sea level on maps

[woody234]

How do you find the sea level of your house in france using the michelin maps website, is it the numbers in red that are the sea levels or is it the blue numbers, im looking at deux sevres area

[Alex H]

Usually marked by a triangle with the height in metres next to it (black print)

I can't see any blue numbers

[Iceni]

Just open the window and look out - if you're at or below sea level you will soon know. 

John [8-)]

[AnOther]

You can find it on GEOPORTAIL

Select CARTES/Carte IGN on the left and then slide the opacity slider to the right. You then have a contour map on which spot heights are marked.

PS: Also gives you Northings and Eastings or SatNav coordinates [;-)]

 

[woody234]

thankyou for replys, geoportail is very good, i wonder how they work out the sea level for a particuler village or town

[cassis]

I think it used to be calculated using differences in barometric pressure from a fixed point close to mean sea level (sea level itself varies, of course).  Each country had its own location at which mean sea level was fixed.  Now many agencies use radar measurements taken from the shuttle Endeavour in 2000.

http://srtm.usgs.gov/

[Clair]

[quote user="woody234"]thankyou for replys, geoportail is very good, i wonder how they work out the sea level for a particuler village or town[/quote]

The IGN (Institut Géographique National) ihas "repères de nivellement" on public buildings and/or road sides.

You can see a map and find the nearest ones to your location (indicated by a yellow dot) here: http://geodesie.ign.fr/

(a long click on the hand tool brings a selection menu, go to the second option (Aller à...) and enter the name of your commune under field 1, choose a map scale and search)

More details (in French) here: http://www.ign.fr/telechargement/education/fiches/relief/R%C3%A9seaux_nivellement.pdf and here: http://www.ign.fr/telechargement/education/fiches/relief/Reperes_nivellement.pdf

[Quillan]

Re barometric pressure. Officially, as many aviators will know, the pressure is deemed to be 1013.2 millibars at sea level except when landing and taking off for obvious reasons. This is the 'magic number' that is used during flight for measuring height.

I was wondering if there was a specific reason the OP was asking. If it's because of flooding and building permission for instance there are other things that come in to play which is why you need a 'cut through' survey of the land to present to the DDE. This is then forwarded to have a computer model created. If its for something else then I appologise.

[Ian]

Google World has a figure for height above sea level - just glide your cursor over the area and the numbers scroll by - likewise the northings and eastings.

[woody234]

so ive checked geoportail and I think the sea level is 204 metres for vernoux en gatine deux sevres, do you think this is correct

[Mik from LA]

The contours shown on the Geoprtail site give heights of contours at 5 metre intervals. (i.e. height difference between contours is 5 metres). Every 25 m  contour (e.g. 100; 125; 150; 175) is labelled somewhere along its length. To get your exact height simply interpolate between contours. In your case the position you want to height should be 4/5 ths of the distance from the 200 metre contour (which should be labelled somewhere) towards the 205 metre contour). There is a 200 metre contour wandering around your village so my guess is that you are correct. If you want me to check this for you, go to Geoportail or Google and place the cursor over the position you want the height of. Read off and send me the lat and long of that position and e-mail or pm me the results and I will confirm the height.

These heights were established by IGN (French equivalent of Ordnance Survey) and are relative to observations of mean sea level at Marsaille averaged over something like a 10 year period at the end of the 1800's. As has been said, it depends on what you need the height information for. Observations of height from GPS, as used in Google for example, use a different  reference spheroid so will be slightly different from those shown on IGN maps. In practise you are usually only interested in differences in height so as long as you only use one system (GPS or IGN) you will get a useable result.

Mik

[Quillan]

I thought I might add that if you want to know the height for any official reason get a geometric to measure it for you. I say this having had a CU turned down initially because the plot was 28cm below the flood line. Simple enough to deal with as the plot is at an angle, just move the location on the plot or add a mound. They do work down to exact centimeters for this sort of thing. If its just for 'fun' or curiosity then by all means use the systems given below.

[woody234]

I need the height level because sea levels are going to rise by 7 metres when the ice caps melt so the low laying lands will probably be flooded 

[powerdesal]

Do you have a timescale ? [8-)]

[Just Chris]

Woody234, I hope you're not serious about the melting of the ice caps!  Water-borne ice melt will make absolutely no difference to sea levels (Arctica), and whilst in some areas in Antarctica land-borne ice  may be melting, it is thickening in others, and probably cancelling out any possible sea level change.

Chris

[Mik from LA]

Woody. It is not just a case of adding 7 metres to mean sea level to

see which parts are affected. You must add the highest astronomical

tide to the height of mean sea level then allow a bit more for weather

variations. But don't worry, nowhere within many miles of Vernoux en

Gatine is threatened by rising sea levels.

Quillan, that is very interesting. In UK the EA publish flood plain maps. Do you know if the same thing is available here?

[Quillan]

[quote user="Mik from LA"]
Quillan, that is very interesting. In UK the EA publish flood plain maps. Do you know if the same thing is available here?[/quote]

Yes they do and I think there is a website you can visit. You should also be able to get a copy from your mayors office or the DDE office but it's only a rough guide. If you live near any water you will have to have a survey done which is passed on to the DDE who in turn pass it on to another department who run a computer model. I did ask the DDE for the heights but it is like extracting blood from a stone. It's probably to do with the French way of thinking as in somebody gives you a figure and you put your application in and it's refused. You then say 'hang on a minute Mr Bloggs said the height was XXX and my house will be above it' and then it all gets complicated.

[AnOther]

[quote user="woody234"]

I need the height level because sea levels are going to rise by 7 metres when the ice caps melt so the low laying lands will probably be flooded 

[/quote]Woody, I'm sure a great many of us would be very interested in either the source or the recipe for whatever elixir you're taking to ensure your survival until this event happens (if ever that is) [:-))]

[:D][:D][:D]

 

[Quillan]

Actually there are some quite respected scientists that say this will happen. Likewise there are some equally respected scientists that say its a load of old cobblers. It depends on who you want to believe. Personally, if it is true, I wont be around to see it, I'll be long gone. However I rather think the comment was made tongue in cheek. [;-)]