GeoEarthScope vertical datums?

Yet another question received via email - this is a good one on vertical datums:

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I downloaded my DEM from Google Earth and brought it into GIS. The area I am looking at is giving me pixel readings of around 6 and we believe the elevation at this point is over 100. I have read and read and from what I can find, the data is in WG84, Zone 10 and the vertical datum is NAVD88. Is that right? I am assuming that if the horizontal datum is in meters the vertical is in meters? That still leaves me with a huge vertical elevation difference.

Any help is appreciated!

Reply:
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The GeoEarthScope LiDAR data are in ellipsoidal elevations, not the more commonly used orthometric (height above sea-level) elevations. You can calculate the geoid separation here: http://sps.unavco.org/geoid/ Once you have the separation value, you can "move" the DEM into the correct orthometric location with a quick bit of raster math (using something like the raster calculator in ArcGIS).

The vertical datum of the GeoEarthScope data set was not fully addressed in the first generation of the metadata document produced by NCALM. Since this issue comes up occationally, I asked NCALM to include a discussion of vertical datums in all future metadata reports on GeoES data they deliver to OpenTopo. I've added a revised NoCal metadata report to the Metadata & Files section of OpenTopo. The text of the new addition to the metadata report reads:

The users should also be aware that the elevation values of all datasets are heights above the ellipsoid (WGS84) and not orthometric heights. The ellipsoid-heights are measured along the ellipsoid normal in contrast to the orthometric heights which follow the direction of the gravity. In many applications the term elevation most commonly refers to the orthometric height of a point. Ellipsoid heights from GPS surveys are converted to traditional orthometric values by applying a geoid height using the latest geoid model from the National Geodetic Survey (NGS).

The Corps of Engineers Coordinate Conversion (CORPSCON, currently at v.6.0) tool can be used to transform the point data (XYZ ASCII tiles) ellipsoid heights into NAVD88 elevations using various GEOID models, including the latest iteration - GEOID03. The converted point data files can be then re-grided to ArcInfo raster format using your preferred interpolation technique. CORPSCON can be downloaded from this address:
http://crunch.tec.army.mil/software/corpscon/corpscon.html