This is for our county vertical control network.
Our crew doesn't like square cuts. One time we found two square cuts on top of a concrete curb inlet. The location description wasn't precise enough to differentiate between the two. We like identifiable BMs.
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There could be a datum issue involved here.
What is the typical difference between NGVD29?ÿ and NAVD88 in that region?
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I'm not sure 0.023 feet would be a datum difference.
I did some research this afternoon concerning this BM; it was set in 1999. There's no doubt it could've settled over the years. The BM was set by running third order levels. The original field notes show a distance of 30' +/- to the road centerline; we measured the same distance. This is the original BM. The median-shaped cracks in the asphalt had us scratching our heads as did the discrepancy in elevation. The discrepancy became smaller after we continued our run to an NGS BM and then ran (on paper) backwards to our beginning BM. We plan on running a new loop through our beginning BM because of other elevation discrepancies en route. Some of these discrepancies might be from running between NGVD 1929 and NAVD 1988 BMs.
Most 88 BM's are converted 29 BM's, I don't know about other parts of the country, but except for resets there are no exclusively 88 BM's around here. If your tie is .02' to a third order Bench, that's probably about as good as you will do. Be really happy you've got vertical control that tight.?ÿ
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In my world .023' is balls on, but without knowing how well it compares to the surrounding BM's in the network and having some reasonable confidence on which to hold... it's sort of a broken clock is right twice a day thing.
Most 88 BM's are converted 29 BM's, I don't know about other parts of the country,
Here in Oregon the difference between '88 and '29 is about 3.5 feet. In Oklahoma it was on the order of 0.5 feet. On the east coast I believe that the difference is all but negligible.
Interestingly, early indications (per OPUS extended reports) are that NATRF2022 orthometric elevations will be nearly equal to '29 (within 0.02') in my area.?ÿ?ÿ
This is for our county vertical control network.
I've recently begun running levels for a vertical control network in my fair city.?ÿ I wrote about the existing situation in another thread a couple days ago.?ÿ My strategy has been to select a single NGS benchmark to hold, double run a series of level loops from that, and adjust. Of course I check into other benchmarks as?ÿ I encounter them, plus checks based on OPUS results. But these are checks only. So far the checks have been good.
My city's GIS/IT department has built me a web page. Check it out. It went on line this Monday. The data shown has been collected since August. I expect to update it quarterly and it to be multiple years before we have the whole city covered.?ÿ ?ÿ
Most 88 BM's are converted 29 BM's, I don't know about other parts of the country,
Here in Oregon the difference between '88 and '29 is about 3.5 feet. In Oklahoma it was on the order of 0.5 feet. On the east coast I believe that the difference is all but negligible.
Interestingly, early indications (per OPUS extended reports) are that NATRF2022 orthometric elevations will be nearly equal to '29 (within 0.02') in my area.?ÿ?ÿ
I went to a couple of presentations by the NGS. They insisted that the new 2022 elevations ARE NOT?ÿNGVD29 elevations. No one there brought up that issue, so it was kinda weird.?ÿ
Here 88 is 2.4XXX feet higher than 29. The new 2022 numbers are said to be 2' lower than 88. As long as 2022 can be set using GPS and leveled accurately between those GPS numbers I figure it's a really good change.?ÿ
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They insisted that the new 2022 elevations ARE NOT?ÿNGVD29 elevations. No one there brought up that issue, so it was kinda weird.
Tides (and tidal station datums) are greatly influenced by gravity so it's not too farfetched to say that there's some correlation between the two.
But we're still talking different datasets, different adjustment mechanisms, and different delta values across comparable distances. Even if pockets of the country have "similar" values, the datums are still different.
Not to mention that NGVD29 held 26 coastal station values fixed. Between those stations, across some short distances, some pretty large residuals were distributed. The effect is much less the further away from the coast we get.
This is for our county vertical control network.
