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Presentation of the plugin concepts#

The RoadNetwork plugin helps to manage a topological county road network and to perform linear referencing with QGIS and PostGIS.

Road network in QGIS

Road network#

Each road:

  • has an identification, e.g. D907,
  • is composed of a series of ordered edges (simple linestrings).

Markers are positioned along the roads. Their position is often measured with a GPS (X,Y).

Marker on the ground

Topology#

The road network is a graph built with edges and nodes:

  • each edge has a start node and an end node,
  • an edge can have a previous edge and a next edge,
  • intersecting edges are cut by a node (if they are on the same level).

Edges and nodes graph

Position and references#

This simple organization of roads, edges and markers allows to determine the position of any object based on:

  • a road code: D902,
  • a marker code: 1,
  • an abscissa: 500m.

You can also provide an optional offset, e.g. 3m, and a side, e.g. left.

References schema

Any point within a distance of 50m of any road will have its references: D902 PR 1 + 500m (3m left).

Linear referencing#

Each data object, point or (multi)linestring, is defined by its references calculated from the road network:

  • Points: trees, street lights, car accidents, etc.
  • Linestrings: paintings, safety barriers, roadworks, etc.

We must be able to:

  • calculate the references from the object geometry,
  • create a geometry based on given references:
    • a point,
    • a multi-linestring between two references.

Linear referencing illustration

Corner cases#

Discontinuity: edges could have been deleted (or have changed status).

Discontinuity

Roundabouts have their own road code, e.g. GIR D35-D80, and cut the other roads.

Roundabout principles

Glossary#

Concept Definition
Graph A directed graph made of a set of edges and nodes representing the road network as a road centerline network, matching the axis of the carriageway.
Road A consecutive sequence of edges forming a road / a way (e.g. D115).
Edge Portion of a road. An edge is a directed segment linking two nodes (simple line).
Node The nodes represent the start and end vertices of each edge.
Marker Point created by the road network manager, physically materialized on the ground by painted marks associated with kilometer markers.
Virtual marker Point created by the GIS administrator to make the administration of the graph easier, especially the computation of the reference point + abscissa.
Reference point (PR, point de repère) Formerly known as kilometer point.
Business or thematic data Set of thematic data overlaid on the graph and topologically linked to it. It can be point data (e.g. road signs) or linear data (e.g. pavement surface).