dr. bo wu · lsgi 521: principles of gis lecture 4: data modelling 1. learning outcomes 2. vector...

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LSGI 521: Principles of GIS Lecture 4: Data ModellingLSGI 521: Principles of GIS Lecture 4: Data Modelling

LSGI 521: Principles of GIS

Lecture 4: Spatial Data Modelling in GIS

Dr. Bo Wu [email protected]

Spatial Data Modelling in GIS

@p y

Department of Land Surveying & Geo-InformaticsThe Hong Kong Polytechnic University

LSGI 521: Principles of GIS Lecture 4: Data Modelling

1. Learning outcomes2. Vector model

Contents

3. Raster model4. Comparison of vector and raster models5. Modeling terrain surface using vector and

raster models6. Object-oriented Modelingj g7. Network Modeling

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• By the end of this lecture you should be able to:– Provide a definition of spatial data model

E l i h ti l bj t d t t d t

Learning Outcomes

– Explain how spatial objects are used to create a data model

– Distinguish between vectors and rasters– Describe a spatial data model structure– Explain how topology is stored in the model– List the advantages and disadvantages of differentList the advantages and disadvantages of different

spatial data models– Know how to model a network

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Spatial Objects

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Vector and Raster Spatial Data

Objects in Real WorldVector View Raster View

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• Vector data model • The basic logic unit in a

geographical context d t li

Basic Types of Geographic Data Models

corresponds to a line, e.g. contour lines

• Raster (Tesselation) data model • The basic logic units is a

single cell or unit of space in the mesh

• Hybrid data model• Possesses characteristics

of both vector and raster data model

• Others9/28/2011 6

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• Spaghetti Model• Chaincodes Model

Typical Vector Models

• GBF/DIME Model• Topologic Model• POLYVRT Model

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• Each object on the map becomes one logic record in the digital file, and is defined as strings

Spaghetti Model

and is defined as strings of x, y coordinates.

• The spaghetti model remains the conceptual map model - direct line to line translation.

Spaghetti Noodles

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p g

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• Advantages– Good for computer-assisted cartographic production

where no topologic relation is required

Advantages & Disadvantages of Spaghetti Model

p g q• Disadvantages

– No spatial relationships are reordered– Common boundary of two adjacent polygon are

stored twice– Inefficient for most type of spatial analyses– Spatial relationships which are implicit in the original

analog document must be derived through computation

– Correction and updates of the data must rely on visual checks of graphic output

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• Classical chaincode model was presented by Freeman-Hoffman in1974

• Unique direction code

Chaincodes Model

• Encode line data on grid unit• Record x, y coordinates only for beginning point of each line

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An Example of Chaincodes Model

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• Advantages– Compactness

Advantages & Disadvantages of Chaincodes Model

– Using unit vector direction code - easier forsome measurement and analyticalprocedures (distance, shape analysis)

• Disadvantages– No spatial relationships are retained - aNo spatial relationships are retained a

compact spaghetti– Coordinate transformations, particularly

rotation, are more difficult

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• GDF/DIME (Geographic Base File/ Dual Independent Map Encoding) from US

GBF/DIME Model

p g)Census Bureau

• Each street, river, railroad line etc is represented as a series of straight line segments

• Usage:

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Usage:– Digitally storing street maps– Providing geographically

referenced address information in computerized form

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• Advantages– As a directed graph, missing segments and other

b t ti ll h k d

Advantages & Disadvantages of GBF/DIME Model

errors can be automatically checked.– Some location systems (e.g. street address) can be

directly derived from the model.

• Disadvantages– Line segments are not stored in any particular order;

to retrieve any particular line segment exhaustiveto retrieve any particular line segment, exhaustive search must be performed on the entire file.

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• Definition– Basic logic entity is straight line segment– A line segment begins or ends at the

Topologic Model

A line segment begins or ends at the intersection with another line or at a bend

• Files– X, Y coordinate node file– Line segment file– Topologically coded network polygon file

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An Example of Topologic Model

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• Advantages– Most popular method of retaining spatial

Advantages & Disadvantages of Topologic Model

relationships among objects– Explicitly record adjacency information– Points, line, polygons are stored in a non-

redundant manner• DisadvantagesDisadvantages

– It is a time consuming process to properly define the topology depending on the size and complexity of the data set

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• POLYVRT (POLYgon conVERTer) presented byPeucker and Chrisman in 1975.

