What is georeference?

The georeferencing of a graphic element (line, text, point,...) corresponds to each point of the latter a couple of geographical coordinates (in degrees, in meters...). This process makes it possible to display information in a space context. The advantages are a juxtaposition with other layers of data occupying the same geographical space. It is then possible to measure actual coordinates, distances and surfaces.

Each country has its own georeference system(s). These are thegeographical coordinate systems.

Several coordinate systems may exist:

• to cover areas of different territories
• because the parameters governing the geographic coordinate system have been revised.

Examples

In Belgium we mainly have Lambert 1972, 2005 and 2008 systems for projected coordinate systems (X, Y coordinates) as well as systems in geographical coordinates (latitude, longitude). Given the size of the territory, each of its systems covers its entire territory.

In metropolitan France, however, in addition to variations appearing over time, planned coordinate systems exist on portions of the territory: from Lambert 1 to Lambert 4 to the south. More recently, global systems have emerged, such as Lambert 93, which covers the entire metropolitan area (with the help ofConical conformancespread over the entire north-south extent of the territory – CC42 to CC50).

How to coexist with othersCoordinate systems?

Geographic coordinates (spherical) systems define a location by coordinates of latitude and longitude. In a given system one could therefore designate any point of the globe. This is why the GPS system is based on a system of geographical coordinates: the WGS 84.

Projected coordinate systems (rectangular or cartesian) are always related to a geographic coordinate system. They are easier to grasp because they express the position of a point in unit length (metric). It is therefore very easy to assess the distance between two known points. We usually work here with false coordinates defined by systems. In a system "A" you can imagine that the coordinates have X-values based on 200,000 and Y-values based on 500,000 and that in a system "B" the magnitudes are based on X on 1,000,000 and Y on 2,000,000. In this way it is very easy to recognize which system belongs to coordinates but above all to detect errors.

It will easily be understood that in a plan established in a coordinate system it is strictly prohibited to use coordinates defined in another coordinate system. It is therefore imperative to perform a calculation to transform the position of a known point in the "A" system towards its position in the "B" system: this operation is called "Projection“.

And inMicrostationHow is this going?

As often at Bentley complex operations are done in a very simple way! InMicrostation(and for any other product based on this platform) simply indicate to the main file that the data it contains is defined in a system ("A" for example). The files that will be referenced (in XRef forAutocadien.ne.s) may also have their own coordinate system. When referring, it will suffice to indicate toMicrostation"redraft" the coordinates of all the reference elements in the master file coordinate system.

This is done on the fly and each point is individually transformed without altering the source coordinates. Clean and effective!

Illustration

In the video below we will illustrate how to define the coordinate system of a file. In second intention, we will attach a file in the SHP format (ESRI shapefile) as a reference in XRef. This type of file allows the exchange of geographical data. It therefore natively includes the definition of the coordinate system used by the elements it contains. Finally, we will use a remote data source (WMS server) to display a map background composed of orthophotographs. The latter is proposed in part freely in the form of ageoportalby the Public Service of Wallonia (Belgium). Similar services are available from various organizations around the world.