Elevation of NGS Point

75.33

Agreed 75.33 ft or 22.96 meters

@mightymoe 

Thanks. The fact that is says ortho height got me confused. I thought there was calculation involving geoid height.

@landbutcher464mhz 

Thanks. Glad you could read the data sheet; looks blurry on my iPhone, and I couldn’t edit my post before it was answered.

Ellipsoid height - geoid height = ortho height.

Posted by: @field-dog

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The fact that is says ortho height got me confused. I thought there was calculation involving geoid height.

Posted by: @lurker

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Ellipsoid height - geoid height = ortho height.

This definition is true, but per the data sheet for this vertical control point, "the orthometric height was determined by differential leveling and adjusted by the National Geodetic Survey in November 2003." Ellipsoid height was not used in establishing the point and doesn't appear on the data sheet.

Since you, too, will be leveling, ellipsoid height remains irrelevant. As others said, the orthometric height of 22.960 meters (75.33 feet) is what you want.

Posted by: @bene1

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Since you, too, will be leveling, ellipsoid height remains irrelevant.

Thanks. I'll hard-code that into my brain.

I went back and pulled some OPUS Share data I did for the GPSonBM work to see how it compared using (Ellipsoid height - geoid height = ortho height) compared to the published elevation. These two benchmarks are about 1.5miles apart.

First point:

4 hour data set Ellipsoid : 397.851m +/0.023m

Data Sheet Geoid18 -32.322m

397.851-(-32.322) = 330.173m this is whats published on OPUS Share with +/-0.066m

Published Elevation = 330.193m 2cm difference(0.066')

Second point:

4 hour data set Ellipsoid: 347.008m +/-0.014m

Data Sheet Geoid18 -32.323

347.008-(-32.323)=379.331m this is whats published on OPUS Share with +/-0.064m

Published Elevation=379.333m  2mm difference(0.007')

@gary_g 

That's close for OPUS; 0.06'.

Usually, I get better results by picking the newer CORS and roll my own calculation in TBC.

Slightly better than OPUS submission. RTX is usually closer to NAVD88 than either. 

We occupy the First order benchmark, hold the NAVD88 elevation, apply Geoid18, check into nearby benchmarks, and where the ellipsoid height ends up is irrelevant. 

The weakness in a GNSS derived elevation is, generally, in the geoid height. The ellipsoidal elevation can be measured within a couple hundredths of a foot, but the geoid separation will have an error of several hundredths. Leaving the resultant ortho ht frequently missing a levelled measurement on the same point by quite a lot.

IDK, a couple hundreths and several hundredths are about the same in the general sense.

NGS from past presentations hold the ellipsoid at about 2cm(0.06') when derived by GPS. The estimates for Geoid18 at 95% are about 2.5cm.

My estimates may be off a bit but I find it to be random for sure. My 2mm check is not normal in my experience.

The accuracy of GEOID18 is generally in the range of 0.1 to 0.15 ft  Easily checked by running the utility on the NGS web site.