Week 10: Topology and Network anlysis Flashcards
(31 cards)
Vector topology supports more advanced spatial analysis, in support of
Asset management and modelling of stormwater systems
What is a GIS topology
A set of rules and behaviors that model how points, lines, and polygons share coincident geometry
Three types of relationships are of interest to GIS
- Contiguity
- Area definition
- Connectivity (shared nodes)
Topology uses
- Planar enforcement
- Data validation
- Spatial analysis (e.g network tracing, polygon adjacency and no polygon overlap without creating new polygons)
Generally two main ways that the concept of topology is implemented in a GIS
- Strict reliance on planar enforcement
- Traditional approach used in ESRI’s original coverage data model - New flexible, rule-based methods
- Newer approach based on user selected rules within a geodatabase (e.g the databases you have used in the labs)
Characteristics of Arc-Node topology
- Each Arc starts with a From-Node and ends with a To-Node
- all arcs and nodes are uniquely numbered - Each arc has a direction, based on the From-Node and To-Node
- All connecting arcs share a common node
- Collectively, these allow the software to
- determine paths trhough a network of connected arcs (e.g roads, rivers, electrical lines, etc)
- Ensure closure of polygon features
Characteristics of Polygon-Arc topology
- Polygons are stored as a list of arcs (polylines) that comprise boundaries
- Polygons have a single (one and only one) label point that links to attribute table using a unique identifier
- Based on Arc Direction (From and To Nodes), left and right polygons can be identified
- hence adjacency of polygons is explicitly defined
Why use topology in GIS
Re routing - to streets that connect at an intersection share a common node (no need to inspect the coordinates of each street arc)
Advantages of topology
- Using stored topological data for analysis
- Supports contianment, contiguity and connectivity operations
- Inbuilt validation of geometry
Disadvantages of topology
- Change of geopmetry requires complete rebuild
- More complexity
- Storage
- Editing
The network data model uses
Specialised topology for connectivity
In networks, geometric features are
Points / nodes (street intersections) and lines (streets)
Network topology is in terms of
Lines connected at nodes
Network flow of resources can be
Directional or not
Network flow is defined by
Impedance, or weighting associated with a line or node (think about elevation as an example)
Four applications of networks
- Over an electricity network, e.g, calculating power load drops
- Over a street network, e.g routing emergency response vehicles or optimizing the route of mail deliveries
- Over a stream network, e.g tracing pollution upstream to source
- Plus a raft of applications associated with linear referencing
The connectivity of features is based on
Their geometric coincidence (topological rule)
As edge is
A feature which has a length through which some commodity (resource) flows
Examples of edges
Water mains, electrical transmission lines, gas pipelines, telephone lines
A junction is
A feature that allows two or more edges to connect and facilitates the transfer of flow between edges
Examples of junctions
Fuses, switches, service taps, valves
Simple edge
Connected to exactly two junctions at each end
Complex edge
As above but can be connected to additional junctions in between
User defined junctions
Defined from users point (feature) data source
