In order to estabish plans with satisfactory precision, it was absolutely necessary to determine throughout the territory a certain number of clearly visible points which would serve as support for current topographical measurements.
In France, IGN. opted for the LAMBERT system (conical projection) thus creating a LAMBERT grid.
This grid makes it possible to designate all these points by their rectangular coordinates ( Y , Y ). To avoid excessive deformations, France is divided into four zones:
In France there are nearly 100,000 points making up the geodetic network. They are all visible from each other, and are therfore located high up. All these points are marked on notebooks and data sheet are made available 
₪ 4.2 The IGN69 network
In parallel with the geodesic network, the altimetric canvas was created:
4000,000 landmarks measured at altitude. Any topographic point can be obtained from this canvas con stituting the NGF. ( General leveling of France).
Altitudes are level, determined by the Marseille tide gauge.
IGN map extract at 1/25 000th
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