Questions about the geoid (Split from Lake Balaton thread)

[Mordred]

good break down Studiot.

 

On survey equipment

1) is done internally. Base calculations.

 

2) requires the maps for localized corrections. Though autocorrection software can help. Coupled with various level sensors etc.

 

3)is eliminated by methodology, control points and multiple measurement.

Edited October 1, 2016 by Mordred

[maximillian12]

Could I take it right back to the beginning?

 

In the beginning studiot said that, if the bottom of the lake was flat, the surface of the lake could also be flat.

 

Does studiot stand by that? In the above post it seems that he is now saying something different:

 

"If the bottom is flat the water surface cannot be different from flat by more than the depth."

That makes sense to me - though wouldn't it also be true that the place where the "difference from flat" is equal to the "depth" would be called "the shore"?

 

It does seem that evidence has been presented to show that the shape of the ocean floor does affect the shape of the ocean surface. But it doesn't seem that evidence has been presented to show that a shallow lake which is flat on the bottom would have a surface whose shape is in any way influenced by that.

[studiot]

Could I take it right back to the beginning?

 

In the beginning studiot said that, if the bottom of the lake was flat, the surface of the lake could also be flat.

 

Does studiot stand by that? In the above post it seems that he is now saying something different:

 

"If the bottom is flat the water surface cannot be different from flat by more than the depth."

That makes sense to me - though wouldn't it also be true that the place where the "difference from flat" is equal to the "depth" would be called "the shore"?

 

It does seem that evidence has been presented to show that the shape of the ocean floor does affect the shape of the ocean surface. But it doesn't seem that evidence has been presented to show that a shallow lake which is flat on the bottom would have a surface whose shape is in any way influenced by that.

 

Are you posting this in the right thread?

 

Local surveys of the lake should surely be discussed in the lake Balaton thread?

 

However I did answer your questions in post#19 here in this thread.

 

Was there something there you did not follow?

 

 

Remember that my objective was to do a 'back of an envelope' estimate of how much the water surface might vary so design a suitable survey method.

 

This estimate suggests that gravitational effects will affect the surface by less than 20mm.

[Neil Obstat]

On 10/5/2016 at 11:38 AM, studiot said:

Are you posting this in the right thread?

Local surveys of the lake should surely be discussed in the lake Balaton thread?

The lake Balaton thread is locked as of April Fools' Day, 2017!

Quote

Posted April 1, 2017 

Moderator Note

OK, that's enough

 

 

Why has this simple question been left unanswered for nearly two years? 

"However I did answer your questions in post  #19 here in this thread." (from post immediately above this one)

I read your post #19 and did not see any answer to this question, 

On 10/5/2016 at 11:38 AM, studiot said:

But it doesn't seem that evidence has been presented to show that a shallow lake which is flat on the bottom would have a surface whose shape is in any way influenced by that.

(Actually, that was maximillian12 asking that question but the system inserts the name of studiot apparently because I quoted it from his post which was a quote of maximillian12)

Perhaps it would help if you could provide some links to sites where this is described, either as an anomaly or as an effect of the surface of a shallow body of water mirroring the contour of the earth's surface that forms the bottom of the body of water, whether it's a shallow lake or a shallow portion of the ocean? Doesn't have to be lake Balaton. Pick some other place you're familiar with, if you prefer that. (The depth of Balaton is something like 4 meters average, apparently, not enough for larger boats with much draft.)

It seems to me that there could be some confusion coming from understanding how the surface of a body of water can be measured in the first place. It is a surface in motion, so how do for example satellites measure the distance to a moving surface like that to establish its elevation in reference to an ellipsoid or a geoid? Influences like currents, waves and air pressure to be removed would mean that the very things that are causing the surface to change would have to be somehow deducted. This seems to be rather impossible to achieve with any accuracy. Two different scientists could come up with two entirely different sets of data and who would know which is closer to the truth? And in any case, the conclusions they arrive at would not be applicable to the "present state" of that same body of water because water is constantly changing. What stops them from saying anything they want to say? There would be no physical evidence left of where the surface level of the water was at a previous time, except for the very data that is under question in the first place. 

Edited June 15, 2018 by Neil Obstat

[studiot]

13 hours ago, Neil Obstat said:

The lake Balaton thread is locked as of April Fools' Day, 2017!

So what was your point (especially as it apparently warranted an exclamation mark) ?

 

13 hours ago, Neil Obstat said:

(Actually, that was maximillian12 asking that question but the system inserts the name of studiot apparently because I quoted it from his post which was a quote of maximillian12)

I encounter this problem and worse with the poorly designed entry text box this forum has.

