network analysis sc msc 2013

8/13/2019 Network Analysis Sc Msc 2013

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NETWORK ANALYSIS:

A useful GIS tool for modelling

flows on geographic networks



Networks – What are they?



What is a network?

A network is an arrangement of interconnected

features that form a functional ‘flow’ system

Examples of networks

Electrical circuitsPrinted circuit boards

Phone line systems

But we are interested in networks in GIS...



What is a GIS network?

A GIS network is an arrangement of connected nodes

and edges that link together to form a functionalflow system in geographic space

Examples of GIS networks

Road networks

Railway networks

Drainage / sewerage networks

N.B. Connectivity is critical...



Networks ‘need connectivity’

A network needs to be fully spatially interlinked in

order to support network modelling flows ingeographic space

In a GIS, this means that all vector lines need to snap

onto each other end-to-end

In a GIS system, the interconnectivity of features is

defined using the concept of topology

Topology makes network modelling possible in GIS



Ok, so what is topology?

A GIS topology is a data structure that defines how

points, lines (and polygon features) sharegeometric components. I t also does a few other

things that we wil l come back to shortly

In simple terms this means that the end node (vertex)of one network vector line is (in most cases) the

start of at least one other interlinked network vector

It quantifies (internally in the GIS ) the nodes that

make up all the lines, and the lines that make up all

the polygons (and the spatial relationship of all of

these network components)



Uses of topology in network

models?

Topology enforces data integrity rules that ensure we

have a coherent network (i.e. no gaps, no

overlapping features, and no dangling nodes)

However, a topology also identifies feature adjacency

(the relative positions of features in geographic

space), AND...

It defines the order of the points on a line (which can

be used to define direction rules on a network)



How features share geometry in a

topology

Vertex ‘A’

Vertex ‘B’

Line segments are composed of vertices




topology

Vertex ‘A’

Vertex ‘B’

Vertex ‘C’

Line segments are linked end-to-end by shared vertices




topology

Vertex ‘A’

Vertex ‘B’

Vertex ‘C’

Sets of line segments define polygon faces

Polygon ‘J’




topology

Vertex ‘A’

Vertex ‘B’

Vertex ‘C’

Polygon ‘J’

Links are shared in adjacent polygons

Polygon ‘K’



This is useful in a GIS network

... Because it allows the computer to establish afunctional link between vertex and line GIS features

that enable real world network processes to be

modelled in GIS




topology

A GIS topology defines how network features share

geometric components

It identifies feature adjacency (the relative positions

of features in geographic space)

It defines the order of the points in a line (which can

be used to define direction rules on a network)

I n summary...



Uses of networks in GIS



Networks in GIS are used for...

Route finding, along a network

Optimising distribution of goods, people,perishable goods, etc to-or-from locations ona network

Optimising access to services (fire, police,ambulance…) or from service centres on anetwork



A familiar Route-finding example...

Google maps 1. Road Network




Google maps 1. Pub l ic Trans it




Google maps 1. Footpaths




Google maps 1. Cyc le paths



A familiar example... Google maps

It chooses the shortest route for four modes oftransport

It estimates travel time based on time of day

It also provides alternative route options

However, users cannot interact with thenetwork flow rules



Optimal route finding by distance, duration (andalso by time of day)

It can optimise a route from current location to a

destination via multiple waypoints on theroute

It also provides alternative

route options

Limited interaction withthe network flow rules

Another familiar Route-finding

example... SatNav



Setting up a network so it can be

used for network modelling in GIS



Setting up a network to use in GIS

Network links (lines) and nodes (vertices) needGIS attributes that govern their capacity tosupport flows across the network

These capacities include:

- ‘cost’/impedance criteria

- ‘way points’/stops

- capacity criteria

- avoiding barriers

The GIS computes optimum routes based uponthese flow facilitating / impedance attributes



Node and link attributes that govern networkflows...

Vertices

Junction node

End/Dangling node

One-way flow

Two-way flow

Different types of nodes and links




Vertices

Junction node

End/Dangling node

One-way flow

Two-way flow




Link impedances:

One way flow

Two-way flow

Slow flow

Fast flow

Turn impedances:

junction controlled

by traffic lights

Stops and Stop

impedances:

R route start/end point

S supply point

D demand point R

S

D

D

Multiple attributes on a ‘real’ network...




Link impedances:

One way flow

Two-way flow

Slow flow

Fast flow

Turn impedances:

junction controlled

by traffic lights

Stops and Stop

impedances:


S supply point

D demand point R

S

D

D





How do you find the best route?

