Has anyone noticed published datasheets from NGS where there is a published orthometric value based on old leveling that disagrees with the value derived from calculating from ellipsoid & geoid heights??ÿ ?ÿIs there any published guidance or wisdom on which to use when using marks for FEMA work (finding elevations relative to base flood elevations)??ÿ I've found that using OPUS or using a CORS as a fixed base yields results that are closer to the height derived from calculating from ellipsoid & geoid heights, but the differences are usually less than a tenth of a foot? What's your standard practice?
Thanks,
Dan
Can you please provide a station PID that can be reviewed as an example?
?ÿ
This reminds me of an old Chicago band song. "Does anybody really know what height it is? Does anybody really care?"?ÿ What you are really tying to is datum. What is the best access to datum? The old leveling or using Geoid computations? I would opine that it depends on the area you work in and how one or the other have proven to work out most of the time. I think the official answer is passive heights however all the passive heights must work together in the local area. If time has destroyed that relativity the Geoid model may give more truthful access to datum. 2022 can't get here soon enough for me.?ÿ
I would further opine that if your differences between active and passive control is less than 0.10' tip your hat to the leveling and GPS gods and move on.?ÿ Anyone who thinks they are accessing true datum at better accuracy than that using our current system must be from Austin.?ÿ
?ÿ
Has anyone noticed published datasheets from NGS where there is a published orthometric value based on old leveling that disagrees with the value derived from calculating from ellipsoid & geoid heights??ÿ ?ÿIs there any published guidance or wisdom on which to use when using marks for FEMA work (finding elevations relative to base flood elevations)??ÿ I've found that using OPUS or using a CORS as a fixed base yields results that are closer to the height derived from calculating from ellipsoid & geoid heights, but the differences are usually less than a tenth of a foot? What's your standard practice?
Thanks,
Dan
Yes, I posted one (HARN/1st order bench) with 4 or 5 different ellipsoid heights and using Geoid Models of different flavors would produce any number of different ortho height. However, there?ÿare only two listed elevations a 29 and a 88 one. Been?ÿusing the NAVD88 elevation for close to thirty years now, I haven't paid any attention to all the other elevations, beyond noting them as an academic?ÿexercise.
?ÿ
?ÿ
Dru Smith showed a few examples in the last webinar he gave on the 1st of February: Blueprint for 2022, Part 2: Geopotential Coordinates
They had old leveling information, and in general, he said there's no way to tell on the datasheet itself how good or bad the vertical coordinates could be.
Yes, there is only one NAVD 88 elevation on the datasheet. That elevation does not change when new realizations of NAD 83 are produced (if the station was used to produce the geoid model). What changes are the ellipsoid and geoid heights to mathematically get from the new elliposid height to the "fixed" NAVD 88 surface (surface model constrained to leveled bench marks that also have an ellipsoid height - why GPS on BM's is important). That is why it is incorrect to mix and match geoid models. GEOID12B is directly correlated to NAD83(2011)2010.00 ellipsoid heights. You should not expect to be able to use a 2010.00 ellipsoid height and a GEOID03 geoid height, using H = h-N, and get a correct NAVD 88 orthometric height that matches the one on the datasheet (or very close).
https://www.ngs.noaa.gov/GEOID/GEOID12B/GEOID12B_FAQ.shtml
https://www.ngs.noaa.gov/GEOID/USGG2009/faq_2009.shtml
?ÿ
?ÿ
I would never sweat 0.10' vertical difference on a data sheet, especially for FEMA work where your BFE is a professional guess to begin with.
I believe that when NGS releases a new geoid model they do a best fit between GPS ellipsoidal heights at selected locations (HARN, etc) versus old leveled elevations at those particular places.?ÿ A best fit of the order used may not exactly match the?ÿH = h-N at all those points.?ÿ Also, if a bench mark was not included in that set when the geoid was fitted, then your GPS measurement with a good geoid will not exactly match the leveled value.
Try FB4322 or AF8660.
Regarding not "sweating" it:?ÿ Contractor wants elevation marked. BFE + Community's Freeboard to build to and be in compliance.?ÿ I don't want somebody else coming out and getting a different answer that's out of compliance because they used a different number.?ÿ Without clear guidance on the difference in values, we're using the lower of the 2 values to benchmark from.?ÿ The BFE's are only published to the tenth, but it better be the *right* tenth. We have the benefit here (NC) of the FRIS system, which allows you to get a direct read of the BFE value instead of having to interpolate from the profiles.?ÿ
Regarding not "sweating" it:?ÿ Contractor wants elevation marked. BFE + Community's Freeboard to build to and be in compliance.?ÿ I don't want somebody else coming out and getting a different answer that's out of compliance because they used a different number.?ÿ Without clear guidance on the difference in values, we're using the lower of the 2 values to benchmark from.?ÿ The BFE's are only published to the tenth, but it better be the *right* tenth. We have the benefit here (NC) of the FRIS system, which allows you to get a direct read of the BFE value instead of having to interpolate from the profiles.?ÿ
Then I would run a level. Better to be safe than on the chopping block - and definitely go the route that gives you a larger margin of error.?ÿ
Regarding not "sweating" it:?ÿ Contractor wants elevation marked. BFE + Community's Freeboard to build to and be in compliance.?ÿ I don't want somebody else coming out and getting a different answer that's out of compliance because they used a different number.?ÿ Without clear guidance on the difference in values, we're using the lower of the 2 values to benchmark from.?ÿ The BFE's are only published to the tenth, but it better be the *right* tenth. We have the benefit here (NC) of the FRIS system, which allows you to get a direct read of the BFE value instead of having to interpolate from the profiles.?ÿ
All the FIRMS I deal with list the data basis for determining elevations BFEs.