My solution is to open quote marks, then copy and paste into the quote box, preceding the paste with an acknowledgement to the true author.

 

13 hours ago, Neil Obstat said:

Perhaps it would help if you could provide some links to sites where this is described, either as an anomaly or as an effect of the surface of a shallow body of water mirroring the contour of the earth's surface that forms the bottom of the body of water, whether it's a shallow lake or a shallow portion of the ocean? Doesn't have to be lake Balaton. Pick some other place you're familiar with, if you prefer that. (The depth of Balaton is something like 4 meters average, apparently, not enough for larger boats with much draft.)

It seems to me that there could be some confusion coming from understanding how the surface of a body of water can be measured in the first place. It is a surface in motion, so how do for example satellites measure the distance to a moving surface like that to establish its elevation in reference to an ellipsoid or a geoid? Influences like currents, waves and air pressure to be removed would mean that the very things that are causing the surface to change would have to be somehow deducted. This seems to be rather impossible to achieve with any accuracy. Two different scientists could come up with two entirely different sets of data and who would know which is closer to the truth? And in any case, the conclusions they arrive at would not be applicable to the "present state" of that same body of water because water is constantly changing. What stops them from saying anything they want to say? There would be no physical evidence left of where the surface level of the water was at a previous time, except for the very data that is under question in the first place. 

 

Since you may be asking for an answer to a question that was already discussed in detail please restate your question briefly and succinctly.

 

Note In the previous material I gavel some satellite figures which showed that 1000m of raised oceanic crust produces about 2m of change at the surface.

I don't know of any shallow water lake that has 1000m lumps on the lake floor.

The limits of the word 'flat' were never discussed, but I noted that the change was 1/500 of the vertical size of the bump in the floor.

So a 1m bump (a substantial bump in a nominally flat lake floor) would produce less than 2mm of effect.

I think this is beyond current technology to measure.

 

Have you read the full preceeding material ?

I realise it is more substantial than the average discussion.

 

Points to look for are

The difference between 'flat' and 'level'  - shown to be less than 20mm ie at least 10 times that of the bottom influence.

The discussion of lake hydraulics and its influence on the water surface.

The critique of the experimental method.

[Neil Obstat]

Hi, studiot. Thanks for the response. I'm just now seeing it.

I was afraid a mod might delete my post for being "off topic" or whatever. But I have questions regarding the contents of this thread and thought it would be nice to keep it all in the same place, is all.

It was a while ago when I wrote that and as I recall I came to new insight AS I was typing the post! Sometimes I have to read and re-read posts even on subsequent days to be sure I'm not missing a hidden meaning because lots of posts are made with one or more typos, grammatical errors or whatever which can change how the final product turns out. I wasn't sure if you were saying that a one-thousand meter tall hill or bulge (a rise) on the ocean floor produces a 2 meter rise or a 2 meter dip in the ocean's surface. I'm still not sure which it is. A "change" or "effect" could go either way. A mound of rock or sediment, on the bottom, would have more density than surrounding water, so that should cause increased gravitational attraction making water COLLECT there, I would think. Is that true? Or would it pull the water down making the surface DIP? I'm trying to be brief without being vague.

If the reality is the former, which I suspect is the case, then a so-called flat lake bottom could be one that is gently concave, that is, not really following the ellipsoid very well, which might cause the water surface to be less convex, as the earth's curvature would normally have it, but this effect could be too small for it to have any significant effect on the surface of a lake. What do you think of that idea?

As for the sea's surface (a sea could be smaller than a very large lake, such as the Dead Sea (605 km²), which is smaller than Lake Erie (25,700 km²)), since a body of water is constantly moving (little ripples, waves, undulations), the measurement of the elevation of any water surface must be an enormous problem, especially under agitated conditions like during a storm. It would make sense if some kind of floating GPS receiver would be required so that 4 satellites could send ephemerides to it which could be processed for elevation data which could then be relayed to a nearby monitoring station (since GPS satellites do not RECEIVE information from ground receivers). That would require a battery, and maintenance. Sounds expensive. But I have not seen that described anywhere. Have you? How else could the elevation profile of a large body of water be mapped out? Are there specially-designed satellites capable of receiving data from such a fluid surface as is water? 

I have read about the difference between "flat" and "level." Some forum members have been very helpful with their diagrams. Level, as in the local plane, tangent to a plumb line through the local geoid, is a relative term that only applies to a small zone, and when combined with other nearby zones effectively copies the geoid itself. Any ONE of those local planes are flat, but over a large area, the most you can ask for is an AVERAGE plane in regards to "flatness." Such an average plane would for example, cut right through a mountain or span above a valley.

