Blog GIS & Territories

To state the problem, here is an excerpt of the publication
“Geometer n ° 2087,  December
2011″: The issue of the month “Safety, reliability. Buried networks” by Laurent
Polidori (director of ESGT) and Gille Costa (surveyor) :

” The basement of our cities has become a real Gruyère whose holes serve as a gateway to a growing number of networks.
Electricity, telephone, district heating, gas, optical fibre, water … All these networks intertwine, rub shoulders, overlap and some have been there for so long that even the memory is lost and their managers know more about the exact location. To open a trench on the public road becomes more and more risky. So much so that after very serious accidents, the MEEDTL decided a very large reform. A one-stop shop will list all the network managers, contacts and eventually the areas of implementation. It will be consulted for the declarations of project of work (DT) and the declarations of intention of beginning of work (Dict).
In the process, the regulation of these declarations is recast.

All new network work will be georeferenced and subject to three accuracy classes.
The goal is simple: to reach in 2019 to know with certainty the location of all that runs under our feet … For more security and reliability. ”

All the operators of the various networks are required, by January 2019
to respect the obligation of basemap and georeferenced routes for the sensitive
networks buried in urban units. For networks outside urban areas, the deadline
is January 1, 2026.

The job seems simple at first glance. Three classes have been defined:

Definition of the three<br /> network classes

Class A
A structure or section of a structure is classified in Class A if the maximum
uncertainty of location indicated by its operator is less than or equal to 40
cm if it is rigid, or to 50 cm if it is flexible. The maximum uncertainty is
raised to 80 cm for underground civil engineering works attached to
installations intended for the circulation of railway transport vehicles or
guided when these structures were constructed before 1 January 2011.
Class B
A structure or section of a structure is classified in class B if the maximum
uncertainty of location indicated by its operator is greater than that relative
to class A and less than or equal to 1.5 m.
Class C
A structure or section of a structure is classified in class C if the maximum
uncertainty of location indicated by its operator is greater than 1.5 m, or if
its operator is not able to provide the corresponding location

Let’s leave aside, for a moment, the technical aspects of measuring
uncertainty and move on to the first major problem of this project. It all
boils down to the phrase ” if the maximum uncertainty of location
indicated by its operator is higher … “.

Definition of LOCATION.

Everyone understands this sentence without having to think about it. If
I have a gas pipe under the sidewalk, parallel to the edge of it, I can
classify this pipe in class A if its position is known with less than 40 cm of
uncertainty. In front of me the sidewalk is 70cm wide, so necessarily the
driving will be class A. It would be so simple, only if it was true!

First thing that should put us in the ear: why in the previous example
ask 40 cm of precision for a measure that everyone can do with a centimeter
with a precision of at least 1cm?

The answer is much more complicated than the question. To know what the
meaning of the word location is to go back, stopped to stop far back.

But I give you a part of the answer right away: the location of the pipe
is not made according to the sidewalk, or any other remarkable element near the
pipe.

First text to know:

Order of 16 September 2003 on precision classes applicable
to categories of topographical work carried out by the State, local authorities
and their public establishments or carried out on their behalf

The text defines two types of objects, point objects and linear or
polygon type objects. For these second, the localization concerns the
identifiable points which construct the lines or the polygons. The location of
a point, according to this order, is as follows:

Localization of a<br /> network point

7.1. Point geographic objects

If the specifications indicate, some geographical objects may be
considered as punctual. They are then determined by the planimetric and if
necessary altimetric coordinates of their reference point. The accuracy class
applies to the difference between the coordinates obtained for each point by a
control measure and the coordinates provided for those points; the possible
support points and canvases included in the survey being excluded from the
points tested.

Now our sidewalk has disappeared and we have to provide planimetric
coordinates. In other words, we must provide the latitude / longitude of the
points constituting our conduct, projected on a plane.

With regard to the accuracy of these coordinates, the text indicates:

Location Precision<br /> Measurement

6.1. Total accuracy class

The precision class defined above applies to the differences between the
coordinates provided for each point and those obtained for control measures. The
total error results from the composition of the internal errors, the errors of
attachment, and the error specific to the legal network of reference. Therefore,
the total error can not be less than one of these three sources of error, and
in particular the error of the legal reference network, as specified or as a
result of the discrepancies noted. during attachment.

There I feel that I lose you! The text speaks of three types of error:

• internal errors: these are
  measurement errors, for example the accuracy of the GPS used.
• the errors of attachment: they
  are, for example, the errors of seizure during the integration in the database
  of the network.
• the error specific to the legal reference
  network: it is the positioning error due to the projection of geographical
  coordinates (latitude / longitude) in planimetric coordinates (X / Y).

The legal reference network is defined by the following Decree:
Decree No. 2000-1276 of 26 December 2000 implementing
Article 89 of Law No. 95-115 of 4 February 1995, as amended, for the
development and development of the territory relating to the conditions of
execution and publication of plan surveys undertaken by public services –
Article 1

This article defines the planimetric coordinate system to be used by all
utilities:

Planimetric Reference<br /> Systems

The national reference system of geographic, planimetric and altimetric
coordinates cited in article 89 of the aforementioned law of 4 February 1995 is
defined as follows:

A. – Geographic and planimetric reference systems:

In the previous table, the “conic conforming 9 zones” are added to the list of projections, in the case of metropolitan France.

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Atilio Francois