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Mean Sea Level
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ihbpah
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 Posted: Thu Sep 29th, 2011 07:40 am

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The TWLWG have asked us to review the definition of Mean Sea Level. The attachemnet is self explanatory.

Attachment: Mean sea level.zip (Downloaded 3 times)

JerryMills
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 Posted: Thu Sep 29th, 2011 09:09 pm

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I agree with the proposed new definition.

Jean Laporte
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 Posted: Wed Aug 14th, 2013 10:39 am

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After consulting the latest TWLWG5/9 report and considering Jerry Mills' and the Secretary supportive opinions, I suggest that the HDWG report to HSS5 be drafted as follows:
 
Task B8: Mean Sea Level


In view of the self-explanatory paper submitted by TWLWG, the new definition of Mean Seal Level is accepted.

Subject closed

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The HD could then read as follows:

S-32 New English definition:

mean sea level

The average HEIGHT of the surface of the SEA at a TIDE STATION for all stages of the TIDE over a 19-year period, usually determined from hourly height readings measured from a fixed predetermined geospatial reference level.

It should be noted that:

The measured value of mean sea level is tied to the location and the observation period. Therefore the tide gauge location and the observation period should always be quoted where the value of mean sea level is to be used as a precise reference level.
 
The measured mean sea level is only an apparent level. Vertical movements of the earth’s crust at the location of the tide gauge will affect the mean value measured.
 
Sea level also has long period variations (the global rise of the oceans). These also affect the mean value measured.

Where modelling is used to eliminate the movement of the earth’s crust (post glacial rebound for example) and/ or the effect of the global rise of the oceans, the resulting mean sea level is relevant to a specific epoch. This epoch (year) and the modelled annual change of mean sea level should always be quoted when mean sea level is used as a precise reference level.

S-32 New French definition:

 niveau moyen de la mer

Moyenne des hauteurs de marée observées à un marégraphe pendant une période de 19 ans, les hauteurs de marée étant prises habituellement toutes les heures et mesurées à partir d'un niveau de référence géospatial prédéterminé.
 
Il convient de noter que:

- le niveau moyen de la mer mesuré est fonction du lieu et de la période d'observation, aussi ces derniers doivent-ils toujours être mentionnés quand le niveau de la mer est destiné à servir de niveau de référence précis.

- le niveau moyen de la mer n'est qu'un niveau apparent. Les mouvements verticaux de la croûte terrestre au marégraphe affectent la moyenne mesurée.

- le niveau de la mer est sujet à des variations à long terme (élévation du niveau des océans) qui affectent également la moyenne mesurée.

- quand l'élévation du niveau des océans et/ou les mouvements de la croûte terrestre sont filtrés par modélisation (par ex. rebond post-glaciaire), le niveau moyen de la mer résultant est spécifique d'une époque (année). Cette dernière et les variations modélisées doivent toujours être mentionnées quand le niveau de la mer est destiné à servir de niveau de référence précis.

 S-32 New Spanish definition:

(to be completed)

:

ihbpah
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 Posted: Tue Aug 20th, 2013 09:20 am

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I have passed this proposed definition to the TWLWG for comment by 28 August.

ihbpah
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 Posted: Tue Aug 27th, 2013 10:22 am

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Further refinements to the definition from the TWLWG, see below:

The average HEIGHT of the surface of the SEA at a TIDE STATION for all stages of the TIDE over a 19-year period, usually determined from hourly height readings measured from a fixed predetermined geospatial reference level.

It should be noted that:

According to GLOSS Program to define the mean sea level for one place in order to absorb all meteorological effects it is needed  at least 40 years of observations is needed.
The measured value of mean sea level is tied to the location and the observation period. Therefore the tide gauge location, the location and ellipsoidal height  (Datum and Epoch)  of the reference bench mark and the observation period should always be quoted where the value of mean sea level is to be used as a precise reference level.

The measured mean sea level is only an apparent level, the relative mean sea level. Vertical movements of the earth’s crust at the location of the tide gauge will affect the mean value measured mean sea level value.

Sea level also has long period variations (the global rise of the oceans). These also affect the mean value measured mean sea level value.

The absolute mean sea level is obtained from the corrections of  tides by correcting the tide series for the vertical  movements of the earth´s crust  through the as determined by a Continuous GPS control station installed  close to the tide gauge after at least 10 years of GPS measurements. Therefore the CGPS location and observation period should always be quoted where the value of absolute mean sea level is to be used as a precise reference level.

Where modelling is used to eliminate the movement of the earth’s crust (post glacial rebound for example) and/ or the effect of the global rise of the oceans, the resulting mean sea level is relevant to a specific epoch. This epoch (year) and the modeled annual change of mean sea level should always be quoted when mean sea level is used as a precise reference level.

