So I just came across a survey control sheet for a highway project and I have a pretty amateur question:
On the survey notes it reads: "elevations reported are NAVD88 orthometric heights, reconciled to published existing project control".
What does 'reconciled' mean in this context? Was there a previous calibration or control in the area of the project?
This is just my assumption but here it goes. ?ÿI believe what they are trying to say is that they are using a specific geoid and adjusted their actual NAVD88 Geoid12 (name your geoid) to match a specific benchmark which should be listed on the plans.?ÿ
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They are doing something similar around here for work on a local lake. All elevations are based on a benchmark set on the dam. We use our existing geoid model (geoid12B) and adjust all elevations down to match the benchmark on the dam.?ÿ
I would say that they know what they are doing. Holding the good elevations and using the Geoid to get ellipsoid height so GPS will work correctly.
Probably as said, an adjustment up or down to the local NAVD88 basis. What many forgot is the geoid model has the word "model in it. You can have very good leveled heights and very good ellipsoid heights and instead of "faking" one of those, what should be done is adjusting the error that is inherent in the geoid model. Every time I see an ellipsoid height listed that is nothing more than a derived quantity from the geoid model and the orthometric height, I label that as a "pseudo ellipsoid height" it is just easier to get fake ellipsoid heights that mean nothing than model the geoid errors, just saying...
SHG
While I see a number of posts presuming to divine from the ambiguous term ??reconciled? reference to performing an adjustment wherein the geoid-ellipsoid separation is allowed to take adjustment (and variants of this theory), I do not see how a definitive answer can be provided other than from the source.
The dictionary.cambridge.org site defines reconciliation as follows: ??the process of making two opposite beliefs, ideas, or situations?ÿagree?.?ÿ
In this case, it appears the survey notes reported encountered a discrepancy between NAVD88 heights (from his survey?) and existing project control. This could as easily be a VERTCON transformation of NGVD29 heights and those in NAVD88. It could mean that some ersatz approach, e.g. applying a constant offset based on a comparison of one point, was used. Other possibilities exist.?ÿ
One aspect of a profession is the use of clear and unambiguous terminology. I do not see that here. I have on occasion had to deal with data provided by private and governmental entities where in-house terminology caused a lot of extra work.?ÿ
My question to those posting replies to this thread is ??how would you document the source of heights for these ??reconciled? points should be choose to use them.
Hope this contributes,
DMM
You ever notice when control moves? I do.
We have a 1995 NGS level run with control.?ÿ I have a level run through tied into multiple GPS (rtk, static and opus). The old cors reported a z velocity of 2.2 mm per year and when you take that into account the GPS and BMs are pretty darned close.?ÿ?ÿ
If you are looking at the Z-velocity reported for a CORS network point, you are not looking at a change in height. That would be the DU value. See the graphic. A pet peeve of mine. The transformation of the differences in the ECEF Cartesian coordinates (DX,DY,DZ) to the more useful local geodetic horizon system (East, North and Up) is pretty straightforward. ?ÿSee second graphic.
It may have been better to state that elevations are NAVD88 elevations based on published existing project control values.
But the intent is clear, they are not bringing in GPS values and overriding existing control points.
Nor should they, that would be wrong.?ÿ