Many of my GPSonBM submissions have been in big gaps between HARN and other shares.?ÿ But I noticed quite a change in the geoid models between two HARN stations, so did some measurements in that area.?ÿ I'm still not quite sure what it means, but I think things are off by a few cm.
Regardless of the specifics, there is a surprising slope to the geoid models in this area, and it flattens out a lot to the north.?ÿ I don't see any obvious landform features to explain that (terrain is flat or very gently rolling hills), but wonder if the limits of glaciation had something to do with compressing things underneath.
The mark at the 12 km point in the graph was tilted when its HARN data was taken, probably due to insufficient supporting soil on one side.?ÿ I estimate that the tilt would lower it by 2 cm after the old level run and there is the possibility that it could have settled as well.
The HARN on the right side didn't have anything to check it for 20 km or more all around.?ÿ It is a reset, so subject to some suspicion already from that.?ÿ I did a 1-hour OPUS-RS on it and came up 4 cm lower.?ÿ So I went to the next mark in the line and found that my measurements would give a similar slope but at significantly different height.?ÿ If I share those sessions and the HARN data is kept, it could put a kink in the geoid model, which would be worse than having the right slope at a slightly off elevation.
Would it be worthwhile for me to get longer sessions on the HARN point and share them, or would the HARN data be accepted over some yahoo's share data?
Slightly off the topic...
Have you tried extending your occupations and submitting whole as OPUS and split 1 hour chunks as OPUS-RS?
In our neck of the woods the heights routinely vary by several cm. The variations usually run 2 to 3 times the quality indicators. The implication being your comparisons may change with additional observations...
In addition to topography have you looked at the geology of your area?
I am not sure what NGS would do with submissions on what you indicate to be suspect or disturbed monuments. As you will see in the rejected lists associated with earlier hybrid geoid models, submitted data is subject to numerous statistical tests when generating models.
i tried to post the link rather than the map. Home page of source is:?ÿ http://www.iihr.uiowa.edu/blog/2016/01/05/geologic-maps-show-iowa-counties-what-lies-beneath/
See:?ÿ
Here (I hope) is a Gravity Map of Iowa, which definitely factors into the "shape/slope/etc." of the GEOID.
Loyal
Are you trying to create a GPS-Geoid from GPS and Leveling?
You're going to have to account for errors due to skew normals which may account for your vertical deviations.
?ÿ
Gps and leveling is what geoid models are; the imprecise measurments of GPS called heights corrected to the precise measurments of leveling called elevations.?ÿ
GPS and leveling contribute to the development and validation of the models, but gravity observations are the meat of the matter...
The 2022 geoid model will be the first one based solely on gravity and not tied to physical benchmarks.
The 2022 geoid model will be the first one based solely on gravity and not tied to physical benchmarks.
emmmmm...yeah, BUT that's not entirely true. It will be "tied" to the CORS Network (and thereby to the NSRS), and we will use GPS to "access & utilize" it (at least in the macro sense). It will still be necessary to run levels around "small" projects when (locally precise) Orthometric Heights are required.
Loyal
Lee, don't get me wrong here, your statement is essentially TRUE. The 2022 Vertical Datum will be in essence a Geoid Model based on a Gravity model, and legacy Bench Marks will not realize the Datum (locally) as they have in the past.
I probably didn't state that very well either here or?ÿin the post above. I sure that YOU know what I'm trying to get at, even though I haven't stated it very well.
Loyal?ÿ
You're going to have to account for errors due to skew normals which may account for your vertical deviations.
With a geoid slope of 2 cm per km and an elevation less than 300 meters, the offset in paths is 0.6 cm and the height correction is entirely negligible.
?ÿ
Here (I hope) is a Gravity Map of Iowa, which definitely factors into the "shape/slope/etc." of the GEOID
When I get home I'll plot these on the picture you posted, but eyeballing it I think I see that my line is about on a sharp transition from red to blue green.
I'm not exactly clear on what it represents-gravity at geoid or surface?
Is this when the party chief shows up to see everybody standing around bullshitting (on a very relevant topic, min you). The newest guy gets a bush axe handle pushed into his hand with the quick order of, ??cut down that tree.? No questions. Everyone gets the point. That conversation will continue, just some time, some place, else.?ÿ
Forgive my intrusion. This is a very interesting discussion but, I couldn??t get that scene out of my head. Another example of how vast and varied ??surveying?? is. ?ÿ?ÿ
Forgive my intrusion. This is a very interesting discussion but, I couldn??t get that scene out of my head. Another example of how vast and varied ??surveying?? is. ?ÿ?ÿ
Good comment, and all too true in many cases. HOWEVER, that attitude concerning the "science side" of surveying is no longer valid in MOST surveying situations anymore. The advent of EDMI technology was the beginning of the end, and GNSS put fork in it! Granted, many small/flat surveys can be done without worrying too much about geodesy, but using technology that one doesn't understand very well (or at all), is a recipe for disaster.
Just my opinion,
Loyal
I did a presentation last year on tech trends in Surveying. A good bit of it was dedicated to reminding us to know what our tools are doing. The EDM gave us bad info when slope reductions and target offsets were applied wrong. GPS gives crap data when you abuse localizations and ignore bad environments. Lidar returns are misleading when the surface sheen is wrong. Photo data with poor ground control doesnt work.
Every tool we've ever used and EVERY TOOL WE WILL EVER USE requires a certain amount of understanding. The more complex the tool, the broader and deeper the knowledge base required to run it. Surveying isn't for those who don't keep up. The exception is upper management, where little to no surveying happens anyway.
