Finding that elusive court case where there is a dispute between the Geoid Model derived orthometric NAVD88, and the level run from Bench Mark NAVD88 elevation probably isn't going to happen. The local FIRM mapping isn't up to the challenge of finding the tenth of separation between the two results. I've been very impressed by the location of the 100 year flood line when a big event happens and where the mapping shows it. But a tenth is a bridge too far for the mapping.
Imagine the glossed over eyes of the judge and or jury in the court as two experts argue about an inch where a building was submerged by a couple of feet of water.
Using 2011/Geoid2018 should work to get close, but I'd still want to know how it relates to local Bench Marks. I do know in a number of communities and I figure most surveyors know if they've been working with it for a while.
Within my fair city, which is about 20 square miles in area, there are exactly 2 NGS benchmarks with levelled elevation. Both were set prior to 1960. I very much doubt that either has been reobserved by the NGS since. One is in a sidewalk near a building doorway (RD0310), the other in a culvert headwall (RD0313), so neither is impervious to settlement. Nevertheless, they look great.
The local county surveyor also has several benchmarks in the area, a couple of which are within a mile of one of the NGS marks. These were probably set in the 1970's or earlier. But nobody seems to know for sure. I've levelled between one of the the NGS marks and a few of the county marks and found good correllation - so far. So I'm confident that these marks are reasonably locally stable.
Meanwhile, elevations determined by long duration OPUS sessions consistently come out about 0.1' higher than those from the levelling relative to those passive marks. So I'm left wondering if the whole area hasn't subsided a bit in the last 60-90 years. This is the PNW, so we know that the whole thing is moving horizontally at a substantial rate. Why not some in the vertical as well? On a clear day I can see Mt. St. Helens from here, and Mt. Hood in the other direction. And what about ground water depletion?
I'm left wondering if these passive marks are really stable - compared to .... what? And if I wanted to really, finally determine the true elevation of my area how would I go about that? Is relying on a 90 year old passive mark -not in solid bedrock - really better than long duration GPS observations relative to CORS? What are the implications for mass of as-built documents of my city's utility infrastructure? Realizing that the 0.1' difference I've seen isn't all that significant for that last, but what if it was?
"What are the implications for mass of as-built documents of my city’s utility infrastructure?"
Those of us in subsiding areas have long since gotten used to the fact that legacy elevations aren't worth much. If the vertical movement is substantial and widespread, a program that monitors critical infrastructure is a best practice. But most discrete engineering projects can be managed successfully as long as the critical vertical elements are identified and measured prior to design.
Pick whatever vertical reference seems most appropriate, document it clearly, and move ahead. Personally, I prefer to reference NAVD88 rather than a 50-year-old NGVD29 mark, but the way you realize NAVD88 can vary as long as it's documented and reproduceable.
I often use a single RTN observation on small-scale standalone projects, and show a few control points on the site for those who follow later. For large-scale jobs I like to use OPUS-Projects for the RTK base stations, picking CORS that I know to be (relatively) stable. I call if NAVD88 in both cases, but I also describe how I got there.
Apparently there is an easier way. The Washington Post had a laudatory article on a young man in Miami who is raising awareness of climate change by posting signs in neighborhoods with the flood elevation on them. And how does he get this elevation? He "downloads" it. No survey crew needed. Ha, who knew?
"Personally, I prefer to reference NAVD88 rather than a 50-year-old NGVD29 mark..."
Fun fact - Early indications are that NATRF2022 elevations in my fair city are going to very nearly equal NGVD29. The difference looks to be well under 0.1 feet, probably close to ±0.01 feet.
Both city and county has continued to specify the use NGVD29 up to the present day because of the legacy as-built data. When we switch to the new datum it will be like '88 never existed. The only agencies anywhere in Oregon using '88 that I know of is the DOT and, I think, the Port of Portland. And FEMA, of course.
Before 2010 all FIRM maps were NGVD29. Once FEMA went through the grinder of approvals that switched in 2010 to NAVD88. NAVD88 is higher in elevation for the same Bench Marks by about 2.4 feet. It depends on the location but that about covers most of the populated areas in the county. The new elevation control will sink down approximately 2'. So close to the 29 elevation. When new mapping will show up is anyone's guess. There will be little stomach to switch because the 2010 mapping created havoc for many homeowners and the local commissioners will not have forgotten. So I know I will be working in NAVD88 until I retire, all the infrastructure is dependent on those FEMA maps and as long as they remain NAVD88 all design and construction will stay attached to them.
Our delta was about 5 inches higher going from 29 to 88.
Roughly 1/2 foot is what it is in Oklahoma as well.
'88 is 3-1/2 feet greater than '29 here in Beaverton, Oregon.
MightyMoe is right on – we will all likely be dust and long forgotten before FEMA does anything systematic about updating the maps. Regrettably the agency has almost no one who can even spell the word datum much less really know what it means, they rely on their contractors to take care of that menial suff!! All too often I’ve found that any number of their contractors know only slightly more than FEMA does. The conversations I’ve had with FEMA over the years about how they will handle the North American-Pacific Geopotential Datum 2022 is that they will rely completely on the NGS developed vertical transformation tool. I hope surveyors across the country will take a seriously hard look at the Web Map on the NGS GPS on BM page - https://geodesy.noaa.gov/GPSonBM/ . While NGS will likely accept additional observations until the total network cutoff 2/29/24, I suspect it will probably be minimal from what’s already there. Therefore, you can make some pretty good assumptions about what the actual accuracy of that transformation tool will be by looking at the number of requested stations that have NOT been submitted. States/Territories like CT, DE, MA, MN, NJ, NC, PR, VT, WI will likely be relatively accurate at the couple of cm/.1 ft level. With some regional exceptions in a few states the actual uncertainty in the rest of the country is going to be higher, could be in the 4-6 cm/.2 ft range, perhaps more. If requirements are to stay on NAVD 88 at a tolerable level say .1 ft this might be complicated without directly connecting to NAVD 88 passive control when you also consider the additional uncertainty in any surveyors’ ellipsoid height accuracy. Unfortunately the vast majority of surveyors have paid little attention to the NGS requests for data to address this issue so it will be what it will be.
Unfortunately the vast majority of surveyors have paid little attention to the NGS requests for data to address this issue so it will be what it will be.
While there certainly could have been more data gathered, it's not even remotely accurate to pin this problem on "surveyors". This is a national issue with significant economic consequences. If federal, state, or local governments thought it was important enough, they could have implemented programs beyond "hey surveyors, go spend your time, money and resources on this critical thing in your off-hours, because we said so".
Hats off to the NGS for spearheading the effort, and hats off to all the surveyors who did contribute. I'm impressed that we got as much data as we did. But it was still a volunteer effort, across the entire country.
The days of NGS having any meaningful field force have been gone for decades due to the development of GPS technology. What they use to do at the local level has long since been taken over by the local government and the private side. When I said surveyors I meant across the board not just the private sector. You can easily see the support that state government surveyors gave if you look at the list of states I indicated looked pretty good. The entire requested set of stations in Puerto Rico was observed by private surveyors. While NGS certainly understands the importance of datum transformation tools, with the exception of ITRF to NAD 83 they play virtually no role in the development and enhancement of the National Spatial Reference System. These are tools that have the greatest importance to agencies and groups that manage legacy data.