I think we are close to a preferred definition, any further input or comment?

ihbpah
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 Posted: Wed Aug 28th, 2013 05:47 am

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A further refinement to the definition from the TWLWG, see below:

The average HEIGHT of the surface of the SEA at a TIDE STATION for all stages of the TIDE over a 19-year period, usually determined from… replace:“hourly height readings measured from a fixed predetermined geospatial reference level.” with: hourly height readings measured relative to a bench mark.

It should be noted that:

According to GLOSS Program to define the mean sea level for one place in order to absorb all meteorological effects it is needed  at least 40 years of observations is needed.
The measured value of mean sea level is tied to the location and the observation period. Therefore the tide gauge location, the location and ellipsoidal height  (Datum and Epoch)  of the reference bench mark and the observation period should always be quoted where the value of mean sea level is to be used as a precise reference level.

The measured mean sea level is only an apparent level, the relative mean sea level. Vertical movements of the earth’s crust at the location of the tide gauge will affect the mean value measured mean sea level value.

Sea level also has long period variations (the global rise of the oceans). These also affect the mean value measured mean sea level value.

The absolute mean sea level is obtained from the corrections of  tides by correcting the tide series for the vertical  movements of the earth´s crust  through the as determined by a Continuous GPS control station installed  close to the tide gauge after at least 10 years of GPS measurements. Therefore the CGPS location and observation period should always be quoted where the value of absolute mean sea level is to be used as a precise reference level.

Where modelling is used to eliminate the movement of the earth’s crust (post glacial rebound for example) and/ or the effect of the global rise of the oceans, the resulting mean sea level is relevant to a specific epoch. This epoch (year) and the modeled annual change of mean sea level should always be quoted when mean sea level is used as a precise reference level.

I think we are even closer to a preferred definition, any further input or comment?

jwootton
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 Posted: Mon Sep 2nd, 2013 07:36 am

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I do not think that the Hydrographic Dictionary is the place to include a whole lot of technical information related to the determination of Mean Sea Level - the Dictionary should contain only the definition of the term (i.e. "what it is" rather than "how is it determined").  The additional notes proposed would be more appropriately included in a technical manual or specification.  Additionally, in terms of the suggeted additional notes:

-  The first note immediately contradicts the definitions by stating that at least 40 years of observations are needed.

-  The 2nd paragraph of the notes introduces a new term "relative mean sea level", and the 4th paragraph yet another term "absolute mean sea level" - the notes state that by using the term "mean sea level" we are really saying it is the "relative mean sea level" and then states how to derive values for the "absolute mean sea level" - what are we trying to define here?

I support that refinement of the definition to relate the measurements obtained to a bench mark, but do not support the additional notes.  Additionally, HDWG will need to consider similar definitions related to tidal observations in the Dictionary such as Mean Low Water Springs, Mean Lower Low Water Springs, Mean Low Water, Indian Spring Low Water, Mean Lower Low Water, Mean High Water, Mean High Water Springs, Mean Higher High Water, Lowest Astronomical Tide, Highest Astronomical Tide, ....

Jean Laporte
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 Posted: Tue Oct 1st, 2013 01:08 pm

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Good point! (which I shall pass to the TWLWG Chair lady who happens to be one of my SHOM colleague).
The HDWG report to HSSC5 has been modified accordingly.

Jean Laporte
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 Posted: Fri Oct 11th, 2013 04:37 pm

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1. Gwenaelle Jan, the new TWLGWG chair lady, explains that the apparent contradiction between the 19 and 40 years observation are due to the fact that her WG is now tasked with addressing chart datums in tidal and non-tidal areas.
Whilst 19 years is standard in tidal analysis, it is irrelevant in non-tidal areas where secular variations are predominant. The Resolution 6/1932 actually recommends that the IHB should… “encourage Member States to carry out systematic, long-term tidal observations, records of typically 40 years or longer, in view of the importance of monthly and secular variations of mean sea level in connection with tidal prediction".

Ms Jan further specifies that (quote):
“The reason why 2 cases in the chart datum definition resolution is the following:
One "generic" definition should be defined and be coherent with the two cases identified by the TWLWG.
Two subdefinitions and not one …… due to distinct dynamics and  differents hydrographics practices: Ocean areas and inland waters. If we define chart datum as MSL, hydrographic centers would be in trouble for marine chart datum. If we define LAW as chart datum for inland waters where there is no tidal signal or a very weak one, hydrographic centers would be in trouble (ex Amazonian River, Sea area near Finland coast, etc.)”.

 
2. Re the very long definition, Ms Jan is thinking (quote) “to divide the [amended HD definition] in 2 parts one clearly identified as Definition and one identified as Technical recommendation to get a coherent chart datum relative to physics …. for areas under tidal influence (tidal height> 30cm) and those where tides represent less than 30 cm of water height”.
 
I am afraid this will have to be further clarified.


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