Network Analyst

Lecture 4

What is network?• A network is a system of interconnected elements, such as edges

(lines) and connecting junctions (points), that represent possible routes from one location to another.

• People, resources, and goods tend to travel along networks: cars and trucks travel on roads, airliners fly on predetermined flight paths, oil flows in pipelines. By modeling potential travel paths with a network, it is possible to perform analyses related to the movement of the oil, trucks, or other agents on the network.

• The most common network analysis is finding the shortest path between two points.

• ArcGIS groups networks into two categories: – geometric networks, and – network datasets.

Geometric networks

• River networks and utility networks—like electrical, gas, sewer, and water lines—allow travel on edges in only one direction at a time. – The agent in the network—for instance, the oil flowing in a pipeline—can't choose

which direction to travel; rather, the path it takes is determined by external forces: gravity, electromagnetism, water pressure, and so on. An engineer can control the flow of the agent by controlling how external forces act on the agent.

• In ArcGIS, utility and river networks are best modeled by geometric networks.

• Geodatabase feature classes (or feature dataset) are used as data sources to define the geometric network– two types of features: edges and junctions– A geometric network can be built in the Catalog tree

• Import data into new or existing feature classes. • Build a geometric network from the feature classes. • Establish connectivity rules for the geometric network.

• This is a not a focus of today’s lecture

Network datasets• Transportation networks—like street, pedestrian, and railroad

networks—can allow travel on edges in both directions. – The agent on the network—for instance, a truck driver traveling on

roads—is generally free to decide the direction of traversal as well as the destination.

• In ArcGIS, transportation networks are best modeled by network datasets. (require the Network Analyst extension)– A single mode of transportation, roads or railroads,– A multimodal of transportations: roads, railroads, waterways, etc.– 3D network datasets, enable model the interior pathways of

buildings, mines, caves, etc.• This is the focus of today’s lecture and lab

Types of network analysis layers• ArcGIS Network Analyst allows you to solve common network

problems, such as finding the best route across a city, finding the closest emergency vehicle or facility, identifying a service area around a location, servicing a set of orders with a fleet of vehicles, or choosing the best facilities to open or close. 6 types or solvers:– Route– Closest facility– Service areas– OD cost matrix (an origin-destination (OD) cost matrix from multiple

origins to multiple destinations)– Vehicle routing problem– Location-allocation

Source: ArcUser 2012 issue

Dijkstra's algorithm• The routing solvers within Network Analyst—namely the Route, Closest

Facility, and OD Cost Matrix solvers—are based on the well-known Dijkstra's algorithm for finding shortest paths. Each of these three solvers implements two types of path-finding algorithms. – The first type is the exact shortest path, and the second is a hierarchical path

solver for faster performance. – The classic Dijkstra's algorithm solves a shortest-path problem on an

undirected, nonnegative, weighted graph. To use it within the context of real-world transportation data, this algorithm is modified to respect user settings such as one-way restrictions, turn restrictions, junction impedance, barriers, and side-of-street constraints while minimizing a user-specified cost attribute.

– The performance of Dijkstra's algorithm is further improved by using better data structures such as d-heaps. In addition, the algorithm needs to be able to model the locations anywhere along an edge, not just on junctions.

• The other three solvers are based on above three solvers or directly on Dijkstra’s algorithm.

Today’s free seminar from ESRI

• http://video.esri.com/watch/92/network-analyst-_dash_-an-introduction

By Patrick Stevens and Robert Garrity

Description of the seminar• Businesses and government agencies perform network

analysis to find the most efficient routes–those that meet customer service requirements while minimizing fuel, labor, and other costs–and to find optimal locations for facilities to attract or serve the most customers.

• ArcGIS Network Analyst is used to model transportation networks and solve problems in which network travel time or cost must be minimized. This seminar discusses different types of network problems and demonstrates a variety of real-world network analysis applications

Key points• Find the closest facility to a location to ensure the best response time.• Find optimal routes for vehicles making deliveries to multiple stops.• Account for impedances such as one-way streets, height and turn

restrictions, construction zones, and historical traffic conditions in a vehicle routing analysis.

• Perform a location-allocation analysis to create service areas based on drive time.

• Determine the optimal location for a new facility based on drive time.• Given a group of facilities serving an area, identify which facility is the

best candidate to close while maintaining acceptable service area coverage.