Is there a standard vertical datum for meters below the WGS84 ellipsoid, or below MSL?

Question

I have data I am publishing a WMS layer representing data under the surface of the water, say temperature at 100m depth. What is the accepted way to represent this in WMS GetCapabilities document?

Do I define the dimension like:

<Dimension name="elevation" 
           units="EPSG:5030" 
           unitSymbol="m">0,50,100</Dimension>

or

<Dimension name="elevation" 
           units="EPSG:5030" 
           unitSymbol="m">0,-50,-100</Dimension>

or something else or should a custom dimension be used in this case?

EPSG:5030 is a length parameter, it doesn't indicate that it's related to any projection or that it might indicate distance from mean sea level for that matter. see http://trac.osgeo.org/geotiff/wiki/VerticalCS
EPSG:5030 represents height above the WGS85 ellipsoid. I found a reference to EPSG:5715, but it doesn't say much other than it's 'msl depth' - does this mean values below msl should be positive?

Answer 1

After some further reading, it looks like the specifics of which orientation z values increase in depends on the specific vertical coordinate system (VCS) being used. Vertical systems oriented with 'z up' are height-based VCSes, as opposed to depth-based VCS where z is down. EPSG:5715 is an example of one such depth-based system, which when examined in the EPSG registry includes this information:

Vertical CS. Axis: depth (D). Orientation: down. UoM: m

The ArcGIS 10 help on vertical coordinate systems similarly states:

Another important part is whether the z-values represent heights (elevations) or depths. For each type, the z-axis direction is positive "up" or "down," respectively.

Finally, a proposed revision to the ISO 19111 standard includes this information on vertical datums:

4.55
vertical datum
Datum describing the relation of gravity-related heights or depths to the Earth

NOTE
In most cases the vertical datum will be related to mean sea level. Ellipsoidal heights are treated as related to a three-dimensional ellipsoidal coordinate system referenced to a geodetic datum. Vertical datums include sounding datums (used for hydrographic purposes), in which case the heights may be negative heights or depths.