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Sea level in France


mint
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I know that in the UK, we have an ordnance datum for sea level in Newlyn in Cornwall.

Here, in France, I have seen a benchmark in our local church so I would like to know what the French equivalent to sea level is in France.

Any ideas, please?

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Is this a silly question, but is sea level the same in Marseille as Brest?

I always look upon the Med as a big lake, which it almost is, yet not quite and wondered if this could make a difference.

If  I don't ask, I'll never learn[;-)]

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Thank you, Pacha.  Totally fascinating and I can now look at our local IGN map and determine our sea level.

Idun, I asked your question of OH who says that, for all intents and purposes, the sea level is the same in both those places.

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I am not at all sure that it is as simple as you think.

 

I am fairly sure I have seen different IGN  maps referinging to different sea level points.

Further the concept of water finding its own level is a somewhat flawed idea.  I am sure I have seen a report somewhere where it has ben shown that somewhere in the Indian Ocean (or maybe Pacific) has its water level at several times lower than anywhere in Holland compared with "sea level" - if that makes any sense at all.

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Yes, andy, the maps will show different sea level points but all the points should use Marseille as the benchmark.

For example, the UK sea level reference point was at Birkenhead but Newlyn was then chosen as it is as far away as possible from large rivers which could affect the "mean" sea level.

Where we lived in South Wales, the high and low points of the tide in the Severn estuary could be as much as 11 metres difference.

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Sorry that is not quite what I meant Sweet17.

 

I am sure I have seen IGN maps that state that the heights shown relate to somewhere other than the Mediterranean (Marseille).

 

It may be related to the 3 regions used for the Lambertin coordinates used throughout the IGN maps - Lambertin coordinates are used by gunners to help range and inclination (shades of the Ordinance Survey in the UK but taken one step further).  Please do not ask me to explain it.  It have seen it expalined 3 times and it all made perfect sence until half an hour later, but it all has something to do with conical projections - or something.

 

PPP is probably the font of knowledge.

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Andyh4 I understood perfectly what you meant, well about not remembering[;-)]

I was getting confused, but  have just realised that I have a good friend who will know this. They will be able to explain it in simple terms to a simple lady and I will believe that I have grasped what they have said, until ten minutes later, when it will sort of evaporate and I will be left with a residue of knowledge that will be skewed enough for me not to be able to explain it to another soul. However, I will ask them next time I see them as I would like to know, even if it is fleeting.

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[quote user="idun"]

Is this a silly question, but is sea level the same in Marseille as Brest?

I always look upon the Med as a big lake, which it almost is, yet not quite and wondered if this could make a difference.

If  I don't ask, I'll never learn[;-)]

[/quote]

Not a silly question at all. A bit of Googling and I found this:

"Because sea level is an average of the varying heights of the ocean's surface, it is the same all over the world. Sea level measurements are averaged between low and high tide and between the changes in depth caused by wind and waves. When a mountain is described as rising a certain number of feet above sea level, it is not exactly that number of feet above to waters at its nearest shore. It is that much higher than the average level of the ocean."

I'm not really sure, though, that this answers your question. I suppose that, if the Earth were a perfect sphere (which it is not) and if we didn't have moon (which makes tides) and if we didn't have winds (which make waves) then sea level would be the same all over the world. Two thoughts have struck me - one is the Coriolis effect, caused by the Earth rotating on its axis (supposedly causing bath water to flow in a fixed direction as it goes down the plug hole) and the other is theoretically weaker gravity at the equator (because of the Earth's shape) perhaps sea level is different in different parts of the world. I seem to recall being told when I was at school that the ocean levels at each end of the Panama Canal were different - but since this was not very much it could have been the result of a cumulative measurement error.

 

All things being equal, Marseille and Brest should have the same mean sea level.

 



 
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[quote user="idun"]

Is this a silly question, but is sea level the same in Marseille as Brest?

I always look upon the Med as a big lake, which it almost is, yet not quite and wondered if this could make a difference.

