[Seavox] Vertical CRS Datums (length dimension)

Fri Feb 16 15:10:04 GMT 2007

Hello Eric,

If you give me a spreadsheet of entries that you wish to be added (name abbreviation and definition) then I will add them - easily done.

 I am also not a specialist (to the level that I did not know there was an EPSG vertical CRS datum list) and so depend upon help from people like yourself.  Using EPSG codes for keys where possible would have been a nice idea, but unfortunately I have already loaded the vocabulary into the system and the one thing the system is designed to do is stop me changing keys.  However, it does allow me to change definitions so I could add EPSG references to the appropriate definitions.  Could you possibly let me have a map along the lines of EPSG code against the key I've used (the Dnn) numbers and I'll do this.

The 'French Naval Chart Datum' came from the EDIOS database as the vertical datum label for one dataset located between 33.75 and 34.83N, 35.25 to 35.92 east (Lebanese Hydrobiology Monitoring Programme) . The actual description for the datum on the MIF was 'French Navy-Navigation Maps'.  If you can give me a better description to use for this in the vocabulary then I will use it.

I will leave it to Pieter to answer the question on the definition for ED50/EPSG4230 in the L101 list.

I hadn't really considered the maintenance issue as I assumed that EPSG entries were stable.  If new entries are made to EPSG that we feel are relevant to our work then they can be added.  No matter what, for anything linked to EPSG I would consider them to be the authoritative governance. Having what is effectively a SeaDataNet subset to EPSG has been done in attempt to simplify vocabulary usage for partners (I find EPSG impossible to use as a non-specialist) and to provide users with a single uniformly managed source for all vocabularies used in SeaDataNet metadata.

Cheers, Roy. 

>>> Eric MOUSSAT <Eric.Moussat at ifremer.fr> 2/16/2007 1:09 pm >>>
Dear Roy

  sorry for my long silence (too much busy)

I have some comments relazted to CRS lists ( 110 and 111).

1/111
Vertical  French CRS are missing in your list. So they should be added.

For example, elevations above sea level in French mainland, even on 
nautic charts refer to IGN69 vertical CRS and in Corsica to IGN78
.

2/EPSG codes for vertical CRS
Among the vertical CRS, they are vertical system which are used for 
elevation which have EPSG code.

For France
IGN69 vertical CRS = EPSG code 5720
IGN78 with EPSG code = 5721

They are other CRS for oversea territories that I will transmit to you 
later as I don't have the whole list at present.

Anyway I think we should have some consistency in the way we described 
the CRS in list 110 and 111 as we mention EPSG code with (partial) EPSG 
defintion in the first one and not in the second.

3/Naval chart datum
It is not enough to mention  "naval chart datum" .

France is divided in tide zone (see attached document from French HO). 
When some body has to correct water height to depth, he has to use the 
tide correction of the nearest refering port of the zone where 
measurements were carried out. These correction relates to an 
hydrographic "zero" level specific to the zone and to the port within 
the zone.

 From one zone to another you may have an offset of several tens of cm.

This is due to the evolution of method and technology used to define the 
0, to the fact that it is a conventionnal (and not an observed) level to 
ensure the safety of navigation, to the fact that some references are 
very old (first one definesd in Brest 200 y ago) and to the sea level rise.

As a consequence, you have to mention :
- the refering port used when mentionning the naval chart datum (this is 
done in our Environment Dpt)

- the date of definition of the zero. In Brest, the present 0 (defined 
in 1996) is 50cm above the previous one . In St Nazaire, 40 cm below the 
previous one.

This is probably somewhat which concerns other countries with high tides 
or/and old datum.

3/11O list
I gave a look to the description of CRS and noticed , at least for ED50 
(4230 EPSG code) that the definition does not fit completiley the EPSG 
one. The original defintion precises that ED50 was used for French 
hydrographic chart. This should be mentionned as numerous data still 
refer to this system in spite of the fact that RGF 93 is the new legal 
ref.system.

This raise the question of the maintenance of these information , 
already avalilable in EPSG DB. How it is planed to do it ?

I am going to leave for one week due to the holidays of my children. I 
will see your reply not before the 26th. I hope this mail be helpfull 
and I thank you for your understanding

Regards

Eric

-----------------------------

Roy Lowry a écrit:
> Dear All,
> 
> Another draft vocabulary for comment, covering CRS datums assuming the dimension of 'length'. Further lists (L112, L113 etc.) will eventually be set up for other non-length based CRS (pressure, density etc.).  
> 
> I appreciate that the content is currently very Euro-centric.  Relevant additions for the USA (including definitions please) welcome.
> 
> Cheers, Roy.
> 
> 
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                           Eric MOUSSAT

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