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Vertical Datum; WGS84 GEOID and NGVD29   Terry Staggs Jan 30, 2008
Re: Vertical Datum; WGS84 GEOID and NGVD29   Melita Kennedy Jan 30, 2008
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Subject Vertical Datum; WGS84 GEOID and NGVD29 
Author Terry Staggs 
Date Jan 30, 2008 
Message I downloaded SRTM data from USGS. Metadata states the vertical datum is WGS84 GEOID (not ellipsoid). I need this in NGVD1929. can't find anything concerning differences between the two, although a couple of things I've read suggest they may be identical Geoids. Anybody know anything about the diffences between these two GEOIDs and how to convert them(I.E. difference grid).

Thanks,

Terry! 
   
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Subject Re: Vertical Datum; WGS84 GEOID and NGVD29 
Author Melita Kennedy 
Date Jan 30, 2008 
Message Hi Terry,

NGVD29 is definitely not the same as the WGS84 geoid. A geoid is often realized through a model like WGS84 has its EGM96 geoid model.

I don't believe there's any direct converter between EGM96 and NGVD29, but I found a paper that calculated the EGM96-NAVD88 and NAVD88-NGVD29 differences and adjusted the area of interest data that way.

http://www.ucgis.org/summer03/studentpapers/yongxindeng.pdf

VERTCON which converts between NGVD29 and NAVD88 is at

http://www.ngs.noaa.gov/TOOLS/Vertcon/vertcon.html

You can also pull control point data sheets for your area of interest here:

http://www.ngs.noaa.gov/cgi-bin/datasheet.prl

(steps: WGS84 geoid aka EGM96 to ellipsoid height, ellipsoid height to NAVD88 height using GEOID03 (see NGS website), NAVD88 to NGVD29 using VERTCON)

Melita 
  Melita Kennedy
ESRI Product Specialist