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
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.
Can you please provide a station PID that can be reviewed as an example?
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.
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
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 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.
