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NGS Datasheet orthometric height discrepancies
Posted by Dan Henry on February 9, 2018 at 3:43 pmHas 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? Is there any published guidance or wisdom on which to use when using marks for FEMA work (finding elevations relative to base flood elevations)? 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
MightyMoe replied 6 years, 2 months ago 11 Members · 30 Replies- 30 Replies
Can you please provide a station PID that can be reviewed as an example?
This reminds me of an old Chicago band song. “Does anybody really know what height it is? Does anybody really care?” 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.
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. Anyone who thinks they are accessing true datum at better accuracy than that using our current system must be from Austin.
- Posted by: Dan Henry
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? Is there any published guidance or wisdom on which to use when using marks for FEMA work (finding elevations relative to base flood elevations)? 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 are only two listed elevations a 29 and a 88 one. Been using 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 exercise.
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.
I’m just one of those evil GIS people. Bwah-hah-hah! Seriously, I do coordinate systems and transformations at Esri.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
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. A best fit of the order used may not exactly match the H = h-N at all those points. 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: Contractor wants elevation marked. BFE + Community’s Freeboard to build to and be in compliance. I don’t want somebody else coming out and getting a different answer that’s out of compliance because they used a different number. Without clear guidance on the difference in values, we’re using the lower of the 2 values to benchmark from. 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.
- Posted by: Dan Henry
Regarding not “sweating” it: Contractor wants elevation marked. BFE + Community’s Freeboard to build to and be in compliance. I don’t want somebody else coming out and getting a different answer that’s out of compliance because they used a different number. Without clear guidance on the difference in values, we’re using the lower of the 2 values to benchmark from. 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.
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.
- Posted by: Dan Henry
Regarding not “sweating” it: Contractor wants elevation marked. BFE + Community’s Freeboard to build to and be in compliance. I don’t want somebody else coming out and getting a different answer that’s out of compliance because they used a different number. Without clear guidance on the difference in values, we’re using the lower of the 2 values to benchmark from. 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.
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 the bench marks and basis 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
Chief, NC Geodetic Survey
Claude T. Bowers Building
NC National Guard Complex
4105 Reedy Creek Road
Raleigh, NC 27607
Telephone: (919) 948-7844
Fax: (919) 733-4407Yes, running levels and using the benchmarks shown on FIRM, and the mark has but one published NAVD88 value on the datasheet. However, you get a different answer if you use active control, an answer that is closer to h=H+N from the datasheet. I 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.
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.
.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?)
http://gpsworld.com/ngs-2018-gps-on-bms-program-in-support-of-napgd2022-part-5/
Thanks, John! That’s a great source of information. 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.
-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? 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 because the measurement says it is far off elevation. 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?
.There will be a webinar on this topic next week, which should clarify the details regarding the double observations.
Finally, we are holding our 2018 GPSonBM webinar next Thursday, February 15th at 2pm Eastern. We will go over the analysis that we went through to derive the prioritized list, show some maps of the geoid analysis, and answer questions you may have.FWIW,
The original post asked why the sum of the three different heights: NAD83 ellipsoid (h), NAVD88 orthometric (H) and the ellipsoid-geoid separation (N) did not sum to zero using the relationship h – H – N = 0.
The reason is that the three heights are retrieved from separate locations in the NGS data base and not forced to agree.
If there is a valid H, determined from differential leveling, it is shown in the CURRENT SURVEY CONTROL block. The value of N is from the most recent hybrid geoid model (now GEOID12B). The value of h is from the most recent least squares adjustment.
When there no valid differentially determined H, it is computed from N and h.
Read the file:dsdata.pdf linked from every datasheet retrieval. It explains the meaning of all values in the NGS datasheet.
NOAA Technical Memorandum NOS NGS 59, ??Guidelines for Establishing GPS-Derived Orthometric Heights,? Zilkoski, et al (2008) describes in great detail the process. It can be viewed on line or downloaded at: https://www.ngs.noaa.gov/PUBS_LIB/NGS592008069FINAL2.pdf
The adjustment process is described step-by-step in the document ??CONSTRAINED ADJUSTMENT GUIDELINES? (also a PDF) is found by following the links ??Tools?, then ??Download PC Software? then ??ADJUST AND UTILITIES? from the NGS home page.
As the adjustment guidelines document describes, a series of adjustments are performed using the observations and their statistics in combination with various constraints to verify project observations, verify fixed control (constraints) and to determine values for new points and to update coordinate values for those shown to be inconsistent with verified constraints.
What is loaded into the data base are the combined results of a horizontally constrained adjustment which provides the positions and ellipsoid heights and the results of a vertical constrained adjustment which yields orthometric heights (H). The modeled ellipsoid-geoid separations (N) are not adjusted in this process but used in the adjustment to compute the H at the unknowns.
My description is drastically simplified; read the adjustment guidelines.
Sounds pretty straightforward? In an ideal world with perfect observations and fixed control it would be easy. Unfortunately monuments are subject to all manner of disturbances often difficult to detect. Measurement systems like GNSS are imperfect. Modeling is improving but there remain problems with data quality/quantity/spacial distribution as well as with the theory underlying the modeling.
The official guidance on what constitutes a ??good? OPUS solution includes reference to better than 5 cm peak-to-peak errors (see: https://geodesy.noaa.gov/OPUS/about.jsp ). IMO, achieving agreement between an OPUS solution and published values at the two-centimeter level can be considered a verification of the published values.
For an insight into the issues related to ??updating? a network, I??ve always liked Dr Charles Schwarz??s http://geodesyattamucc.pbworks.com/w/file/fetch/52782335/ConstrainedAdjustments1.pdf (a better scan than the one on the NGS site).
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