Sorvey Definitions ₪ 4. CANVAS

Sorvey Definitions ₪ 4. CANVAS


₪ 4 .1 The LAMBERT system   
Definitions, Topography, plan, topographic, terrain,

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  Definitions, Topography, plan, topographic, terrain,

₪ 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  
Definitions, Topography, plan, topographic, terrain,

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