• Explicitly and separately storing each type of

POLYVRT

• Explicitly and separately storing each type ofdata entity in a hierarchical data structure– A chain is denoted as the basic line entity comprised

from a sequence of straight line segments– A line segment begins and ends at a node– A node is defined as the intersection point between

two chains and its position is indicated by x ytwo chains and its position is indicated by x, ycoordinates

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• Polygons• Chain List• Chains

An Example of POLYVRT Model

• Chains• Points• Nodes• Files are linked

by pointers

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• Advantages– More efficient in retrieving than simpler topologic structures;– Selective retrieval of only specific classes in the data model at a

Advantages & Disadvantages of POLYVRT Model

S y ptime;

– The queries concerning the adjacency of polygons need only deal with the polygons of interest. Only individual chains which bound the polygons of interest are retrieved

• Disadvantages– The physical separation of files cause the need for a link or

pointer structurepointer structure– Incorrect pointers can be extremely difficult to detect or correct– Initial generation of this structure can be cumbersome and time-

consuming

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Raster Models

For Vector Models:Individual entities are the basic data units for which geometric information is explicitly recorded.

While Raster Models:The basic unit become a unit of space for which entity information is explicitly recorded.

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A Simple Raster Data Model

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Another Example of Raster Model

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• Run-Length Encoding• Block Encoding

Typical Raster Models

g• Chain Encoding• Quadtree Model• Point Quadtree Model• K-D Tree

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• Run-length encoding allow the points in each mapping unit to be stored per row in terms, from left to right, of a begin cell and an end cell.

Run-Length Encoding

g

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• Block encoding extends the run-length encoding idea to two dimensions by using a series of square blocks to store data

Block Encoding

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• Chain encoding works by defining the boundary of the entity. The boundary is defined as a sequence of unit cells starting from and returning to a given origin. The direction of travel around the boundary is usually given using a numbering system (e g 0=North

Chain Encoding

boundary is usually given using a numbering system (e.g., 0=North, 1=East, 2=South, and 3=West).

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• Advantage– more compact than square grid

it bl f li ll l l ith i

Advantages & Disadvantages of Encoding Models

– suitable for linear or parallel algorithms processing

• Disadvantage– not suitable for processes which depend upon

neighborhood effects

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• Recursive subdivision of space in quadtreemodel results in a

Quadtree Model

model results in a regular, balanced tree structure of degree 4– A hierarchical or tree

data model– Each node has 4 sons

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• Advantages– Tree file structure - well developed already

Advantages & Disadvantages of Quadtree Model

– Suitable for handling a very large database– Compatible with conventional cartesian

coordinate systems• Disadvantages

There is a trade off between the complexity of– There is a trade-off between the complexity of the object and the dimensions of the smallest grid cell.

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• Point quadtree is a special quadtree that divides space based on the location of ordered points, rather than regular spatial decomposition

Point Quadtree Model

g p p

TORONTO

BUFFALODENVER

OMAHACHICAGO

MOBILEATLANTA

MIAMI

(0,100) (100,100)

(60,75)

(5,45)

(25,35)(35,40)

(50,10)Y

(80,65)

(85,15)

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(90,5)

X (100,0)(0,0)

CHICAGO

DENVER TORONTO OMAHA MOBILE

BUFFALO ATLANTA MIAMI


• Advantages– Location based

Advantages & Disadvantages of Point Quadtree Model

– Save space• Disadvantages

– The shape of the tree is highly dependent on the order in which the points are addedAdditions and deletions are impossible except– Additions and deletions are impossible except at the leaves of the tree

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• K-D tree divides the area into two parts instead of four, yielding a tree of degree 2

K-D Tree

(0,100) (100,100)

TORONTO

BUFFALODENVER

OMAHACHICAGO

MOBILEATLANTA

MIAMI

(60,75)

(5,45)

(25,35)(35,40)

(50,10)Y

(80,65)

(85,15)

(90,5)X (100,0)(0,0)

CHICAGO

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CHICAGO

DENVER

TORONTOOMAHAMOBILE

BUFFALO

ATLANTA

MIAMI


Modeling Terrain Surface

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• Raster-based Digital Terrain Model (DTM) is simply a grid of height values in which each cell contains a single value representative of the height of the terrain that is covered by that cell.

