to other earth surface reference points would move."

What part of that idea are you having problems with???


**The GPS satellite's ephemeris is adjusted after the fact, which is why post-processing of data is used for control surveys instead of less accurate real-time processing. GRACE will increase the accuracy of the ephemeris even farther and should drop elevations down to 1mm with lat-lon dropping into sub-mm accuracy - assuming GRACE is successful.



3. The center of the earth doesn't matter, as I've already taken great pains to point out. All that counts is the reference ellipsoid, which has many, many control points scattered all over the earth. If the control points in Australia drop 3 mm and control points in the US rise 3 mm then it's obvious the center has moved and that movement can be measured and taken into account. We're talking about a 3 dimensional, spherical (ellipsoidal) model, not a plane surface. What happens on one side will show up in the opposite direction on the other side and in various stages of horizontal and vertical movement elsewhere - all discernible because the reference ellipsoid is a relatively simple mathematical construct.

I should also note if the center of the earth moves, the satellite positions will not "suddenly move" to their new orbital positions, it'll take time for them to adjust, which is why we use post-processing and ephemerides.

 

[MoSurveyor]

Seriously?

I doubt we can do that with any accuracy that surveying requires. Just how accurate is this GPS measurement suppose to be, and what reference do these satellites use them?

If post #27 doesn't answer your question then ask again and be more specific. I was writing it when you posted.


And, yes, we can measure the movement of the North American plate. How do you think we know that it's moving at all?

 

[Lord of Planar]

If post #27 doesn't answer your question then ask again and be more specific. I was writing it when you posted.


And, yes, we can measure the movement of the North American plate. How do you think we know that it's moving at all?

The thing is, GPS uses frequencies that have around a 20 meter or greater wavelength. To resolve down to millimeters is only possible with an exceptional signal to noise ratio, which simply does not exist. We are technically advanced enough to resolve such levels with sub picoseconds gate technology... if we have no noise interfering... but then we also have atmospheric variations which also change the timing of a signal from so many miles away.

 

[MoSurveyor]

The thing is, GPS uses frequencies that have around a 20 meter or greater wavelength. To resolve down to millimeters is only possible with an exceptional signal to noise ratio, which simply does not exist. We are technically advanced enough to resolve such levels with sub picoseconds gate technology... if we have no noise interfering... but then we also have atmospheric variations which also change the timing of a signal from so many miles away.

That's why we have to observe points for such a long time to get high order control. Eventually, the initial error ellipse reduces to ±3 mm for each direction horizontally and (roughly) ±5 mm for elevations. Taking more observations on different days reduces the atmospheric issues to almost nothing and further post-processing and adjustment of the final network completes the task to bring in ±1 mm horizontally and roughly ± 2 mm vertically.


I was a surveyor when we started using GPS - classes on the subject and almost two decades of field work with it. Originally, civilians couldn't use the military P-code, which increased observation times dramatically (over today) for high order control points. Being able to use that code - actually, a complex analysis of the L1 and L2 signals - reduced observation times. The military finally released access to the P-code (for awhile) since we were getting around their restrictions, anyway. In the end, the military retained access to the P-code, which is why military hand-helds are far superior to civilian models. However, survey grade units can still interpret the L1/L2 in such a way that we don't need it.

 

[Lord of Planar]

That's why we have to observe points for such a long time to get high order control. Eventually, the initial error ellipse reduces to ±3 mm for each direction horizontally and (roughly) ±5 mm for elevations. Taking more observations on different days reduces the atmospheric issues to almost nothing and further post-processing and adjustment of the final network completes the task to bring in ±1 mm horizontally and roughly ± 2 mm vertically.


I was a surveyor when we started using GPS - classes on the subject and almost two decades of field work with it. Originally, civilians couldn't use the military P-code, which increased observation times dramatically (over today) for high order control points. Being able to use that code - actually, a complex analysis of that signal - reduced observation times. The military finally released access to the P-code since we were getting around their restrictions, anyway, and that allowed for relatively cheap consumer GPS units, though they're far, far from being as accurate as survey grade GPS.

