Reference points for data collection

 

The land surveyor is first involved in a project at the survey stage when information related to distances, horizontal and vertical angles and altitude is collected using a variety of different methods, eg laser equipment, survey rods (long vertical rulers) and GPS/GLONASS satellite signals etc. The data so obtained enables 3-dimensional, flat or altimetric models to be made almost immediately.

Network of control points for positioning

France has different GNSS networks permanently in place such as the RGP. Above is the RGP's PANA station (Paris Nation) : permanent aerial and control station
France has different GNSS networks permanently in place such as the RGP. Above is the RGP’s PANA station (Paris Nation) : permanent aerial and control station
(© JF Delarue)

In metropolitan France, the geodesic network, established by the IGN (National Geographic Institute) and used as a legal reference, is RGF 93 (French Geodesic Network set up in 1993). This is a group of 1023 points spread over the whole country, with a point every 20 or 30 km. This in turn is linked to European (ETRS) and world (ITRS) networks. Part of this network is made up of permanent GPS stations (the National Geographic Institute’s RGP network).

Positioning himself using these reference points, the surveyor is able to obtain GNSS observations or measurements of angles and distances using modern (post 1980) survey equipment. In doing so the surveyor uses a method called traversing to supplement the IGN points, ie intermediate points (stations) are determined in the immediate vicinity of the details to be surveyed.

The measurement of altitude is called levelling and is carried out within the confines of another specific network. The system of reference for France is IGN 69 where level 0 is situated at the Marseilles tide gauge. IGN 78 is the system of reference for Corsica. These reference points are cast iron moulds fixed to various structures, usually public buildings. There are more than 400,000 of these in the whole of the country!

Absolute precision

Until recently (the 1980s), relatively crude and time consuming methods of measurements were used, eg chain measurement, ie measurement of distances with the aid of a chain but, today the use of modern instruments (tacheometers, levels, 3D laser scanners etc) has considerably shortened the time involved in carrying out surveys.

GNSS (GPS + GLONASS) positioning is the perfect solution to determine traversing stations which is then used for data collection and installations
GNSS (GPS + GLONASS) positioning is the perfect solution to determine traversing stations which is then used for data collection and installations
(© JF Delarue)

 

Great progress has also been made during the last few decades in obtaining and specifying more precise measurements, especially over distances. For example, when the surveyor states that the movement of a building could be 5mm, he must specify the accuracy of this figure. A movement of 10mm ±1 mm will have nowhere near the same effects as a movement of 10mm ±10mm !

The introduction of computer science (for databases, calculations and models) and the growth of geographic information systems has profoundly transformed the surveying profession. Computers and instruments with integrated electronics have provided speed of calculation and enabled, for example, databases being available onsite as well as the option for the automatic correction of errors.