lecture 23 (mini-lecture): a brief introduction to network analysis parts of the network analysis...

Lecture 23 (mini-lecture): A Brief Introduction to Network Analysis

Parts of the Network Analysis section of this lecture were borrowed from a UC Berkeley GIS Center Short Course, written by Anders Flodmark: slides are noted

------Using GIS--Introduction to GIS

Network Analysis

• Networks defined as a set of interconnected line entities, generally arcs, whose attributes share some common theme primarily related to flow.

• Arcs in a network must share the attributes necessary for analyzing these flows (speed limits, frictions, etc.)


Adapted from UC Berkeley GIS Center

Network Arcs Can Have Direction

• Directed - flows allowed to move in a single direction (down stream, one way street)

• Undirected - flows allowed in both directions (two way streets)



A network is a system of linear features connected at intersections and interchanges.

These intersections and interchanges are called nodes

Arc

Node


Basic elements of a network


The linear feature connecting any given pair of nodes is called an arc.

Network Attributes of interestAttributes that can be attached to a network:

–origin, destination coordinates

–shape point coordinates

–street name/highway number

–aliases on street name

–directionality: one way traffic?

–classification: freeway, arterial, collector, residential

–speed limit, congestion (impedance) or travel time

–traffic volume

–length: driven length vs digitized length

–scenic value

–connectivity (turn tables)



Network ProblemsPath finding

paths

tours

Allocation

supply and demand

impedance

Spatial interaction

accessibility

interaction



Path findingFinding the shortest or least-cost manner in which to visit a series of locations in a network. The cost may be determined by distance or by travel-time or a combination of factors calculated as a cost value. Distance that factors in things that slow you down is known as functional distance

Often the parameter that is minimized in path finding is travel time. This factors in things like topography, traffic volume, average speed, stops etc.



Computing best routes at the continental scale (e.g. how do I get from LA to New York?) is easy, because small variations in distance measures are relatively unimportant.

At the intra-city level, the optimization criterion is travel time, which depends on legal restrictions (stop signs, traffic signals, one ways) and congestion (which varies by the minute), hence the margin of uncertainty in routing is far greater.


Path finding


This would not happen if Ponch and John had Path finding software


“Ponch, there’s a 4-12 going on at the video arcade, what’s the fastest way to get there?”

“Gee, I dunno John, let’s get off our bikes so we can look at our big, cumbersome map, and try to figure it out?”

Location - Allocation

Allocation assigns portions of a network to a location based on predetermined criteria. Essentially an area of the network is assigned to be serviced or served by a facility at a given location. It is based on Supply, Demand, and Impedance.



Allocation Assigns Portions of a Network to a Center


These street segments can be used to define a service area


Allocation Assigns Portions of a Network to a Center


Source: http://www.geo.umass.edu/courses/geo594a/warmup/warmup3/

Allocation analysis yields service areas for each of these facilities: that is, the areas they can reach within a specified driving time.

Allocation



Spatial interaction

Accessibility = How Connected is a Node

Accessibility is an aggregate measure of how reachable a location is from other locations.



Accessibility



Usefulness of Network Analysis

Used by retailers in market studies for siting new facilities

Used by utility company in managing their infrastructure: water, sewer, power

Used by consumers to get directions

Used by agencies to map out service areas: fire, police, public transportation facilities.


lecture 23 (mini-lecture): a brief introduction to network analysis parts of the network analysis...

Documents

gis slide

gis path

gis ponch

uc berkeley gis center

gis basic elements

network attributes

network arcs

network problems path