I have a hard time comprehending a GPS being accurate to millimeters. I understand how statistical analysis can give multiple data points highly updated results, but one... its still never 100%, just mid 90's, and two... we are dealing with for than a couple unknowns. I;m sorry, but the timing accuracy of the multiplexed carriers would have to be too precise to get down into the millimeters. The timing is about 3 picoseconds to the millimeter. I'm not up with the latest electronics, but I have a working knowledge of this stuff. I would like to see some technical data on the theory of operation if you know a quick reference. I cannot fathom resolving a wavelength into 10,000 parts, with any accuracy. Again, signal to noise and atmospheric variations. A third yet... satellite orbits vary more than millimeters, from calculated, regularly with the changing temperature of the upper atmosphere...

 

[Lord of Planar]

To add, I can accept that surveying with a pair of matched GPS receivers between two points not far from each other can be resolved to such levels. Determining one point from the other. They would both receive the effectively the same timing changes and noise. If your claim is receiving only to a receiver on the ground, from the GPS network, I don't buy it and would love to see the theory of operation if it's real.

 

[MoSurveyor]

To add, I can accept that surveying with a pair of matched GPS receivers between two points not far from each other can be resolved to such levels. Determining one point from the other. They would both receive the effectively the same timing changes and noise. If your claim is receiving only to a receiver on the ground, from the GPS network, I don't buy it and would love to see the theory of operation if it's real.

It's called differential GPS (DGPS) and that's what surveyors use. Stand alone observations would take days and days of constant observation to resolve - and that's how the main WGS84 control points are observed, if I understand correctly. (I've only dealt with the NAD83 reference.) There are control points out there that are constantly active, taking GPS measurements 24 hours a day, every day for years except maybe 2 hours twice a year for maintenance. In fact, that was becoming the trend on local surveying, too, when I retired from surveying five years ago. Local survey groups pay for active stations so they can all use them as their differential reference points - though I should point out that (at least in Missouri and Arkansas) we are required by law to use two control points for land and control surveying (as opposed to topological and construction surveying). Taking reading 24/7 from multiple stations makes it's own refinements and the data is still post-processed to further increase the accuracy.

 

[MoSurveyor]

I have a hard time comprehending a GPS being accurate to millimeters. I understand how statistical analysis can give multiple data points highly updated results, but one... its still never 100%, just mid 90's, and two... we are dealing with for than a couple unknowns. I;m sorry, but the timing accuracy of the multiplexed carriers would have to be too precise to get down into the millimeters. The timing is about 3 picoseconds to the millimeter. I'm not up with the latest electronics, but I have a working knowledge of this stuff. I would like to see some technical data on the theory of operation if you know a quick reference. I cannot fathom resolving a wavelength into 10,000 parts, with any accuracy. Again, signal to noise and atmospheric variations. A third yet... satellite orbits vary more than millimeters, from calculated, regularly with the changing temperature of the upper atmosphere...

Statistical analysis is a large part of it. Observations are generally made at 1 second intervals so a common, local control point session (15 min followed by another 15 min a few hours later) will have two sets of 900 observations each. But it's not JUST statistical analysis that increases the accuracy in post-processing. Post-processing also takes into account the updated ephemerides from the GPS satellites.

I think you may have also neglected to account for multiple satellites. Minimum requirements are four satellites for N-E-El (can't trick the rules of geometry!) but surveyors seldom take measurements without having a 6 satellite constellation to observe. Using 6 satellites scattered over various points in the sky reduces a lot of error from the 3D geometry, itself. Once you add in the precise locations of those satellites from the ephemerides you end up with a very precise reference from which to calculate your one point --- 900 (x2) observations per satellite which yields 5400 (x2) lines from which you derive your position. Add in the second point (always located on a known control point) for atmospheric corrections and it's not hard to get ±5 mm for a local CP. High order marks require 4 hours of observation on two separate days for a total of 14400 (x2) observations per satellite with the (required in this case) 6 satellites yields 86400 (x2) lines for calculations. Easy to see ±1 mm accuracy from that much data over 8 hours of observations.



PS
My mistake. High order observations are taken in 4 hour sessions on three separate days, not two. It's been a few years since I helped MoDNR/NGS update our local high-order control points. Writing about it brought back memories ...

