Gioidal Vertical Shifts all zero?

Discussion created by alanboatman on Jun 30, 2014
I have a DSM that was generated in UTM WGS84 with the default Elipsoidal 'Height Above Ellipsoid' vertical coordinate system (datum).

I would like to Project this raster to the same xy UTM WGS84, but with a local vertical datum.  When I do this with raster projection tool the z values remain exactly the same.

Indeed when I look at the vertical shifts of all of the Vertical Datums that you can choose from the shift is always zero.

Can anyone explain this to me, since I thought the whole point of a local vertical datum was to apply local MSL, rather than a global ellipsoidal datum.

Also does anyone know how I can find what the vertical shift of a particular Vertical Datum should be, from say UTM WGS84 Ellipsoid, for say,  Hon Dau 1992 Vertical datum?

Any help would be much appreciated.