# Converting to Australian Height Datum from Google Earth elevation

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.

http://www.dse.vic.gov.au/property-titles-and-maps/geodesy/geocentric-datum-of-australia-gda

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.

Currently, the z values from Google Earth are PROBABLY relative to the EGM96 geoid model. AHD (Australian Height Datum) is another gravity-related vertical datum.

I think you should do some comparisons. Check the values you can get from Google Earth against some existing AHD values.

Secondly, use the EGM96 geoid model to convert the GE values back to WGS84/ITRF ellipsoidal heights (h). Then use the AusGeoid09 model to convert them to AHD. In addition, you might also try converting from WGS84/ITRF lat/lon/h to GDA94 lat/lon/h to try to remove that 1 m offset. However, the AusGeoid09's resolution may be such that this extra just wouldn't matter.

## Background information

I like to think that there are two types of vertical datums. One is ellipsoid-based heights that are really part of a 3D geographic coordinate reference system. The other set comprises a wide range of coordinate reference systems that are all gravity-related. You can have a geoid or quasi-geoid which is a surface of equipotential (loosely--gravity is the same at every point on the surface). A geoid model often approximates this surface as a raster (grid of points) where each point's value is the difference between the ellipsoid surface and the geoid surface.

A country's vertical datum was originally a leveling datum. That is, you identified a point on the coast, figured out its elevation relative to local mean sea level and then measure inland using relative measurements. Later, countries would take all that information, plus recent surveying done with GPS information and gravity readings and re-adjust the height datum.

Generally, when someone has a z value, it's an elevation, and somehow gravity-related, but that value may have been determined by several different ways (including different geoid models!). GPS natively returns ellipsoidal heights, but most GPS have or can accept a geoid model to convert it, at least loosely, into an elevation.