I've recently begun running levels for a vertical control network in my fair city.?ÿ I wrote about the existing situation in another thread a couple days ago.?ÿ My strategy has been to select a single NGS benchmark to hold, double run a series of level loops from that, and adjust. Of course I check into other benchmarks as?ÿ I encounter them, plus checks based on OPUS results. But these are checks only. So far the checks have been good.
My city's GIS/IT department has built me a web page. Check it out. It went on line this Monday. The data shown has been collected since August. I expect to update it quarterly and it to be multiple years before we have the whole city covered.?ÿ ?ÿ
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Very nice.?ÿ Uh, what's the history behind the BM called "KATHY?"
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They insisted that the new 2022 elevations ARE NOT?ÿNGVD29 elevations. No one there brought up that issue, so it was kinda weird.
Tides (and tidal station datums) are greatly influenced by gravity so it's not too farfetched to say that there's some correlation between the two.
But we're still talking different datasets, different adjustment mechanisms, and different delta values across comparable distances. Even if pockets of the country have "similar" values, the datums are still different.
Not to mention that NGVD29 held 26 coastal station values fixed. Between those stations, across some short distances, some pretty large residuals were distributed. The effect is much less the further away from the coast we get.
I chatted with Pam who used to give presentations for the NGS and it sounds like people were giving the NGS a hard time about going back to NGVD 29 numbers. I wasn't thinking that way but it is interesting that the old guys running levels across the continent will be so close to the new data sets using more modern gravity measurements. It's remarkable really.?ÿ
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Very nice.?ÿ Uh, what's the history behind the BM called "KATHY?"
The monuments in this first "pilot" set were all in place before I got here, save for two (LOMBARD_TC & FLAGPOLE).?ÿ I have no idea when "KATHY" was set, or by whom. The stamping was on it when found, so I've rolled with it. That street was rebuilt in the 1990's. So it has probably been out there since then.
I have a milk crate in the store room with about 90 of these monuments standing by, and I have another monument of the same character in another part of town that is shown as found on a Record of Survey filed in 1994.?ÿ There is another milk crate with 45 of the nippled benchmarks. So I've got a few years worth of work to do just to eat down the stock.?ÿ Meanwhile I'm keeping my eyes open for "naturals", like CHB, HALL1, & 350903, of which I've already identified several.
it is interesting that the old guys running levels across the continent will be so close to the new data sets using more modern gravity measurements. It's remarkable really.?ÿ
The old work was indeed remarkable, but there was something on the order of a meter tilt across the USA relative to the new datum, which is only close to 29 near parts of the west coast.
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it is interesting that the old guys running levels across the continent will be so close to the new data sets using more modern gravity measurements.
NAVD88 is based on the same levelling measurements that '29 was.?ÿ
it is interesting that the old guys running levels across the continent will be so close to the new data sets using more modern gravity measurements.
NAVD88 is based on the same levelling measurements that '29 was.?ÿ
Yes, but 88 isn't as close to the new 2022 as 29, that's why surveyors have been really teasing the NGS guys about the new datum. For instance I have a bench mark with 3920.0 from NGVD29, 3922.5 for NAVD88, and the new datum will be 3920.4. Since I'm about as far from an ocean as you can be, it begs credibility to think it's a random occurrence.?ÿ
I suppose there are places where the reverse is true.?ÿ
Establishing a consistent, reliable and accurate continent-wide vertical datum is challenging. A meter-level cross-CONUS tilt was hard to detect until the advent of systems like GRACE.
The conscious decision to NOT fix the NAVD88 to tide gauges was due to the recognition that heights at these gauges did not represent a single geopotential surface.?ÿ
One meter in the 4.4 million meters from Miami to Seattle sounds pretty good. Of course it isn??t now with all the additional data, models and understanding of the geoid we now enjoy.
The first graphic shows the many surfaces and height systems used in modern geodesy. The second shows the cross-continent shift.
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