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Bill93

Favorite Tutorials on Elevation and Gravity?

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I have again been trying to understand what elevation means. Probably not one person in 100,000, and very few professional surveyors, understand the answers to these questions:

 

What does Orthometric mean?

Why wouldn't a large loop of perfect optical leveling measurements come back to the same elevation at the starting point?

Dynamic height?

What is the geoid?

Why isn't gravitational force the same everywhere on the geoid?

How do you find the geoid under a land mass?

Bouguer plate?

What approximations are made in the definition of NAVD88?

Does water flow uphill some places? (That comes back to the definition and measurement of "uphill.")

 

I have _Physical Geodesy_ by Hoffman-Wellenhof and Moritz. I can almost read it with what college calculus and vector math I remember, but they give almost no examples so I'm not always sure I know the implications.

 

I have Elementary Surveying by Wolf and Ghilani, but their chapter on the subject is a cut and paste of some relevant material that is not complete enough for continuity.

 

A brief search has not found the ideal tutorial, although I did find some helpful stuff including a powerpoint by someone at NGS. Maybe what I'm looking for is a college sophomore level explanation of a graduate level subject.

 

So does anybody have any recommendations?

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Bill, I had saved the following link in our benchmarking 101 forum in our local geocaching forum a few years back. It is from the NOAA and has all kind of easy to understand readings. It states that the lessons are oriented for students in the 9-12th grade range.

 

Hoever, it didn't say if that was American schools or Japanese schools. LOL

 

http://oceanservice.noaa.gov/education/tutorial_geodesy/lessons/global_pos_tutor.pdf

Edited by LSUFan

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A concise (perhaps too concise) summary of orthometric, dynamic, geoidal, and ellipsoidal heights is on page 75 of "Introduction to Geodesy" by James R. Smith.

Edited by holograph

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A nice overview, good pictures, but it doesn't tell you how to calculate anything.

 

I'm going to check out some of these and other things found by search on the NGS site:

http://www.ngs.noaa.gov/PUBS_LIB/pub_vert.shtml

 

Side comment: Interestingly enough, they say NAVD88 is tied to one benchmark TY5255 but you can't get a data sheet for it.

Edited by Bill93

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A nice overview, good pictures, but it doesn't tell you how to calculate anything.

 

I'm going to check out some of these:

http://www.ngs.noaa.gov/PUBS_LIB/pub_vert.shtml

 

What are you hoping to calculate? As far as I know, the various heights are measured, or calculated through the models that the NGS has. I'm not sure there is any kind of reasonable do-it-yourself calculation, other than making use of the NGS models and tools.

 

For instance, you asked "How do you find the geoid under a land mass?" I think one answer is "first, you launch some satellites, then you build a network of laser and doppler ranging stations that can precisely measure the paths of the satellites, and you then run some computer models on supercomputers to calculate the location of the geoid, taking into consideration tides and seasons."

 

Or another kind of answer would be "you look at the datasheet to see the geoidal height" or "you use the NGS geoid model tools".

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There are lots of interesting things going on behind the data sheets, a few of which a user might need to understand.

 

Foremost of those would be the Orthometric Correction. As they processed leveling measurements decades ago (no GPS), they had to take into account the fact that perfect optical leveling around a loop of many miles will not close on itself. You need the orthometric correction to compensate for the variations in gravity. Working surveyors/engineers need to use that correction in their current work when a project has a large north-south extent or a large change in elevation.

 

Furthermore, some theory behind what elevations mean is very relevant to understanding the current GRAV-D project that will end up, not with just more accurate vertical values, but a different kind of vertical datum.

 

I found a tutorial that would be quite good if it didn't have so many confusions in it, like unlabeled changes in units, different symbols in the text and the equations, and a few misused terms.

http://surveying.wb.psu.edu/sur351/Chapter6/c6-new.htm

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On a lighter note you can filter a dat file for stations with less than 1 meter difference between orthometric and geoid heights - find those stations and stand on or touch a mathematical construct - the ellipsoid. how cool is that. a project i've been itching to do.

Edited by billwallace

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