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
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.
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.