Conversion from EGM96 to NAVD88

Confused. EGM96 is a geoid that is usually applied to the WGS84 ellipsoid for ortho heights. NAVD88 is a vertical datum with heights related to the GRS80 ellipsoid, NAD83(2011) horizontal datum location and the Geoid18. Kind of apples and oranges.

You should be able, if you have the software, to apply the EGM96 model to the GRS80 ellipsoid to get an ortho height with a position either in NAD83 or ITRF, then compare to a NAVD88 height.

Here’s what you do

1: back out of your EGM98 height to you original WGS 84 ellipsoid height.  Make sure you know which iteration of WGS 84 you have - the current realization is WGS 84 (G2296).

2: Go the the Nation Geodetic Survey web site - www.NGS.NOAA.gov and open the utility HTDP from their Geodetic Tool Kit.  This will allow you to transform your WGS 84 lay,long,ellipsoid ht to NAD 83 (2011) values.

3: Open GEOID18 under the Science and Education banner from the home page and perform Interactive Computation which will give you the geoid ht.

4: Add the geoid ht to the NAD 83 (2011) e ht from step 3 and you have NAVD 88 at the uncertainty stated in the Geoid18 computation.

If you have any problems with this get in touch directly at [email protected]

Forgot to mention also make sure you know the epoch of your WGS 84 values, the epoch for NAD 83 (2011) is 2010.0, you will need these to properly use HTDP.

I have several projects going on right now (military) where they want NAD83 (2011) epoch 2010.00 and NAVD88 as well as WGS84 (G2139) current epoch and EGM08. I know that G2139 is not the latest, but that is what they requested and the difference from G2139 is very small. 

Some are done with VRS, others with RTX, and some with a local base positioned by post processed RTX. When using a VRS I use HTDP to transform the CORS coordinates to WGS84 (G2139). I do separate adjustments for NAD83/NAVD88 and WGS84/EGM08. I would say that whether NAVD88 is above or below EGM08 would depend on location. 

Base9 describes the best way to go from NAVD88 to EGM08 without doing a readjustment. You need to be careful about what geoid was used for NAVD88 and what realization. Also what realization of WGS84, although the recent realizations have not differed much at all in ellipsoid height.