This document was published with and applies to ArcGIS 9.3.
A 9.2 version also exists.
A 9.2 version also exists.
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Networks pervade your daily life. You drive cars from home to work on a street network, cook your dinners with natural gas delivered through gas utility lines, catch up with news and send e-mails through the Internet, and visit relatives by flying on an airline route system.
Networks channel flow. Certain phenomena flow in a continuous field across a region, such as rainfall or temperature. But rainfall collects into streams, and nearly all resources that you process or goods that are manufactured flow in a constrained way, carried along networks of streets, pipes, cables, and channels.
A network is a one-dimensional system of edges that connect at junctions that transport resources, communications, and people. The following are the three common types of networks:
Transportation involves the movement of people and the shipment of goods from one location to another.
Streets are the ubiquitous network—you spend a fraction of every day traversing this network. Streets have two-way flow, except for situations such as one-way streets, divided highways, and transition ramps.
Streets form a multilevel network—while most roads are at surface level, bridges, tunnels, and highway interchanges cross each other in elevation; a simple overpass has two levels, and a highway interchange typically has four.
You make your daily travel optimal by hopping from one mode of transport to another, switching between walking, driving, riding a bus or train, and flying. You also use natural hierarchies in the transportation network. Trips of any distance usually begin by driving to the closest freeway on-ramp and proceeding to the off-ramp closest to the destination.
Street, rail, and subway systems have well-defined geometry for the edges of the network, but transportation systems such as airline routes and shipping lanes have indeterminate or variable edges with geographic junctions at airports and harbors.
Some transportation tasks include the following:
Calculating the quickest path between two locations
Determining a trade area based on travel time
Dispatching the closest ambulance to an accident
Finding the best path and sequence to visit customers
Routing a garbage truck efficiently
With geographic information system (GIS) software, you can analyze a transportation network to support planning goals such as relieving congestion, mitigating pollution, optimizing delivery of goods, and forecasting demand for transportation. See the following illustration of a transportation network:
Rainfall on the landscape accumulates from rivulets to streams, rivers, and finally, an ocean. The shape of the surface directs water to a stream network. Gravity drives river flow from higher elevations to sea level.
A hydrologic network usually models a river as a connected set of stream reaches (edges) and their confluences (junctions). When a stream drains into a lake, hydrologic models continue the flow along an arbitrary line midway between shores until an outlet is reached.
Special large-scale hydrologic project models may include 3D analysis of flow lines through a channel volume, but simplifying a river to a one-dimension line network is suitable for most applications.
Most parts of a river network form well-drained dendritic networks with distinct channels and flow directions.
In flat terrain, river flow becomes more complicated—a large river near an ocean often forms a delta with a complex braided network, and tidal effects can reverse flow near the shore.
Some hydrologic tasks include the following:
Deriving catchments on a surface model for each stream reach
Accumulating rainfall on catchments and transferring flow to reach
Using gauge values to predict flood surge along a river
Designing a system of channels and holding ponds for high water
Managing diversion of water for agriculture or city water works
The basic analysis done on river networks starts with estimating peak and average rainfall (from radar or model assumptions), determining how water gathers from catchment areas to river reaches, and how it accumulates downstream at confluences. See the following illustration of a river network:
Utility networks are the built environment that supplies energy, water, and communications and removes effluent and storm water. Water utilities are gravity driven or pressurized, depending on terrain. Flow in a gas utility is driven by pressure in pipes. Electric power flows from high voltage potential to low. Pulses of light carry communications in a fiber-optic network.
Utility networks have a nominal flow condition, with a few sources delivering a resource to many points of consumption. Some utility networks tolerate loops, such as a water network. For other utilities, a loop is a fault condition, such as an electrical short circuit. All utility networks contain dynamic devices, such as valves and switches, that can interrupt or redirect flow in the event of an outage or system maintenance.
Some utilities, such as telecommunications and electrical networks, have multiple circuits on a common carrier (edge), such as electric lines with three phases of power or twisted-pair lines in telephony.
Some utility network tasks include the following:
Establishing the direction of a commodity flow
Finding what is upstream of a point
Closing switches or valves to redirect flow
Identifying isolated parts of the network
Finding facilities that serve a set of customers
Utilities are concerned first with the safety of customers and employees, followed by the reliability of the system, and then cost efficiency in system operations. GIS technology is an effective tool to reach these goals. See the following illustration of a utility network:
See Also:How to draw networks on a map
About networks and graphs
Two core network models