VERTCON Change GEOID Question ?

VertCon does NOT use a geoid model. It is an interpolation program that uses the original NGVD29 and the original NAVD88 elevations as published in about 1990. It has NOT been updated. The interpolation is a bi-linear system identical to what is performed with NADCon, and is only a snapshot of what existed twenty-two years ago.

Cliff ? The Elevation Difference = The Geoid Difference

That is all I can figure out from last week to this week.

If VERTCON does not use a GEOID then it should note that fact for the NAVD 88 input/output. Otherwise it is apples and pomegranates.

All I want to know is what in VERTCONN changed.

Paul in PA

Try these:

Geoid09 Geoid Height Computations

Geoid03 Geoid Height Computation

Geoid99 Geoid Height Computation

DDSM

Perhaps Paul is using CORPSCON? That program calls NADCON for the NGCD29-NAVD88 computation, and also use a geoid model to go from GRS1980 to NAVD88. It can use Geoid90, geoid93, geoid96, geoid99, geoid03, or any custom geoid (such as geoid09)

Cliff ? The Elevation Difference = The Geoid Difference

Howdy,

Perhaps I am missing something in this thread. I over explain with significant simplifications below.

Paul in PA states that he is not getting the same results when mixing results using Geoid03 and Geoid09. He should not get the same result. Geoid03 and Geoid09 are different models with the most recent model indicated by NGS to be a better fit to NAVD88 heights.

OPUS and all GPS data reductions are computed in three-dimensional geometric space. The Earth-Centered-Earth-Fixed (ECEF) cartesian XYZ coordinates are transformed into latitude, longitude and ellipsoid heights. Ellipsoid heights are geometric and do not include the impact of gravity as do NAVD88 heights. Remember that the direction of the plumb line at an instrument setup is towards gravity; the instrument station is normal to the ellipsoid at the same point.

OPUS shows the ellipsoid height and indicates that the NAVD88 value is computed by applying a geoid model. It indicates which model it uses. The geoid value is the distance between the geoid and ellipsoid at the point (geoid-ellipsoid separation). The geoid is a geopotential surface of the earth's gravity field best representing global mean sea level in a least squares sense.

Geoid 09 is a hybrid geoid model that takes a gravimetric mode (USGG 09) and applies some datum transformation (e.g NAD83 is not geocentric at the meter level) as well as applying a "corrector surface" based on GPS observations (yielding ellipsoid heights) on points with published NAVD88 height.

The equation h (ellipsoid height) - H (NAVD88 height) - N (geoid-ellipsoid separation) = 0 (zero +/- errors in h,H and N) can be used to get a sense for how well the model fits in your area. If you have the time and interest you can retrieve data for all BM that also have GPS observations. Compute the geoid-ellipsoid separation and compare the resulting values with the geoid model values.

To summarize, comparing an NAVD88 value derived from an ellipsoid height and GEOID09 with an NAVD88 value derived from and ellipsoid height and GEOID03 should not be expected to agree. The ellipsoid heights are computed by OPUS and will not change.

VERTCON was intended for mappers and allows users to transform between NAVD88 and NGVD29. It is based on the creation of a grid of differences based on points with values in both systems.

Computing an NAVD88 height from an NAD 83 GPS-derived ellipsoid height requires the use of the hybrid geoid model currently GEOID09.

Hope this is responsive.

Cheers,

DMM

I don't know what you put into VERTCON, but when I use your position and the NAVD 88 height of 162.367 m you got from OPUS-RS I get an NGVD 29 value of 162.578 m. Which is exactly what you would expect if you enter a different height for the same point. As Cliff correctly pointed out VERTCON does not use a geoid model and it has not been updated in any way.

OK, Let Me Explain

Last Wednesday I used VERTCON to change my elevation to NGVD 29 for a flood elevation project.
I had a GEOID03 elevation from OPUS-RS, 162.350 m.
I was on NJGS "21 W 1", PID = KV7129.

I input N 40 52 21.617 W 74 54 3.176 NAVD 88 162.350 m into VERTCON
VERTCON told me the datum shift at that point was (-0.211 m) = NGVD 29 162.561 m

Thursday I resubmitted to OPUS-RS twice and got GEOID 03 elevation 162.350 m and GEOID 09 162.367 m but did no further analysis.

Today I submitted both elevations to VERTCON to change my elevation to NGVD 29

For the GEOID 03 elevation of 162.350 m into VERTCON
VERTCON told me the datum shift at that point was (-0.194 m) = NGVD 29 162.544 m

For the GEOID 09 elevation of 162.367 m into VERTCON
VERTCON told me the datum shift at that point was (-0.194 m) = NGVD 29 162.561 m

Since I cannot have two different NGVD 29 elevations at the same point, and since the difference in datum shifts from last week to this week is 0.017 m and since the difference in OPUS-RS elevations from GEOID03 to GEOID09 is 0.017 m, and since my GEOID03 submission results last week equals my GEOID09 submission results this week, I can only conclude that last week VERTCON was using GEOID03 and this week it began using GEOID09.

Paul in PA

OK, Let Me Explain

Paul: do you not read what people are saying? VERTCON does not in any way shape or form use a geoid model. It is strictly a program to transform between two vertical datums. It is a model created from a data set consisting of a set of benchmakrs where both adjusted NAVD88 and adjusted NGVD29 values are available. It used the differences at the common points to create a grid of datum shifts, and then it interpolates for your position. There is no way it can come up with two different values for the same point. It uses a NAD83 position as input. The fact is, you could use a NAD27 position and you are not likely to see any difference, maybe a mm or so.

As I pointed out above, you must be confusing VERTCON with CORPSCON.

NEVER MIND, I CAN'T BELIEVE I DID THAT

4 TIMES TODAY I WENT BACK TO VERTCON, AND EVERY TIME I TYPED IN LONGITUDE 75 INSTEAD OF 74. The 1 DEGREE OFF NEVER CAUGHT MY ATTENTION ONLY THE FACT THAT THE DIFFERENCE WAS EXACTLY THE SAME AS MY DIFFERENCE IN GEOIDS.

I wasted a day and I wasted some of your time, for which I apologize.

Paul in PA