Calm down Lily, it’ll be much better in the morning. 
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Good luck with the fencing today 
Thanks, up and ready for him, he’ll be here in half an hour.
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Our village neighbour has angrily told us that ownership of their property includes air rights to the stars and our mailbox sticking out of our garden wall is in their air space. I just laughed and shook my head.
Apparently, after due research I found the legal concept lies encoded in the Latin phrase Cuius est solum, eius est usque ad coelum et ad (“Whoever owns the soil, it is theirs up to Heaven and down to Hell”). However, as in UK modern times have reduced this to an ‘upper stratem’ for air traffic and ‘lower stratem’.
I expect our neighbour will be using the sublevel stratem for her eventual decent.
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Brilliant, I’ll write that down, for later use. 
As a point of interest, in the uk you cannot own land. All land belongs to the crown. All you can have is an “interest” in the land.
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Hi Mik,
In France we have two different altimetrique 0 points NGF and in Paris NVP. The NGF 0 point is the sea level in Marseille and I forget the 0 point in Paris.
How many does the UK have and where is or are the 0 points .
Really? The Scottish government talks quite specifically about who owns land in Scotland.
My grovelling apologies John, I only studied English Property law as part of my course and even then paid scant attention to such a boring subject for a youngster chomping at the bit to get out and map the world.
In Scotland, they are a law unto themselves.
No worries, I’m not a Scottish nationalist by any means but one of my hot buttons is the casual use of England for the UK and vice versa.
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Mia culpa
She’s a funny girl…
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Gosh Wozza, now you are opening up a can of worms.
I guess that by “altimetrique 0 points” you mean mean sea level for mapping purposes. As I understand it, in France, Marseilles is the reference point for the whole of France. The second one mentioned still uses Marseilles but is older, slightly different and used for early mapping of Paris.
In UK the reference point is Newlyn.
BUT this is all ancient history from a mapping point of view. These days, mapping is achieved using satellite observations. The height information is related to a geoid. This is a surface of equipotential gravity surrounding the earth. The problem is a geoid is an irregular object with lumps and bumps according to the minerals in the ground at any locality.
For practical purpose a “best fit” spheroid is used to “simplify ” mathematical calculations. This is a triaxial oblate spheroid (or squashed rugby ball in less posh language). The height reference point is the centre of that spheroid.
Just to complicate matters, there are four major satellite positioning systems and they all use different spheroids so any navigation system has to make corrections to incorporate measurements when using all four systems together.
When all that is accomplished, it then has to be made to fit existing but old mapping that was measured on a mish mash of systems and presented on various map projections depending on what the mapping is for (another deep rabbit hole).
TL/DR one – Newlyn
Well you did ask Wozza.
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If I owned land in England I would definitely be taking it up with the Blessed Oliver, or his spirit, as if I wasn’t already Republican enough. 
Yes. Thanks for the in-depth response.
I just wondered as I receive plans, section cuts and 3D models of parisien buildings and topographies with either NGF or NVP. Alignment XYZ can be fun on the software I use. But most is due to lazy modeling.
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Marseille. No S 
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If you are talking about mismatches between models and topographies the NGF?NVP question will only relate to the Z component of XYZ coordinates. NGF is 31.3 cm higher than NVP
When it comes to XY mismatch in XYZ coordinates you have to be aware of the projection on which the coordinates are based. In revolution times a Cassini projection was used. Then there was a change to Lamberts Conical Orthomorphic but nowadays Lambert 93 is used because it is more compatible with satellite observations.
But, when it comes to mismatches, it could, as you say, just be lazy modelling
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Mareille? 
I stand corrected Vero. My ancient textbook calls it Marseilles but it is written in English!
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Same here for Wales. And no doubt Ulster. And Eire, come to that.