Testing all possible options is VERY SLOW – a

network of only 9 links can be traversed in 9!

(factorial 9) ways

This amounts to 9*8*7*6*5*4*3*2*1= 362880 potential combinations to choose from

Any there alternatives to this?



The A* (A-star) Algorithm

A* uses rules of thumb (heuristics) to speed up

selection of the most likely optimal route




First or all... What is an Algorithm?

An algorithm is a step-by-step procedure for solving

a computational problem

The A* algorithm follows a step-by-step procedure

to choose the best network path

It is quite easy to understand (and is actually very

commonly used in real world network modelling )

How does it work?




Start




End

Which is the best route?

Start




End


Start



A-star Algorithm

(http://en.wikipedia.org/wiki/A-star_search_algorithm)

Step 1 Which is shorter? h1 + g1 ……?

h1

g1

The A* Algorithm



A-star Algorithm


h2

Step 1 Which is shorter? h2 + g2 .…?

The A* Algorithm

g2



A-star Algorithm


Step 1 Which is shorter? h3 + g3 ....?

h3

g3

The A* Algorithm



A-star Algorithm


h2

Result 1 h3 + g3 is longest, so is excluded

h1

g1

g2

The A* Algorithm

*



A-star Algorithm


Step 2 move to next nodes and new g1 & g2

g1

g2

The A* Algorithm

Th A* Al ith



A-star Algorithm


Step 3 .... etc to choose likely shortest path

The A* Algorithm



The A* Algorithm – barriers

The algorithm

recognises barriers,

and chooses an

optimal path thatcircumvents them

In this network each

dot represents anetwork node

Is the shortest distance

always best?Start

Goal



Using a weighted edge network (weights are

impedances by time, dist or cost) speeds things up

The A* Algorithm – edge weigh t ing

EndStart

H

B

I

„A‟



1. Network links are weighted according to an

impedance value ( speed limit, two-way traffic,

street parking, traffic lights etc)

2. Lots of options are possible

3. Step 1 AB? AI? AH? Least impedance?




EndStart

H

B

I

„A‟




1. AI least impedance and equal shortest

2. Next step... IB, IJ, IK, IG, IH?




EndStart

H

B

I

„A‟




4. IJ & IK equal in terms of impedance. Problem?

5. No. Just compare distance of IJ & IK




EndStart

K

JI

„A‟




6. IK slightly shorter, so best option

7. Next step examines following node options... etc

8. Eventual chosen route AIKLE




Optimal route chosen based upon weights

(impedances) and distances

End„E‟

Start

LK

I

„A‟




GIS Networks:

More than just A to B

R ti i l



Routing in a more general sense….

Networks are not just about getting from A to B...

Networks can connect people to the nearest

CENTRE (school, hospital, depot, etc.) with

enough CAPABILITY (desks, beds, delivery vans)

So... you also need to have this info in your GIS:

• database of CENTRES and their CAPABILITIES

• distances from any address to potential centres

• GEOCODING facility

• ability to generate travel directions

G di



Geocoding

Generating travel directions requires a process

called Geocoding

Geocoding assigns an address to a map coordinate

Addresses can be geocoded using software that

accesses postal data files

G di dd t d



Geocoding (see: http://geocoder.us/)

Geocoding – address to coords.

G di dd i t l ti

http://geocoder.us/

http://geocoder.us/



Addresses can be geocoded from points in GIS by

including the postal data as a table in your GIS

GIS street vectors are associated with the addressranges attached to each side of the street vector

The GIS estimates the location of an address basedon the length of the segment and the addressrange assigned to the segment.

For example, an address of 49 Main Street would plot half way down Main street of 200 houses

Geocoding – address interpolation

G di dd i t l ti




Step 1 – GIS finds coordinate location

G di dd i t l ti




Step 2 – GIS finds network street vectors



G di dd i t l ti




Step 4 – GIS interpolates an address location

R f



AA

http://www.theaa.com/travelwatch/planner_main.jsp

ESRI

http://training.esri.com/gateway/index.cfm?fa=catalog.webCourse

Detail&courseid=274

Longley, P., Goodchild, M., Maguire, D., Rhind, D. “Geographic

Information Systems and Science. Ed 3”, 2011. p 395-397

Jones, Christopher, “Geographic Information Systems and

Computer Cartography”, Longman, Harlow, 1997. P224-229

Worboys, M. and Duckham, M., “GIS A Computing Perspective”,

CRC Press, London, 2004. P211-218

References



End of lecture

Lab with Dr. Drummond nextweek

network analysis sc msc 2013

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