[Seavox] Depth, height and L111

[Roy Lowry]

Dear Eric,

An off-list e-mail reminded me that I hadn't responded to your point that the L111 list contains a mixture of depth datums (essentially references to sea surface such as LAT or mean sea level - effectively a GEOID) and height datums (dry land surveying benchmarks such as ODN or NAP).  At the moment L111 includes both because we get the same metadata - say current meter depth - from different sources to different datum types.  For example all Dutch 'depths' are converted through local mean sea level determinations to heights (so they are negative) above NAP (a bolt in the middle of Amsterdam).  However, BODC 'depths' are mostly measurements relative to sea surface at unspecified state of the tide.

It is obvious from your comments that further work on L111 definitions is needed and L111 possibly needs splitting in two, but I feel I need detailed guidance from you for this.  There are two possible ways I can see to make this happen.  One (if you can find the time) is for you to take the list as it is now and produce a version including your suggested improvements.  The other would be for me to spend a couple of hours with you next time I am in Brest, which I think is inevitable some time soon as SeaDataNet progresses (and much easier now FlyBe have started direct flights from Manchester).

Cheers, Roy.