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Forward Star Network Structure: Centroid Nodes and Connectors

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ASU Trans+AI Lab edited this page Mar 8, 2025 · 4 revisions

Forward Star Network Structure: Centroid Nodes and Connectors

This guide covers the addition of Centroid Nodes and Centroid Connectors to streamline the building of GMNS networks compatible with various traffic assignment packages like TransCAD, Cube, VISUM, DTALite, and path4gmns.

Feature Overview

The forward star structure uses centroid nodes as central points in each zone, connected to other network nodes via connectors, enhancing the efficiency and compatibility of network files across various traffic assignment tools. This guide explains how to structure node.csv, link.csv, and demand.csv to incorporate centroid nodes and connectors.

1. Centroid Node Creation

Each zone will have a dedicated centroid node:

  • Centroid Nodes are assigned sequential IDs, starting from 1, 2, 3... based on zone_id.
  • These nodes should be listed first in node.csv for easier sorting and compatibility.

Example Format in node.csv

node_id zone_id x_coord y_coord node_type
1 1 100.0 200.0 centroid
2 2 150.0 250.0 centroid
... ... ... ... ...

2. Connector Creation

Connectors link each centroid node to its zone's activity nodes. This ensures internal zone connectivity.

  • From each centroid node, create connectors to each activity node within the same zone.
  • Link IDs and from_node_id/to_node_id pairs should reflect the new structure.

Example Format in link.csv

link_id from_node_id to_node_id length capacity free_speed link_type
1 1 100 0.5 1000 60 connector
2 1 101 0.5 1000 60 connector
... ... ... ... ... ... ...

3. Node Numbering and Sequencing

After listing the centroid nodes:

  • Physical nodes (activity nodes) are listed sequentially following centroid nodes in node.csv and those physical nodes including activity nodes should not have zone_id associated.
  • Ensure that node numbers are updated consistently across both node.csv and link.csv.

Updated node.csv Layout

node_id zone_id x_coord y_coord node_type
1 1 100.0 200.0 centroid
2 2 150.0 250.0 centroid
100 110.0 210.0 activity
101 120.0 220.0 activity
... ... ... ... ...

Updated link.csv Layout

link_id from_node_id to_node_id length capacity free_speed link_type
1 1 100 0.5 1000 60 connector
2 1 101 0.5 1000 60 connector
3 100 101 1.2 1000 60 roadway
... ... ... ... ... ... ...

4. Demand File Consistency

To match the new centroid structure:

  • demand.csv should follow the zone_id order of centroid nodes.

Example in demand.csv

o_zone_id d_zone_id volume
1 2 150
1 3 100
2 1 80
... ... ...

Purpose and Benefits

This forward star structure with centroid nodes and connectors improves:

  1. Compatibility: Ensures networks are easily importable into commercial and open-source assignment packages like TransCAD, Cube, VISUM, DTALite, and path4gmns.
  2. Efficiency: Optimizes node-link lookups, as used in industry-standard planning packages.
  3. Consistency: Supports seamless integration with traffic demand models, as shown in the Chicago Sketch example in DTALite and recent unified datasets.

Reference

This structure is also documented in:

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