Just an eyeball look at the two different combined scale factors shows 1 unit difference in the 7th place; that roughly translates to 2 feet in the elevation factor.
Shawn Billings, post: 369241, member: 6521 wrote: The gravitational anomaly is real though.
You seem convinced. Can you point me to any reliable source material in support? Everything I've ever read that seems remotely reliable points in the other direction (that's kind of a pun).
P.S. I've been to Mystery Spot twice, once as a child and once as an adult. The most recent visit didn't offer me anything to suggest that it's more than a clever tourist attraction, but I didn't have any equipment with me to actually test anything.
That West Coast stuff is all fake. Here's the real one: http://www.mysteryhill-nc.com/
Jim Frame, post: 369245, member: 10 wrote: You seem convinced. Can you point me to any reliable source material in support? Everything I've ever read that seems remotely reliable points in the other direction (that's kind of a pun).
P.S. I've been to Mystery Spot twice, once as a child and once as an adult. The most recent visit didn't offer me anything to suggest that it's more than a clever tourist attraction, but I didn't have any equipment with me to actually test anything.
Not reliable, no. I'd like to go there with some equipment myself.
I did observe the flagpole in the parking lot was not parallel with the runners on a hand rail near the shack. The hand rail wasn't part of the attraction, as far as I could tell and the difference between the runners and the flagpole was small but discernable.
The story goes that the "spot" was discovered by a survey party that traversed through the area (of course that didn't come from a reliable source, so take it for what it's worth).
The likelihood of gravitational anomalies seems pretty credible to me. It only requires a significant difference in mass in a small area compared to the surrounding area. No need for aliens or the supernatural.
Just bring a plumb bob.
It would be easy to check, they probably wouldn't let you do it.
Too far away from me to go back. Maybe we could have a go-fund-me account set up to send Jim back with a plumb bob and report back. 🙂
Shawn Billings, post: 369266, member: 6521 wrote: Too far away from me to go back. Maybe we could have a go-fund-me account set up to send Jim back with a plumb bob and report back. 🙂
you could just call them and say you would like to map their anomaly, if they say no way, you probably have the answer:whistle:
NWFL50, post: 369055, member: 11623 wrote: The Reason for 4 points is to be able to calculate error. Take a sheet of paper and lay it on a table. Now fix one point to the table. This is a one point localization. If you are lucky and the table is level then you will hit close to the rest of your points on the sheet and it appears correct. But what if the table is not level? Then your one point wont work. Two points still might not. What if the table is out east and west but not and south is level. well if the two points you localize on are also north and south, everything in between those two points will be good. The further east or west you go the worse they get. The only way to know you are right is to make sure all 4 corners of the sheet are correct. 3 corners will work but you need 4 to see your precision.
When you create a three point vertical calibration you are FORCING the creation of that plane to fit the measurements. So it's not like a sheet of paper, but more like a sheet of glass. Of course it will show Zero error, since there are only three points. The fourth will show the errors, and adjust the plane to a least squares solution to the observed control points. But it will still be fixed plane, with all the possible tilt errors others have discussed. A One point calibration fixes the plane to that control point and the plane has NO tilt.
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.
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
