switching from geoid 12b to geoid 18

The differences in geoid heights between 12B and 18 vary by location, primarily because of the densification of available data used for GEOID18 resulting from the GPS on Bench Marks campaign(s). You can use this tool to compute a GEOID18 geoid height (N) for your project location, then compare that to what the geoid height was for 12B. The difference will be the amount your derived orthometric height will change depending on which geoid model you apply to the ellipsoid height(s).

https://www.ngs.noaa.gov/GEOID/GEOID18/computation.html

The area where you work has almost no NAVD88 bench marks, so it is unlikely any GPS on BM observations were done that would have changed the geoid heights in your area between 12B and 18.?ÿ

@spmpls i got an output N of -31.095 M, or about 102 feet for a random spot on my drawing where i inserted a point. what does that number represent? elevation? Because both in the contours provided by napa county and our seperate survey data, the elevation we have is around 112 feet. both use navd88 i believe

@casualsurveyor0192

H (NAVD88 elevation) = h (NAD83 ellipsoid height) - N (geoid height).

Your computation returned a GEOID18 geoid height for your entered location of -31.095 meters. You need to compare that value with what the 12B geoid height was for the same location. The difference, if any, will be the change in your derived NAVD88 orthometric height (elevation).

?ÿHere is the GEOID12B computation utility:

https://geodesy.noaa.gov/GEOID/GEOID12B/computation.html

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@spmpls thank you very much. Lastly, is there a magazine or publication I can use to stay up to date on changes? I heard that opus may stop processsing data for 12b, is there any truth to that?

The best place to stay informed on these issues is right here:

https://geodesy.noaa.gov/

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@spmpls thanks again. Looks like depending on my location, I tried one on the valley floor and one up on our highest project, the difference is about .08 ft (valley floor) and .12 feet (higher EL.) does this change in geoid height directly correlate to what the change in elevation would show had i been using geoid 18 in my surveying equipment?

@casualsurveyor0192

Yep: H=h-N. 

-----------Ellipsoid

^

|

N = a negative distance in most of California

|

-----------Geoid

@dave-karoly

Actually, negative in all of California and the rest of the Conterminous U.S.

When grading the California PLS exam years ago, one of the questions was something like "at your project location, which is higher, the ellipsoid or geoid?" An examinee answered "ellipsoid", which was correct, but then went on to state "Because the geoid is below the ellipsoid in all of the United States", which is not correct. Last I checked Alaska is in the US. They got zero points for that answer because of that incorrect statement. I always told people taking the exam to just answer the question and move on. Don't spend any time fortifying your answer and I used that situation as an example. All risk and no reward.

When I apply Geoid 18 in my area to NAD83 (2011) ellipsoid heights I tend to be closer to NAVD88 elevations than using Geoid 12b.

I normally correct the ellipsoid height to NAVD88 elevations with which ever flavor of Geoid I'm using.

In other words I fix my elevation and let the Geoid model move the ellipsoid height up or down.

Most people use the ellipsoid height and shift the elevation which causes you to have different elevations for each Geoid model, 90, 96, 99, 03, 09, 12, 18. So there would be at least seven different elevations for the same point through the last 28 years of GPS usage, or a change every four years or so. Of course each Epoch also changes the ellipsoid height and that adds to the number of elevation changes, 86, 93, 01, 07, 11. So mixing the seven Geoid model heights with the five ellipsoid heights can produce a large number of different elevations for the same point.?ÿ

Basically, you need to stay on top of the changes, if you have an ongoing design like many engineering designs are, keep good records of how you derive elevations and let the next guy know where it all came from.?ÿ

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@casualsurveyor0192

OPUS has been returning GEOID18 on results for a while now - as of September, I believe.

Keep tabs on the NGS homepage, they will always post up changes to products and services, often long in advance.

Or, if you don't mind being notified by email, I highly recommend signing up for NGS alerts.

Posted by: @mightymoe

Basically, you need to stay on top of the changes, if you have an ongoing design like many engineering designs are, keep good records of how you derive elevations and let the next guy know where it all came from.

For sure. I can't tell you how many times I have seen design surveys and plansets stating "Vertical datum is NAVD88" with absolutely no other information.

The lesson is, "Pick a datum realization and geoid and stick with them for your whole project."?ÿ Later ones will be self-consistent over a larger area, but any recent one will probably do for most projects.

On the question of which model of ellipsoid-geoid separation, I always recommend the most recent be used. In any event, the choice of which model to use, like which horizontal and which vertical datums are used must be included in the documentation of the work.

Note that I use the term ellipsoid-geoid separation rather than geoid height. The value labeled ƒ??geoid heightƒ? represents the linear distance from the surface of the reference ellipsoid to the geoid.?ÿ

While the relationship is often expressed otherwise, I always use h - H - N = 0 (+ errors in h, H and N).Note that the NGS datasheets do not use this relationship to sum to zero. It extracts the three quantities separately from their database.

I obnoxiously include an annotated datasheet. I forgot to label H which is the distance from the geoid to the terrain.