GIS networks consist of interconnected lines (known as edges) and intersections (known as junctions) that represent routes upon which people, goods, etc. can travel.
The object traversing the network follows the edges, and junctions appear when at least two edges intersect.
Junctions and edges can have certain attributes affixed to them that increase the cost of traveling in the network, known as impedance. For example, a road network can have speed limits attached to the edges, and a junction can prevent left turns.
Networks are either directed, in which only one direction of travel is allowed within the network, or undirected, in which any direction of travel is allowed.
Networks are widely used for two types of modelling
- Utility networks: including water mains, sewage lines, and electrical circuits. These networks are generally directed.
- Transportation networks: including roads, railroads, and flight paths. These networks are generally undirected.
Types of Operation performed on Network Analysis
One common type of network analysis is finding the shortest path between two points. In a network of streets, the “shortest” route can either refer to different variables, such as: distance, time, and monetary cost (such as purchasing a plane ticket). An ambulance driver looking for the shortest path to his destination will travel the route that will get him or her from point A to point B in the least amount of time.
The traveling salesman problem is defined as reaching every point in a network in the most efficient way possible. It is derived from the idea of a salesperson trying to reach a planned set of cities to sell his or her product in the quickest, most efficient way possible, either through money made, or time. UPS uses a traveling salesman algorithm to efficiently deliver as many packages as possible to their customers every day.
Network partition is a dividing up of regions in a network to zones or subcategories.
These regions are sized based on proximity to specific points in a network. This is common for fire stations in metropolitan areas.
Basic functions already existing in GIS including buffer, overlay, query, etc. are useful in Transportation Planning. However, deeper analysis of network data is available for planning applications. Examples of such higher uses include network flow equilibrium models, travel demand models, trip generation and distribution, as well as activity-based models and transportation/land-use interaction models. The latter use is particularly useful as demand for transportation influences land use, and reciprocally, the changed land-use’s influence on transportation. Limited commercial software exists to perform such tasks, nevertheless, skilled programmers should be able design programs to perform these tasks where software is inaccessible or non-existent.
The two major GIS software’s, ArcGIS and QGIS have tools for solving this problems.
ArcGIS makes use of Network Analyst Toolbar and QGIS makes use of the QGIS Network Analysis Toolbox 3.
Watch this space for how to use this tools for your network analysis.