Postgis topology workshop session

Setting up a boundaries database with PostGIS topology

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Benjamin Deswysen

Benjamin Deswysen

→ Geomatician @GeoPostcodes

Polleur (Liège, Belgium)

ULiege, UNamur

Geo data science & engineering

OSM occasional contributor

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© Taïs Grippa - 2023

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“A collection of vector polygons representing either postal code or administrative division shapes.”

Boundaries database

GeoPostcodes products

“A truly global data set containing all administrative divisions, cities and postal codes for 247 countries.”

Worldwide postal database

GeoPostcodes products

“A collection of vector polygons representing either postal code or administrative division shapes.”

Boundaries database

GeoPostcodes products

“Complete your geo data with street-level information for address validation, autocomplete, and last-mile delivery.”

Streets database

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Topology in GIS

Topology

Geography <=> Territory delimitations and “ownership”

Topology

Geography <=> Territory delimitations and “ownership”

• Political boundaries
• Administrative divisions
• Economic zones
• Urban areas
• Cultural regions
• …

Topology definition

Topology expresses the spatial relationships between connecting or adjacent vector features (points, polylines and polygons) in a GIS.

Topological or topology-based data are useful for detecting and correcting digitising errors (e.g. two lines in a roads vector layer that do not meet perfectly at an intersection). Topology is necessary for carrying out some types of spatial analysis, such as network analysis.

Source: https://docs.qgis.org/3.28/en/docs/gentle_gis_introduction/topology.html

Topology errors

Source: https://docs.qgis.org/3.28/en/docs/gentle_gis_introduction/topology.html

1. Undershoot: vector lines that should connect to each other don’t quite touch
2. Overshoot: line ends beyond the line it should connect to
3. Slivers: vertices of two polygons do not match up on their borders.

Topology rules examples

• Area edges of a municipality map must not overlap.�
• Area edges of a municipality map must not have gaps (slivers).�
• Polygons showing property boundaries must be closed. �Undershoots or overshoots of the border lines are not allowed.�
• Contour lines in a vector line layer must not intersect (cross each other).

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Source: https://docs.qgis.org/3.28/en/docs/gentle_gis_introduction/topology.html

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Postgis topology

= postgresql extension

- 4 Edges

- 4 Nodes

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Example of polygon boundaries

1 Polygon

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GIS topology = outgrowth of graph theory

It’s a �graph!

Postgis topology datamodel

=> Topological primitives

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“lines”

“polygons”

“points”

Postgis topology datamodel

Face

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Edge

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Node

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Topogeometry type

Topogeometry = postgis topology column type

= layers (groups) of primitives

= composed object � (topology_id, layer_id, topogeo id, type)�

• Doesn’t contain any geometry directly, only references
• A table in topology needs a topogeometry column
• Documented in topology.layer and topology.topology metadata tables

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Topology ≠ Geometry

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Advantages of postgis topology

• Storage: store only once the shared edges�
• Edition: edit shared edges, not polygons�
• Detect topologic errors: gaps/overlaps�
• Simplification: simplify edges, not polygons�
• Implement nested boundaries: aggregate features based on an admin hierarchy

Advantages of postgis topology

• Nested boundaries: aggregate features based on hierarchy

Advantages of postgis topology

• Simplification: simplify edges, not polygons

VIZ / HPR boundaries databases

Simplification

Real polygon

Simplification: careful with tolerance value

1 Polygon� - 2 Edges� - 2 Nodes

Simplification

→ Oversimplification = Empty geometry!

Simplification: careful with tolerance value (2)

1 Polygon� - 2 Edges� - 2 Nodes

Densify topology� - 5 Edges� - 5 Nodes

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Simplification

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Q&A

Any questions?

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Workshop

https://bdeswysen.gitlab.io/postgis_topology_workshop

Get your hands dirty!

https://bdeswysen.gitlab.io/postgis_topology_workshop