Change of Datum values

[Christian-Shom]

In the attached file, elements on possible issues when changing a Datum value, but not the data underneath).
Further investigation is needed.
Issue #9 - Datums.pdf
Datums.xlsx

[Annette-BSH]

From a quick research in our data:
Baltic Sea:
• ‚mean sea level‘ (VERDAT = 3) -> will be available for both M_VDAT and M_SDAT
North Sea:
• ‚highest astronomical tide‘ (HAT; VERDAT = 30) for M_VDAT related objects, e.g. BRIDGE
• ‚lowest astronomical tide‘ (LAT; VERDAT = 23) for M_SDAT related objects, e.g. OBSTRN
-> should be ok, as both options will be available for the related datum type

There are also some objects referring to a local datum (VERDAT = 24), which will be available in S-101, too.

I am not sure I understand your concern correctly. I see two possible problems that could arise from the need to change the VERDAT entry.

1. Changing the entry in VERDAT will have to be accompanied by a change in the other attributes connected with it (e.g. VERLEN, HEIGHT for M_VDAT or VALSOU for M_SDAT).

2. Changing the entry in VERDAT (connected to M_VDAT) will also effect VERDAT of other objects (connected to M_SDAT) in the area.

I agree with the first issue. This will cause a lot of work, but it is unavoidable or the data will be ruined.

The second issue is no problem in my opinion, as attributes refer either to M_VDAT or M_SDAT and therefore are independent from each other. The tricky part is to separate the two groups. The definitions in the DCEG for M_VDAT and M_SDAT are confusing, as they both mention ‚datum for sounding reduction‘. For reference (DCEG Vers. 1.0.1):

3.8 Sounding Datum
IHO Definition: SOUNDING DATUM. The horizontal plane or tidal datum to which soundings have been reduced. Also called datum for sounding reduction. (Adapted from IHO Dictionary – S-32).

3.9 Vertical Datum
IHO Definition: VERTICAL DATUM. The reference level used for expressing the vertical measurements of points on the earth's surface. Also called datum level, reference plane, levelling datum, datum for sounding reduction, datum for heights.. (Adapted from IHO Dictionary – S-32).

Maybe the definition of vertical datum can be changed to not include 'datum for sounding reduction' to avoid confusion?

Conclusion: As long as the connected attributes are adjusted according to changes in M_VDAT (or M_SDAT) there should be no problem with attributes referring to the datum definition (besides the effort needed to deal with the amount of objects). If the amount of objects affected by the changes is too high, maybe ‚Local datum‘ (24) could serve as (temporary) wildcard until all objects can be adjusted to fit with the new ‚real‘ vertical datum.