Hey, I never promised you a non-hydro post and here I am. I've recently been e-mailed about the navigational surface for hydro - ie a realisation of the seabed but in a format which is easily exchangeable. One of these is the BAG, Bathymetry Attributed Grid (this is one of many posts associated with this). It's not the only gridded format, but one which is becoming increasingly used. This is a good article about the bathymetric surface project.
It got muddy (in my eyes) with the realisation of depth with respect to WGS84 (or equivalent). Once they started quoting my FIG papers I realised I was in trouble! I'm not so fussed about my papers, but the FIG publication is worth a read if that's what you're into
We have to be so careful that we don't confuse the bathy surface/BAG with referring depths to a geodetic reference frame. OK, perhaps it's only me but I hope not.
Shall I just crawl back into my geo/geeky/hydro box?