[Seavox] Vertical CRS Datums (length dimension)

[Eric MOUSSAT]

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 around 20 tides zones .
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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 > Seavox at mailman.nerc-liv.ac.uk
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-- 


                            Eric MOUSSAT

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