Everyone I've seen lists the bench marks in the area. Locally that was based on NGVD29 Bench marks until the new FIRM maps came out in 2010. After 2010?ÿthe bench marks and?ÿbasis became NAVD88. That info has little to do with ellipsoid heights, geoid heights, it is specifically tied to the given elevations on those bench marks. I would go with whatever your FIRM states, sometimes a FIRM map doesn't show a local bench mark and you need to look at adjoining FIRM maps.
Surveys should be tied to "good" bench marks in the area, if you can do that with GPS and match the elevation using the latest ellipsoid height and Geoid12 great!!! I've never seen that happen but it would be cool, I just use good undisturbed bench marks and the listed NAVD88 elevation.
Now if the FIRM lists OPUS and GEIOD03 surveyed in 2005, then there you go, use that.
If you are in North Carolina, I would recommend you contact Gary Thompson, NC Geodetic Coordinator, if you want further clarification.
Gary W. Thompson, NCDPS?ÿ
Chief,?ÿhttp://www.ncgs.state.nc.u s" target="_blank" rel="noopener">NC Geodetic Survey
Claude T. Bowers Building
NC National Guard Complex
4105 Reedy Creek Road
Raleigh, NC 27607
Telephone: (919) 948-7844
Fax: (919) 733-4407?ÿ
?ÿ
Yes, running levels and using the benchmarks shown on FIRM, and the mark has but one published NAVD88 value on the datasheet.?ÿ However, you get a different answer if you use active control, an answer that is closer to h=H+N from the datasheet.?ÿ ?ÿI think we're set on using the lower value to benchmark from, as it should yield an acceptable answer however the next guy chooses to do it.?ÿ
Yep, the elevation from the bench, just be sure the bench is stable, check to at least one other one. I generally see a pretty good shift, if one bench is 0.09' low using GPS the next one may be 0.11' low, look for consistency.
I got an email last week stating NGS is working on a GEOID2018. They asking for assistance to provide data to OPUS for sharing.
Quoting advice from Base9geodesy in an older thread:
------------------------------------------------------------------------------------------------
https://surveyorconnect.com/community/gnss-geodesy/bench-mark-stability/#post-454561
I have it from a long time friend and colleague at NGS that there will likely be one more hybrid geoid (GEOID18?) to be released. The approximate time line as he provided to me is:
1. January 2018 -- Produce a prototype model with the new data we have now plus outreach material for it including a prioritized list of marks we want new obs on, and (Brian's) maps showing the accuracy and difference from GEOID 12B
2. Engage stakeholders at state surveying conferences and hold regional discussions on mark prioritization to update old data and fill data gaps - to be held with Advisors and regional partners - by end of February to inform Surveyor's Week activity in March
3. Cut off for new data submissions to be included in new model will be ~Aug 31st, 2018.
4. Beta release in December 2018
5. Outreach at state surveying conferences in early 2019 to gather feedback and find problems
6. Final product release April 2019
Look at the NGS GPS on bench mark map to get an idea of where the greatest needs for new observations in your area are:
http://www.arcgis.com/home/webmap/viewer.html?webmap=e9e8b68a75594c85bd4379e790b5dc08&extent=-149.3895,11.6366,-41.3719,59.0097
--------------------------------------------------------------------------------------------------
If the existing geoid models your area don't work well (try xGEOID17b), you should be helping to fill in the gaps using submissions to OPUS Share.
?ÿ
I was pretty sure I posted this earlier, but it does not appear in the thread...
See the article by Dave Zilkoski in GPS World (maybe in their newsletter?)
?ÿ
http://gpsworld.com/ngs-2018-gps-on-bms-program-in-support-of-napgd2022-part-5/
?ÿ
?ÿ
Thanks, John!?ÿ That's a great source of information.?ÿ I particularly liked finding the link to the new priority list for observations.
The statement "Two matching, independent GPS observations are required for each mark" needs clarification.?ÿ
-What are the matching criteria?
-Does independent mean just setups on two different days, or does it require two different people/equipment sets to do the 2nd submission??ÿ I find 5 of my submissions in the list, and don't know I could provide the 2nd one for those or not.
I'm a little surprised to find one on this priority list that I did (for early practice in submitting OPUS shares) on a Reset (hence 3rd order) mark, that is within a few km of GSVS14 stations that must have tons of data.
There's also one that the DOT has submitted with a note saying the mark must have been disturbed?ÿ because the measurement says it is far off elevation.?ÿ I wasn't going to worry about it, but since it's on the list maybe I should get a measurement to confirm their conclusion?
?ÿ
?ÿ
There will be a webinar on this topic next week, which should clarify the details regarding the double observations.
