(above, below???) the underside/soffit of the (floor/ground??) slab at el. 22.10.

NGF means Niveau Générale de la France, i.e. ordnance survey elevation, which is defined in a far more draconian fashion in this country than in the UK, say, where we would simply say 'm asl' (metres above sea level) if pushed. "El. xxx" (for Elevation xxx metres) is sufficient.

Or you could say "x m above datum" or a whole lot of other things.

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Note added at 2004-02-09 13:39:14 (GMT)
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Bugger, my E finger has gone trigger-happy.