 

[Lord of Planar]

I think you may have also neglected to account for multiple satellites. Minimum requirements are four satellites for N-E-El (can't trick the rules of geometry!) but surveyors seldom take measurements without having a 6 satellite constellation to observe.

I do understand this. The differential in time if the satellite signals require a minimum of four signals to get a 3 axis position.

 

[Tim the plumber]

MoSurveyor,

I understand what you are saying but;

If a large part of the earth moves up, say the Kamchatka Peninsula, and there are no reference points there for the GPS system the gravitational center of the earth has just altered. The oceans will move to get the new balance point. If the sea bed moves up the sea level will rise. If the reference point near the sea shore moves down the sea level will rise.

The system has to be useful for practical purposes. If the position and altitude of the site you were surveying was constantly changing making it impossible to say what the position of it was from day to day then the system would not be useful to yourself as a surveyor. It has been set up to be practically useful not as an academic reference study of the changing shape of the earth.

 

[MoSurveyor]

MoSurveyor,

I understand what you are saying but;

If a large part of the earth moves up, say the Kamchatka Peninsula, and there are no reference points there for the GPS system the gravitational center of the earth has just altered. The oceans will move to get the new balance point. If the sea bed moves up the sea level will rise. If the reference point near the sea shore moves down the sea level will rise.

As I've already stated, any significant alteration of the gravitational center of the earth would be detectable either through irregularities in satellite orbits and/or opposite movement of points on opposite sides of the earth.

Look, let's take a simple scenario and say the earth's gravitational center moved 3 feet directly (axial) north. The north pole would "rise" 3' and the south pole would "drop" that same 3' and neither would have a positional change. All points on the equator would "move" north 3' with no change in elevation. Points at various places in between would both move and change elevation. Taking in the "sudden displacement" of ALL the points in the system, we can determine exactly how far and in what direction the center moved - and that's even before "confirmation" from satellite movement because, as I said earlier, the satellites won't move as suddenly, it'll take time for their orbit to stabilize.

The system has to be useful for practical purposes. If the position and altitude of the site you were surveying was constantly changing making it impossible to say what the position of it was from day to day then the system would not be useful to yourself as a surveyor. It has been set up to be practically useful not as an academic reference study of the changing shape of the earth.

Surveyors constantly measure the changing shape of the earth, that's our job. As I've already noted, the system even picks up the drift of the continents. How much more "changing shape" do you need?

 

[Tim the plumber]

You have much more faith in the system than my friend who has just retired from the British army. He was in the surveyors, making and using maps for artillery and guided missiles.

 

[MoSurveyor]

You have much more faith in the system than my friend who has just retired from the British army. He was in the surveyors, making and using maps for artillery and guided missiles.

I spent 25 years surveying, the last 20 as a professional land surveyor using GPS. I also have a life-long friend who's worked for Uncle Sam for over 30 years making maps for NGS (and the military, too, the last 15 years since the offices were combined). Cartographers, at least US government cartographers, know very little about surveying. Maybe the UK is different for some reason, though I can't imagine why that would be the case.


PS
I doubt artillery and guided missiles require ±0.1 feet precision (or 1:20,000 error), which is what urban land corners have to be in most states. Not putting down the British military - I'm sure their work is excellent within their requirements. I just doubt those requirements are as stringent as land corners. If a missile is off a whole foot, even, who'd know? I mean, where do they measure from, the tip of the nose, the rearmost point, the center of the payload, somewhere else? If a building is a foot onto someone else's land, well, now you've got real problems.

 

[Tim the plumber]

15 years ago, or so, he was in Norway doing the survey work for some air bases.

The AA equipment was to be moved to a new position every 3 hours in the event of war so that the Russians would not have time to get back with a photo and work out where to bomb to kill the SAMs.

Each of the many prepared locations needed a post in the ground which had it's location on it. Included in this information was the date it was done and the speed of continental drift to the mm so that the battery would be able to integrate it's fire control with the whole base's defenses.

Apparently the information is now out of date as it's not accurate enough for the latest systems which need to communicate with each other continuously about their position.