I have studied a little in water hydraulics and open channel flow, so I understand how currents can affect the surface of moving water. The center or midstream of an open channel is lower in elevation than the extremities, depending on depth, bottom contour, channel width, and velocity of flow. A lot of factors to keep track of.

Finally, I haven't seen anything here on "critique of the experimental method." Perhaps you can point me in the right direction? 

Thanks again for your help!

 

Edited June 26, 2018 by Neil Obstat

[studiot]

29 minutes ago, Neil Obstat said:

Hi, studiot. Thanks for the response. I'm just now seeing it.

I was afraid a mod might delete my post for being "off topic" or whatever. But I have questions regarding the contents of this thread and thought it would be nice to keep it all in the same place, is all.

It was a while ago when I wrote that and as I recall I came to new insight AS I was typing the post! Sometimes I have to read and re-read posts even on subsequent days to be sure I'm not missing a hidden meaning because lots of posts are made with one or more typos, grammatical errors or whatever which can change how the final product turns out. I wasn't sure if you were saying that a one-thousand meter tall hill or bulge (a rise) on the ocean floor produces a 2 meter rise or a 2 meter dip in the ocean's surface. I'm still not sure which it is. A "change" or "effect" could go either way. A mound of rock or sediment, on the bottom, would have more density than surrounding water, so that should cause increased gravitational attraction making water COLLECT there, I would think. Is that true? Or would it pull the water down making the surface DIP? I'm trying to be brief without being vague.

If the reality is the former, which I suspect is the case, then a so-called flat lake bottom could be one that is gently concave, that is, not really following the ellipsoid very well, which might cause the water surface to be less convex, as the earth's curvature would normally have it, but this effect could be too small for it to have any significant effect on the surface of a lake. What do you think of that idea?

As for the sea's surface (a sea could be smaller than a very large lake, such as the Dead Sea (605 km²), which is smaller than Lake Erie (25,700 km²)), since a body of water is constantly moving (little ripples, waves, undulations), the measurement of the elevation of any water surface must be an enormous problem, especially under agitated conditions like during a storm. It would make sense if some kind of floating GPS receiver would be required so that 4 satellites could send ephemerides to it which could be processed for elevation data which could then be relayed to a nearby monitoring station (since GPS satellites do not RECEIVE information from ground receivers). That would require a battery, and maintenance. Sounds expensive. But I have not seen that described anywhere. Have you? How else could the elevation profile of a large body of water be mapped out? Are there specially-designed satellites capable of receiving data from such a fluid surface as is water? 

I have read about the difference between "flat" and "level." Some forum members have been very helpful with their diagrams. Level, as in the local plane, tangent to a plumb line through the local geoid, is a relative term that only applies to a small zone, and when combined with other nearby zones effectively copies the geoid itself. Any ONE of those local planes are flat, but over a large area, the most you can ask for is an AVERAGE plane in regards to "flatness." Such an average plane would for example, cut right through a mountain or span above a valley.

I have studied a little in water hydraulics and open channel flow, so I understand how currents can affect the surface of moving water. The center or midstream of an open channel is lower in elevation than the extremities, depending on depth, bottom contour, channel width, and velocity of flow. A lot of factors to keep track of.

Finally, I haven't seen anything here on "critique of the experimental method." Perhaps you can point me in the right direction? 

Thanks again for your help!

 

Hi Neil

I'm sorry I thought the original parent thread would be easily referenced in this split off one but I had to dig to find it.

 

The missing material is here, including the satellite maps etc.

 

https://www.scienceforums.net/topic/98386-laser-curvature-test-on-lake-balaton/?page=9

 

I'm sorry the ridiculous new forum software no long numbers posts so referring to individual ones is now a nightmare.

 

[Neil Obstat]

2 hours ago, studiot said:

The missing material is here, including the satellite maps etc.

I found no mention of "satellite maps" anywhere in that thread. Did you post the wrong link?

 

[studiot]

12 hours ago, Neil Obstat said:

I found no mention of "satellite maps" anywhere in that thread. Did you post the wrong link?

 

I went back through it and can't find the maps themselves either, although the link to Seasat was there.

I could have sworn I included the maps themselves but perhaps it was in another thread.

But I can't find them there either finding anything on this benighted site is a real pain.

So I will look out the data for you.

[studiot]

Here are the copies of the Seasat data.