If  I don't ask, I'll never learn[;-)]

[/quote]

The reference is to mean sea level. The mediterreanean will be preferable as tidal variation will be less than on the atlantic coast so easier to measure. Tides are caused by the attraction of the sun and moon  imposed on the gravitational field of the earth, less effect in med.

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The earth bulges at the equator, down to gravitational pull of other bodies (the moon and planets) and the centripetal acceleration caused by the rotation of the earth.  Ask Superman said, gravity at the surface will be slightly less here as it's further from the centre of the earth.

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Slightly off tangent but when I walk down the lane here leading to the beach about half a km from the sea, the horizon is actually above me in the distance. Its a very strange feeling and makes you feel quite vulnerable when you see these movies with giant tidalwaves etc.
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I don't know if this helps, I am interested in what is below the water and not above it but it may help explain.

Everywhere is different so there is no standard sea level. When hydrographic information is applied to charts for navigation the depth is always measured at the lowest astronomical tide (LAT - not to be confused with the abbreviation for Latitude) which is depth you see on a chart. When sailing and to ensure you clear any obstacles you apply the highest astronomical tide (HAT) or in some cases the mean sea level (MSL) which is the mean of the two.

There are three bodies that monitor these levels, PSMSL (Permanent Service for Mean Sea Level), BODC (British Oceanographic Data Center) and UHSLC (University of Hawaii Sea Level Center). Each chart will have on it what the levels are in that area.

To give an example if your traveling down the tidal section of the Thames the measurements of height and time are from London Bridge (Tower Pier to be exact) so when you look at Tilbury on the chart it will say HW London Bridge +6 minutes so whatever the time of high water is at LB you add 6 minutes to it to get the time at Tilbury. Now you might think "Hang on a minute Tilbury is nearer the sea and it's the sea coming in that creates the tide rise so Tilbury should be before LB not after?". Well the reason for this is that the tide can't get past Richmond Lock so it 'backs up' which is why Tilbury is after LB. The height of the tide will also vary depending on position of the moon, the amount of wind and which direction the wind is blowing from. The point of explaining this is to show that all sorts of factor come in to stating what 'Sea Level' actually is at a given point.

So going back to above ground what they do in the UK is take the average of the Mean Sea Level from all 12 measuring points (East, London, North, North East, North West, N. Ireland, Scotland, South, South East, South West, Wales and West) and this is what OS maps are based on when giving heights above sea level. Nowadays however its a lot simpler, you simply use a very accurate version of GPS.

 

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Yes, I know that feeling Val2. I coaxed my other half to write a song about the ships above the town, as from certain points around here, there is the town and then the sea with the ships and then the sky all nicely 'layered'. Love it.

It is easy to sea the curve of the earth too and in olde days the ships must have easily been seen to go over the horizon, even from the higher ground. So much for the suggestion that the earth was ever flat.

I understand that there has to be a 'mean' sea level as the tides have variable high and low levels. I think that I too had heard at some point that the sea at both ends of the Panama Canal were different levels and hence my question. Think I have heard enough now and am struggling with some of it, but that is 'me' rather than needing to be told more.

Thanks for posing such an interesting question Sweet17, is there any reason for it?

 

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In the old days all heights were related to a Geoid. This is a surface where every point has an identical gravitational force. Heights on maps were derived from this surface by spirit leveling. A datum, such as the Marseille tide gauge, simply defined which geoid you were on. Mean sea level was chosen as a convenience to mariners, who did most of the mapping, because all you had to do to find the depth of water under your ship was add the calculated tide height, at your current time and location,to the sounding on the chart.

The geoid is an irregular surface depending on the nature of the underlying geology. It would represent sea level if there were no tidal influences.

Then along came GPS. This derives heights geometrically and does not vary like the geoid. The GPS heights are referenced to a Spheroid which is a regular shape. It is chosen to be a best fit with the current Geoid used for mapping so that any differences are small.