The degree of noise which would result in flooding people like yourself with information other than the location of your spot relative to some relatively local "fixed point" would be vast. It's also not useful as it would still not be enough. How do we know how all of the sea bed has moved? GPS is fantastic for what it is. It is not, however, a measurement of all of the earth's shape to the mm. And that's what you need to measure sea volume to the mm in that way.

There is a good way around it. Day length. We have been measuring that very well for some time. If ice melts at the poles it will add to the oceans. The mass of the water will not be near the poles it will be around the whole globe. That will slow the rotation of the earth and make the day a little longer. Day length shows no sign of such a change. Which is not surprising given that one of the other threads was talking about an annual melt from Greenland of 4 cubic kilometers.

 

[sedan]

​

 

[Earthling]

Geodesists can predict sea level to within one metre.That's comforting. :mrgreen:

 

[Lord of Planar]

Geodesists can predict sea level to within one metre.That's comforting. :mrgreen:

Once they have a baseline for what the level is at a particular latitude and longitude, increases and decreases can be based on that.

 

[Earthling]

Once they have a baseline for what the level is at a particular latitude and longitude, increases and decreases can be based on that.

That's comforting too. :mrgreen:

 

[Tim the plumber]

​

Yep, and when the mountain moves up what happens to sea level? To the foot is OK but to the mm?!!

 

[longview]

The video actually had some good information.
The reality (not so Scientific but practical) is that sea level only matters, where it meets the shore.
While it is likely useful to know the theoretical sea level anywhere in the world,
The practical side of things wants to know if port facilities around the world are in danger.
Thankfully we have shore based tide gauges, which are more accurate than satellites,
which tell us, we are good for several centuries at the current rates of rise.
I suspect we will be off organic hydrocarbons before 2100, because they have more value than to be used as fuel.

 

[Earthling]

I suspect we will be off organic hydrocarbons before 2100, ...

That's a racing certainty and I've been telling catastrophists this for years, but they don't listen.

Our streets aren't ten feet deep in horse manure ......

 

[Tim the plumber]

That's a racing certainty and I've been telling catastrophists this for years, but they don't listen.

Our streets aren't ten feet deep in horse manure ......

I think it's actually a response to the whole idea of change.

For those of us who kind of like new stuff that's not much of an emotion laden word, but for the people who like to know that tomorrow will be the same as today then it's a huge threat that there might be any change at all. Just the idea.

Climate change must therefore be stopped! Technological change is unthinkable unless you know that somebody knows exactly what it is. etc..

 

[Earthling]

I think it's actually a response to the whole idea of change.

For those of us who kind of like new stuff that's not much of an emotion laden word, but for the people who like to know that tomorrow will be the same as today then it's a huge threat that there might be any change at all. Just the idea.

Climate change must therefore be stopped! Technological change is unthinkable unless you know that somebody knows exactly what it is. etc..

I've seen a lot of change in my (days away from) 73 years.
There were still quite a lot of horse drawn vehicles around when I was young, so either I'm just an old fogey who doesn't know what he's talking about, or I'm a visionary, because I know for a fact that the "business as usual" phrase spouted by catastrophists is garbage, there's no such thing, nothing stays the same, and by 2100 the ICE will be a relic of a bygone age.

 

[flogger]

I've seen a lot of change in my (days away from) 73 years.
There were still quite a lot of horse drawn vehicles around when I was young, so either I'm just an old fogey who doesn't know what he's talking about, or I'm a visionary, because I know for a fact that the "business as usual" phrase spouted by catastrophists is garbage, there's no such thing, nothing stays the same, and by 2100 the ICE will be a relic of a bygone age.

I don't doubt you are right. I seriously doubt the AGW hypothesis but I have no doubt that whatever climate does transpire humanity will have the ingenuity to adapt to it. What concerns me most today though is the current environmentalist mission to deny us the resources with which to do so because of their anti human/aspirational/progress stance.

I'm tired of the decades of lecturing and indoctrination by these self righteous asses about how evil I am for wanting a better life for myself and my family. Our species is not some kind of plague needing suppressed because our planet is deemed to precious and fragile for it to exist upon it. We are a legitimate part of the biosphere too