Thanks, Loyal.
"Two “points” with the same geopotential number, will NOT (necessarily) have the same Gravity Value."
Interesting. Can you elaborate on that? I'm still struggling with the concept of Equipotential.
Dave
Hi Dave,
I'm probably completely out to lunch, but here goes.
I think a geoid affects a raw GPS measurement not at all, or only minimally, because the GPS satellites are working with the ellipsoidal model of the earth. Now a GPS device's geoid model will affect the calculated elevations.
I don't think geoid12a is a TIN. It's possible that they created one, but the final product is a regularly gridded surface. Based on some statements, and going back to the geoid09 acknowledgements, etc., data from NGA (who knows), NGS (OPUS/data sheets/GPSBM), USGS (NED), NASA (SRTM) plus Natural Resources Canada were crunched to come up with the final model.
The geoid09 pages report using multi-matrix least squares collocation. Yeah, anyway, here's a link to a presentation (840kb) by Dan Roman (fellow OSU alum) on geoids, although it's from 2006.
Melita
Thanks All!
I greatly appreciate all your replies. It's helped me to better understand this formerly impenetrable subject.
I love this forum!
Dave
😀
Pretty technical explanation, I know, but we are surveyors after all.
Melita,
That exactly answers one of my questions. Thanks for the link.
Dave
> Hi Dave,
>
> I'm probably completely out to lunch, but here goes.
>
> I think a geoid affects a raw GPS measurement not at all, or only minimally, because the GPS satellites are working with the ellipsoidal model of the earth. Now a GPS device's geoid model will affect the calculated elevations.
>
> I don't think geoid12a is a TIN. It's possible that they created one, but the final product is a regularly gridded surface. Based on some statements, and going back to the geoid09 acknowledgements, etc., data from NGA (who knows), NGS (OPUS/data sheets/GPSBM), USGS (NED), NASA (SRTM) plus Natural Resources Canada were crunched to come up with the final model.
>
> The geoid09 pages report using multi-matrix least squares collocation. Yeah, anyway, here's a link to a presentation (840kb) by Dan Roman (fellow OSU alum) on geoids, although it's from 2006.
>
> Melita
:good: :good:
Ralph, thanks for the good link. I was thinking gravity measurements were taken on the ground. In an airplane you could sure cover a lot more earth.
Dave
Here is an explanation I hear once (paraphrased).
If I hold an object (say a ball) in my hand, it has "potential energy". (ie: when I let go of it, it will fall). That amount of P.E. is measurable. If I hold that ball higher it will have a different measurment P.E. If I take that ball and move it around at the same measured P.E. value, it will be in an "equipotential" field (plane sort of, except that plan is not straight like a mathematical plane).
That field is perpendicular to the plumb line. That plumb line might be angled to a simple ellipsoid if I am near a mountain, or if the ground underneath me is pulling the gravity in a different direction.
My apologies if my explanation is not very accurate, and I am sure anything I said wrong will be corrected here.
Anyway I hope that helped.
> Ralph, thanks for the good link. I was thinking gravity measurements were taken on the ground. In an airplane you could sure cover a lot more earth.
>
> Dave
Yes Dave, I think satellites are involved and also believe the Space Shuttle was involved at one point also.
Cheers
Thanks, Tom.
In my reading, someone said that the plumb line to an equipotential surface is not a straight line, but curved. I'm still mulling that one over. :-S
Dave
I think the line is constantly perpendicular to the equipotential surfaces, When the equipotentials aren't parallel the plumb line has to curve as it goes.
Tom,
I even saw a graphic representation of the curved normal, and I still don't understand it. Still mulling. :-S
Dave
There are some videos on YouTube where Bill Henning outlines which geoid to use with which epoch of NAD83.
http://m.youtube.com/user/LSUC4G
This thread may be better suited to the GNSS/Geodesy category.
It might help solicit more responses.
> Tom,
>
> I even saw a graphic representation of the curved normal, and I still don't understand it. Still mulling. :-S
>
> Dave
here's how I understand it...Gravity is a balance between the gravitational force pulling something down counteracted by centrigugal force (the spin of the earth) trying to throw it outwards. At the equator the centrifugal force is at it's maximum and at the poles at it's minimum. So for any given height, the force of gravity is greater at the poles (less Centrigugal force to counteract it) than at the equator.
This bends an equipotential surface (which is a surface of equal gravity, so it has no choice) as it gets closer to the pole. So the infinite number of equipotential surfaces are converging and if a plumb line has to be perpendicular to every equipotential surface it passes through (again, it has no choice) then the result is a curve.
Two points with the same orthometric height, one much farther north or south than the other are not on the same equipotential surface. This is the difference between orthoheights and dynamic heights.
A large, calm lake, extending north/south has the same dynamic height at each end but would have a slightly (lower?) orthometric height at the end nearer the pole.
God I hope I'm right...!
Thanks for the link, Kevin! Those 3 vids look good!
Dave
Colin,
"So the infinite number of equipotential surfaces are converging"
The light bulb comes on! I was just thinking about ONE equipotential surface, the Geoid. You're right, there's actually an infinite number of equipotential surfaces and they would be converging. An infinite number of "bent" plumb lines would be a curve.
Thank you.
Dave
Hi Kevin,
I thought about that when I started the thread, but I decided the subject was too broadly general to slot into GNSS. I'm not sure anyone really uses the categories, anyway. That in itself would be a good question to ask forum members.
Dave
I do. I set my preferences to only display the categories I am interested in.
Thanks All!
I don't recall seeing anyone mention about tidal and MSL stuff.
I attended a few webinars presented by Dave Doyle et. al. about exactly this stuff and how it really does make a difference.
I would suggest looking that up to see if you can find it in the archives. I've been out of surveying for years and barely knew anything then. But, the math-head and physics-head in me got my attention.
If nothing else, it was terribly interesting stuff for me - plus I got many hours of credits and a few certifications out of it. I'm not licensed in anything but found it all terribly interesting.
E.