### ppt

```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)
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 wellknown 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/networkanalyst-_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.
```