I am trying to figure out how to convert from Google Earth elevation to Australian Height Datum (AHD).
Google Earth's elevation / altitude is based on The vertical component (altitude) is measured from the vertical datum, which is the WGS84 EGM96 Geoid.
I assume that WGS84 is Ellipsoidal value.
The absolute difference between GDA94 and ITRF is now approximately one metre. Since the WGS84 reference frame is now aligned to the ITRF at the centimetre level, the absolute difference between WGS84 and GDA94 is also approximately one metre.
I ignore the 1 metre difference.
And I use AUSGeoid09 http://www.ga.gov.au/ausgeoid/nvalcomp.jsp to compute.
I enter the latitude and longitude from Google Earth and into GDA94 Latitude and GDA94 Longitude text boxes. And I select Ellipsoidal for GDA94 Height (m) and enter the elevation value I get from Google Earth and click on compute. Then the next page will show me the AHD value for this location and assuming that the Google Earth elevation value I entered (WGS84) can have 1 metre difference (I don't know if this is + or - but I can try both +1 and -1).
What I am trying to do is to compare the Q100 flood level with the AHD level on any given Australian point. Say the council gives me a Q100 flood level in AHD, say 10m AHD, and I am trying to find out if the given point has a higher AHD value of 10 or not.