Coordinate positioning

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This chapter introduces the concept of locating railway infrastructure elements by using coordinate reference systems.

Besides the intrinsic and the linear track-referenced coordinate system, the RailTopoModel also allows to locate railway infrastructure elements using geodetic and geometric coordinate systems. The most known example in this case is the World Geodetic System WGS84.


A geometric coordinate system can either be a projected X, Y, Z coordinate system, or a λ, ϕ, h geodetic coordinate system, or even an X, Y(, Z) schematic plan coordinate system.
It could be read as "at coordinate X, Y, Z in the system [EPSG:xxxxx] [01/06/2014]4" [4: The exact content of the definition string of the geometric system is not yet final]
The model integrates a link between the linear positioning system and the intrinsic positioning system. It supports the "conversion" of an element position associated with a number between 0 and 1 to a kilometric point on its reference system.


As there are a number of different coordinate reference systems with different parameter sets available, it is important to identify the coordinate system without ambiguity. The International Association of Oil & Gas Producers set up the EPSG <ref>European Petroleum Survey Group</ref> Geodetic Parameter Registry, which provides a unique code for every coordinate system. The websites and can be used to obtain an overview about different coordinate systems by their EPSG code.


Relevant classes in the RTM

The classes, used to define the location and positioning systems and their coordinates, are:

  • PositioningSystem: This class defines the general concept of a Coordinate Reference System used for positioning.
  • LinearPositioningSystem: This class defines a Linear Referencing System. It defines a start and an end coordinate.
  • LinearAnchorPoints: this class describes the reference points within the linear reference system (Milestones, anomaly points...) and their characteristics.
  • LinearCoordinate: This class defines the localization expressed in a Linear Reference System (LRS).
  • GeometricPositioningSystem: This class defines a Geometrical Reference System, so it allows to locate a resource with its geometrical coordinates (x, y, z or λ, ϕ, h). The class provides the parameter crsDefinition to define the Coordinate Reference System. This parameter shall be used to name the EPSG code and thus define all the relevant parameters of the coordinate system.
  • GeometricCoordinate: This class defines the localization expressed in a geometrical (or geographical) Reference System, so it defines the coordinates (x, y, z or λ, ϕ, h).
  • PositioningSystemCoordinate: This class represents a coordinate in either a geometric or linear reference system.
  • IntrinsicCoordinate: This class allows associating an intrinsic coordinate coming from the topology network to another coordinate, either geographic or linear.
  • AssociatedPositioningSystem: This class allows to group couples of coordinates to define the translation parameters between an external (geometric or linear) coordinate system and the element’s intrinsic coordinate system.

What you should have learned
  • Coordinate reference systems are (besides intrinsic and linear referencing) the third option to define positions in the RailTopoModel.
  • All geodetic coordinate reference systems can be defined by a unique identifier: the EPSG code.
  • Coordinate reference systems can also be used to define schematic coordinates as they are used in terms of screen coordinates.

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Positioning - Track-referred positioning