As several have stated above, the [actual] gravitational anomalies that we are [really] talking about, are subtle [almost] to the extreme.
While we "plot" these various maps, images, graphical expressions of what the various geoid models 'look like' in the theoretical (big picture) sense, it sometimes tends to inflate our perception of what we are really talking about.
However, WE should have a good understanding (feel) for the effects of these undulations in our area, AND (more importantly) just how much 'they' may or may not affect our work.
ALSO...even Stability A Bench Marks 'can' move, so blindly tweaking your data to physical points that you haven't VERIFIED as being "valid," is not a good idea in general. Mixing First Order USC&GS/NGS Bench Marks, with USGS Third Order Bench Marks, with "COE, BOR, County, City, whoever, Bench Marks is a recipe for disaster! If things don't FIT the Geoid Model "reasonably well," then you might want to dig a little deeper...
By the same token, don't blindly adopt a modeled value either.
Loyal
Bill93, post: 369278, member: 87 wrote: I doubt that you could generate a 3 degree tilt in a local area with just the difference in density of ordinary rocks. The inclination of the vertical is usually seconds, to maybe minutes when you are near mountains, so degrees would require a an impossibly huge contrast in materials.
I've seen how easy it is to fool the eye with terrain. Driving through the eastern mountains I found several instances where I thought I was going downhill until I noticed how hard the engine was working or uphill but was really gaining speed. As a flatlander who rarely changes elevation more than 300 ft, I had to train myself to watch the speedometer more closely and not rely on the audio and visual cues to control my accelerator foot.
That's true. I hadn't considered the math on that until I posted it, but you are probably very correct. The side of mountains induce about a 30" deflection, if I remember correctly? So yeah, the 3å¡ thing is probably greatly exaggerated. Mea culpa.
Loyal, post: 369305, member: 228 wrote: As several have stated above, the [actual] gravitational anomalies that we are [really] talking about, are subtle [almost] to the extreme.
While we "plot" these various maps, images, graphical expressions of what the various geoid models 'look like' in the theoretical (big picture) sense, it sometimes tends to inflate our perception of what we are really talking about.
However, WE should have a good understanding (feel) for the effects of these undulations in our area, AND (more importantly) just how much 'they' may or may not affect our work.
ALSO...even Stability A Bench Marks 'can' move, so blindly tweaking your data to physical points that you haven't VERIFIED as being "valid," is not a good idea in general. Mixing First Order USC&GS/NGS Bench Marks, with USGS Third Order Bench Marks, with "COE, BOR, County, City, whoever, Bench Marks is a recipe for disaster! If things don't FIT the Geoid Model "reasonably well," then you might want to dig a little deeper...
By the same token, don't blindly adopt a modeled value either.
Loyal
"even Stability A Bench Marks 'can' move"
Everything is moving! Between tectonic movement, subsidence, post-glacial rebound, the effects of the solid/liquid water distribution on the planet and who know's what else there is no such thing as a "fixed point." That's the whole purpose for the development of the new 2022 redefinition and GRAV-D. We have 1st order benchmarks here that have moved 18' vertically in 34 years! Considering how long it takes just to get an environmental clearance on some projects design plans more than a few years old are going to be very difficult to deal with.
MightyMoe, post: 369261, member: 700 wrote: It would be easy to check, they probably wouldn't let you do it.
The Mystery Spot literature invites people to bring their own instruments and make their own findings. I don't know if anyone has ever taken them up on it.
Jim in AZ, post: 369327, member: 249 wrote: "even Stability A Bench Marks 'can' move"
Everything is moving! Between tectonic movement, subsidence, post-glacial rebound, the effects of the solid/liquid water distribution on the planet and who know's what else there is no such thing as a "fixed point." That's the whole purpose for the development of the new 2022 redefinition and GRAV-D. We have 1st order benchmarks here that have moved 18' vertically in 34 years! Considering how long it takes just to get an environmental clearance on some projects design plans more than a few years old are going to be very difficult to deal with.
Boy, ain't that the truth!
Not only that, but I have seen many (somewhat isolated) TOWNS around the Great Basin, in which ALL of the infrastructure is based on the Elevation STAMPED into the 190? Bench Mark in the Court House Steps (or wall). For all practical purposes, a "mini-Datum."
Loyal
Jim Frame, post: 369328, member: 10 wrote: The Mystery Spot literature invites people to bring their own instruments and make their own findings. I don't know if anyone has ever taken them up on it.
Hey, a good level, a GPS, you could determine your own Geoid heights and plot up a map,,,,,,prove or disprove it right there:-)
MightyMoe, post: 369351, member: 700 wrote: Hey, a good level, a GPS, you could determine your own Geoid heights and plot up a map,,,,,,prove or disprove it right there:-)
You won't be doing any good with GPS there, it's all redwood trees.
Um, by the way ... WGS84 is NOT a projection. Not even close. Whopping difference.
As Mr. Steven Estopinal likes to quote the late Will Rogers; "It's not what we don't know is the problem, it's what we think we know and it just ain't so."
Bill93, post: 368856, member: 87 wrote: If you are talking about the handwritten numbers at the bottom of the pdf, they are less than 2 parts per million different. Is that significant in your project?
A likely reason for them to differ is the elevation factor which is used in calculating the combined factor. How well do the elevations match? Was there confusion between orthometric elevation and height above the ellipsoid? How well did the projection point scale factor compare between them?
EDIT: It looks like ellip-ortho does not match the discrepancy.
I note this is an ultra-rapid OPUS solution. I wouldn't pin anything critical on an ultra-rapid solution, but if possible wait a few hours for rapid or even the precise solution you can get in a couple weeks.
Hello Bill,
Thanks for getting back to me, I ran those again after 24 hours and it brought them up out of the ultra-rapid solution. The results were tighter to the observed control shots I took on the same points. The independent surveys helped determine what to hold. Thanks again for your help....I would like to be able to be here more often, a lot of work...Paul
Lee D, post: 368881, member: 7971 wrote: Not sure how you got the scale factor you wrote on there from TBC... here's what TBC reported for the scale values when I keyed that point in. I think the very slight difference from OPUS may be because I keyed it in as Global instead of Local (or Grid).
Hello Lee, Thanks for taking the time, I ran them again and compared them to yours, would like to have more time to be here,
in any event Thank You, Paul