Heights have nothing to do with map projections. Maps deal with position i.e. lat and long. Heights are simply labels on that map

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This is all getting a bit heavy, Idun your question ‘is sea level the same in Marseille as Brest?

Depending on the time of tide I think Marseille would be slightly higher than due to the earth’s rotational forces (nearer the equator)

I’m sure this will be corrected by an expert.

 

All you need to do is have a marine chart (Not a map) and with a tidal atlas add the height of tide; you then have the depth of water. (Bit more to it than that but this will do)

 

Far easier is to go out in a boat and read the depth on a sounder or use a stone attached to a length of string.

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[quote user="audio"]

This is all getting a bit heavy, Idun your question ‘is sea level the same in Marseille as Brest?

Depending on the time of tide I think Marseille would be slightly higher than due to the earth’s rotational forces (nearer the equator)

I’m sure this will be corrected by an expert.

 

All you need to do is have a marine chart (Not a map) and with a tidal atlas add the height of tide; you then have the depth of water. (Bit more to it than that but this will do)

 

Far easier is to go out in a boat and read the depth on a sounder or use a stone attached to a length of string.

[/quote]

The level of the sea will be different but the thread is about a Datum, commonly mean sea level, measured contiuously over several years.

Consider the Panama, quoted above, and forgetting the complementary gravitational effect of the sun, spring tides and all that jazz.

The moon due to its distance from the earth has a greater gravitational effect, inverse square law and all that jazz.

So when the moon is over the middle of the Atlantic it will "suck up" the water as a consequence the Atlantic end of  the Panama  will be lower  than the Pacific end.

Whan the moon is over the middle of the Pacific the level of water will be lower at the Pacific end of the canal, etc , etc.

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Tide Tables for Marseille and Brest today

Brest

               HW      LW      HW      LW

Time       04:48   11:03   17:05   23:30

Height      6.4m   1.7m   6.7m      1.5m

Marseille

               HW      LW      HW      LW

Time       03:00   07:47   14:14   20:44

Height      0.3m   0.4m   0.2m   0.5m

HW = High Water

LW = Low Water

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The Thames tidal saga reminds me of my childhood instruction at the Yacht Club in Swanage as to the importance of double high tides.[:)]

In may I spent an enjoyable moment between Buck Shore and the Life Boat House.

Quote:

 At this point we can get very local and consider what happens at Swanage which is the major coastal town in the Isle of Purbeck.
Swanage is founded around Swanage Bay, but effectively in the English Channel. The tide is funnelled as it passes round Kent and spread as it moves west to the Isle of Wight where it is 'split' each side of the Isle of Wight. Having so done it 'rejoins' aroundabout the Needles, but since it has covered different distances and been subjected to different sea-floor conditions and depths there is a lag on the north side.

http://www.isleofpurbeck.com/tides.html

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[8-|][quote user="pachapapa"][quote user="audio"]

This is all getting a bit heavy, Idun your question ‘is sea level the same in Marseille as Brest?

Depending on the time of tide I think Marseille would be slightly higher than due to the earth’s rotational forces (nearer the equator)

I’m sure this will be corrected by an expert.

 

All you need to do is have a marine chart (Not a map) and with a tidal atlas add the height of tide; you then have the depth of water. (Bit more to it than that but this will do)

 

Far easier is to go out in a boat and read the depth on a sounder or use a stone attached to a length of string.

[/quote]

The level of the sea will be different but the thread is about a Datum, commonly mean sea level, measured contiuously over several years.

Consider the Panama, quoted above, and forgetting the complementary gravitational effect of the sun, spring tides and all that jazz.

The moon due to its distance from the earth has a greater gravitational effect, inverse square law and all that jazz.

So when the moon is over the middle of the Atlantic it will "suck up" the water as a consequence the Atlantic end of  the Panama  will be lower  than the Pacific end.

Whan the moon is over the middle of the Pacific the level of water will be lower at the Pacific end of the canal, etc , etc.

[/quote]

 

I am quite aware that this thread is about datum; my post was directed to Idun in an effort to answer her question.[8-|]

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