Sorry for the rant, but Loyal started it??ÿ
The dichotomy inherent in that scene is something I??ve been a part of. Both ends of the axe, to be sure. I survey solo (now) and I value knowledge over production. However, I sometimes keep moving ahead with strength in process and procedure, letting the understanding carry its own course. It generally gets there ahead of me.
When it doesn??t, I drift over to the RPLStoday and saddle up. If it gets deeper, I head to the books or the web. This topic will have me going to the web as I don??t think any of my books will do it.?ÿ
So, and I think yall got this, that scene wasn??t me cracking a whip. It was just something that comes to the collective conscious of surveying. It was an aside but, touched on the breadth of what we do/ can do.
Last note. I don??t know Bill (op). I am humbled by the mathematical, conceptual and survey specific knowledge he demonstrates and actively pursues (others here, as well). My apologies, again, for the intrusion. And, mostly, thank you all for sharing.?ÿ
In 2020 the new Geoid model will come out which will be based on gravity measurments, this model like recent ones should be simple enough for local surveyors to check as most will have good local level runs to compare it too. That of course is the important issue, will it model the earth so you can tell where water will flow, clearly the ellipsoid models fail terribly at this task and we need something to correct GPS to make it useful. Newer model have continued to improve, I'm hopeful that this one will work even better, but it will only be useful if it passes the level test.
In 2020 the new Geoid model will come out which will be based on gravity measurments, this model like recent ones should be simple enough for local surveyors to check as most will have good local level runs to compare it too. That of course is the important issue, will it model the earth so you can tell where water will flow, clearly the ellipsoid models fail terribly at this task and we need something to correct GPS to make it useful. Newer model have continued to improve, I'm hopeful that this one will work even better, but it will only be useful if it passes the level test.
Mighty,
Saying the elliosoid 'fails' at a task it was never intended to perform is a bit misleading. As Einstein said, 'Everyone is born a genius, but if you judge a fish on it's ability to climb a tree it will live its entire life believing itself a moron'.
Ellipsoid models are constructed as the basis for horizontal reference frames. Geoid models are applied to help these horizontal systems provide a somewhat useful vertical component. As we grow in understanding our dynamic earth, we also grow in the ways we mathematically express its many attributes.
I know you know this, but I couldn't resist the temptation to defend our friends the squished spheres and our current expressions of mathematical violence..
In 2020 the new Geoid model will come out which will be based on gravity measurments, this model like recent ones should be simple enough for local surveyors to check as most will have good local level runs to compare it too. That of course is the important issue, will it model the earth so you can tell where water will flow, clearly the ellipsoid models fail terribly at this task and we need something to correct GPS to make it useful. Newer model have continued to improve, I'm hopeful that this one will work even better, but it will only be useful if it passes the level test.
Mighty,
Saying the elliosoid 'fails' at a task it was never intended to perform is a bit misleading. As Einstein said, 'Everyone is born a genius, but if you judge a fish on it's ability to climb a tree it will live its entire life believing itself a moron'.
Ellipsoid models are constructed as the basis for horizontal reference frames. Geoid models are applied to help these horizontal systems provide a somewhat useful vertical component. As we grow in understanding our dynamic earth, we also grow in the ways we mathematically express its many attributes.
I know you know this, but I couldn't resist the temptation to defend our friends the squished spheres and our current expressions of mathematical violence..
I do feel it's important to say that partly because I've known surveyors that for whatever reason would survey elevations on the ellipsoid (or at least using the ellipsoid shape for elevations), oftentimes doing a simple add to a bench mark and then carrying on using the ellipsoid shape. Possibly other vendors didn't supply a good working Geoid model, and of course calibrations can get wonky real fast in our GPS environment when using bench marks over any large area.
I think of the Geoid as the shape of the earth, which is lumpy and shifting over time. The ellipsoid is a rough math representation of the earth's shape,?ÿbut it's not real, only valid for as you say a horizontal reference where you can over short time periods reproduce accurate measurements.
So the Geoid is real but unknown in a precise sense, the ellipsoid is unreal but can be known in a precise sense.
The accurate measurement of the Geoid up till now was done by the old-timers running bench marks, but they didn't have the horizontal location. Now adding the ellipsoid locations to the bench marks allows for a much better Geoid model, what Bill is doing should be applauded, he is preforming a wonderful service to this endeavor.
?ÿ
?ÿ
?ÿ
The line shown in my plot seems to fall near a region of transition on the picture Loyal posted, but it isn't on the steepest part.
I think I found the source of his picture.?ÿ People may want to see if USGS has similar info for other areas.
https://pubs.usgs.gov/ds/2005/135/ia_iso.htm
The picture appears to be neither at the surface nor ellipsoid (please forgive the dumb mention of geoid where I meant ellipsoid in my question above).?ÿ I don't understand all the geodesy terminology used.?ÿ The data sources in the references are from the 1970's.
The colors on this isostatic residual gravity (Hill and others, 1995) map reflect variations in the Earth's gravity field caused by density variations in the rocks composing the upper part of the Earth's crust. An isostatic residual gravity grid was derived from the Bouguer gravity anomaly data by removing the gravitational effect of the compensating mass that supports topographic loads. The thickness of this compensating mass was calculated using averaged digital topography by assuming a crustal thickness for sea-level topography of 30 km (18.65 mi), a crustal density of 2.67 gm/cc, and a density contrast between the crust and upper mantle of 0.40 gm/cc. The anomalies and patterns on the map reveal features of subsurface geology, including the location of buried faults, sedimentary basins, plutons, uplifted basement rocks, and so forth. Positive anomalies (red colors) delineate rocks more dense than the Bouguer reduction density of 2.67 gm/cc, whereas negative anomalies (blue colors) result from rocks of lower density.
This picture plots the data as points without curve fitting.
https://pubs.usgs.gov/ds/2005/135/ia_boug.htm
?ÿ
