Has anyone used the VDatum web site ( https://vdatum.noaa.gov/) for EGM1996 to NAVD88 conversions??ÿ If so, have you found it to work?
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You got the parenthesis into the URL, which breaks it. ?ÿ I haven't used the tool.
You want to perform an EGM96 ?ÿto NAVD88 height conversion using VDATUM. ?ÿAs seen in one of the images below, it does not appear to be possible using the tool. I get an ??out of bounds? error. I did use coordinates from a nearby CORS in Nevada; far from any coastline.?ÿ
Be advised that an NAVD88 height is a particular type of orthometric height. Hybrid geoid models were created to enable use of GPS-derived ellipsoid heights to derive NAVD88-compatible heights.?ÿ
To obtain an orthometic height using the EGM96 model requires that the ellipsoid height be in WGS84/IGS and NOT in NAD83(2011). ?ÿI was unable to select the EGM96 model in the on-line version of VDATUM. This was my first experience with this version of VDATUM and I may not understand how to correctly use the tool. That said, I found it interesting that the input IGS coordinates yielded incorrect NAD83 values.?ÿ
I was able to derive an orthometric height from a WGS84/IGS ellipsoid height and EGM96 using the UNAVCO page here:?ÿ https://www.unavco.org/software/geodetic-utilities/geoid-height-calculator/geoid-height-calculator.html ?ÿBTW, EGM96 is not the most recent NGA model.
I can also derive an NAVD88 height given an NAD83(2011) ellipsoid heights and the GEOID12B hybrid model of the ellipsoid-geoid separation.
I can also derive an orthometric height from an IGS ellipsoid height and the USGG2012 (or the other ??X?gravimetric geoid models. This is NOT an NAVD88 height.
Unfortunately, the gravimetric model derived heights are NOT NAVD88. See the output from the X gravity model below. ?ÿ
As can be seen from the output of the ??X? gravimetric geoid model there are decimeter-level discrepancies in the derived heights. Note as well that the UNAVCO site tool height disagrees with the X model heights.
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Original post inspired some review of EGM2008. References provided for those with an interest in the topic. BTW, some of the larger files on the NGA site take a long time to load/download and sometimes ??time out.?
http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm2008/egm08_wgs84.html - Overall discussion of EGM2008.
https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2011JB008916 - Journal of Geophysical Research article on EGM2008. Lots of dense maths...
https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/jgrb.50167 - errata for article above. Note reference to restrictions on using data at high resolution.
https://www.mathworks.com/help/aerotbx/ug/geoidheight.html#responsive_offcanvas - Matlab script file description NOT actual Matlab ??m? file. Note statement about accuracy of EGM96 modeled geoid heights: +/- 0.5 to +/- 1.0 meters.
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I don't think that is possible in VDatum. Here is how you need to do it...
1) transform the EGM96 elevation to an ITRF ellipsoidal height by "unapplying" the EGM96 separation.?ÿ
2) Use HTDP to transform the ITRF ellipsoidal height to a NAD83 (2011) ellipsoidal height
3) Use GEOID12B to transform the NAD83 (2011) ellipsoidal height to an NAVD88 elevation. You can use either the NAD83(2011) transformed lat/long or the ITRF lat/long for input to GEOID12B. 1.5 meters error in horizontal position is not going to make any difference in the interpolation?ÿ
While Mr. Hamilton??s approach has the virtue of being straightforward and logical, it relies on the assumption that an EGM96 height refers to the same geoid as NAVD88.?ÿ
In the bottom left of the images posted in my first reply to this thread are shown various orthometric heights computed from a starting IGS08 position. Note that the NAVD88 height derived from GEOID12B significantly differs from those using the gravimetric geoid. The heights for the point I was using for numeric examples indicates a disagreement between the NGS USGG2012 orthometric height and the EGM96 computed orthometric height of greater than two decimeters. Note as well the differences between the hybrid model height and the various gravimetric model heights. ?ÿ
Orthometric heights determined using USGG2012 and EGM96 are determined using IGS08/WGS84 coordinates and a gravity-based geoid model. USGG does not include the corrector surface nor datum transformations incorporated into the hybrid model.
In the link provided to a Matlab site there is mention that the accuracy of the EGM96 modeled heights is ?ÿ+/- 0.5 to +/- 1.0 meters. The latest EGM is EGM2008; a significant improvement.
While the height determined using Mr. Hamilton??s method will certainly better approximate NAVD88, it would not be good enough for me. Of course, the original post did not provide much in the way of details as to intended accuracy nor usage.
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Actually I was assuming that the EGM96 height was derived from an accurate ITRF/WGS84 position, in which case doing the reverse to get back to ITRF and then HTDP to NAD83(2011) would be fairly accurate. Of course maybe that was a faulty assumption.?ÿ