• DEM (Digital Elevation Model)

Raster-based Digital Terrain Model

DEM (Digital Elevation Model)

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An Example of DEM

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1：1,000,000 Map Scale DEM

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1：250,000 Map Scale DEM

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44



45

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• A more advanced, more complex and more common form of vector-based DTM is the Triangulated Irregular Network (TIN)

Vector-based Digital Terrain Model

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TIN Model

16

5

7

11A

BC I

JK

N

2

3

87

910

4

C

D

E

F

G

H

LMEDGES

adjacent

ABCDEFGH

B, KA, C, LB, DC, E, LD, FE, GF, H, MG, I

NODES

node#

ABCDEFGH

1, 6, 71, 7, 81, 2, 82, 8, 92, 3, 93, 4, 94, 9, 104, 5, 10

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X-Y Coordinatesnode# coordinates

123

11. . .

x1, y1x2, y2x3, y3. . .x11, y11

Z Coordinatesnode# z_value

123

11. . .

z1

. . .

z2z3

z11

IJKLMN

H, J, NI, KA, J, NB, D, MG, L, NI, K, M

IJKLMN

5, 10, 115, 6, 116, 7, 117, 8, 97, 9, 107, 10, 11

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An Example of TIN

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• Advantages – Adapt to the nature terrain

Advantages & Disadvantages of TIN Model

– Facilitate the calculation of slope and other terrain-specific parameters

• Disadvantages – More time is required in dividing irregular

space point setspace point set– For some point set, there are many possible

different triangulations

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• An object is a self-contained package of information describing the characteristics and capabilities of an entity

• In a geographical object data model, the real world is

Object-Oriented Modeling

modeled as a collection of objects and relationships between them

Each entity in the GIS is an objectA collection of objects of the same type is called a classEach class has a superclass

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Each class has a superclass from which it can inherit both instance variables and methods


Object-Oriented Data Model

OBJECT

POLYGON LAND PARCEL

Polygon-1 Polygon-1 Land parcel-1

Land parcel-2

Land parcel-3

• Three key hallmarks of object orientation– Polymorphism– Encapsulation– Inheritance

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• Advantages– The ‘natural’ model: directly corresponds to the object

found in reality.

Advantages & Disadvantages of Object-Oriented Model

found in reality.– Completeness: every object is completely bounded

with a defined ‘shell’.– Inheritance: a class can include subclasses that can

inherit both its data and methods.– Openness: allows to modify and expand instance

variables and methodsvariables and methods.• Disadvantages

– Complicated– Difficulty of implementation

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What is a Network?

Any system of interconnected linear features

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How to Represent a Network?

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Network Elements

Links

• Network nodes• Network links

• Impedance and demand• Turn• Stops (intermediate nodes) • Centers (resource provider/center)

School

End point of 8 route bus

Links

Turn

Centers

N l ft t h

Impedance

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Start point of 8 route bus

Barrier

Links

Stops10 people get

on the busdeman

No left turn here

Demand

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The Simplest Node-Link ModelTwo fundamental tables are required in the Node-Link Model that

can be stored in a relational database

• Node table This table contains at least three fields; one to store a unique identifier

• Link table – This table also contains at

least three fields; one to store an unique identifier, one to store the node of origin and one to store the node of

– This table contains at least three fields; one to store a unique identifier and the others to store the node's X and Y coordinates..

destination. A fourth field can be used to state if the link is unidirectional or not.

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• Impedance– Length by default– ‘from-node’ to ‘to-node’ and ‘to-node’ to ‘from-node’ impedances

Different impedance may be assigned to different turn types

Network – Impedance and Demand

– Different impedance may be assigned to different turn types

• Demand– Attributes added to the network

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• There are n2 possible turns at every network node, where n is the number of edges connected at that node

• Four types of action

Network - Turn

– Passing straight through an intersection– Backtracking, i.e. making a U-turn– Turning left– Turning right

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• Node# - internal node no. of the network node at which turn is made• Arc1# - internal node no. of arc entering the node• Arc2# - internal node no. of arc leaving the node• Azimuth – north azimuth of arc entering the node

An Example of a Turntable

g• Angle – angle of turn from Arc1# to Arc2#, left turns have + angles, right

turns have – angles

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• Cover-ID– Unique user-id for each stop

• In-order– Value for the order in which the stops will be visited

Network - Stop

Unlike nodes, stops do not affect or alter path direction

Value for the order in which the stops will be visited

• Route-ID– Feature-id of the route

created• Impedance

– The cost associated with the stop, e.g. time, money cost

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cost• Transfer

– Loading or unloading of resources or commodities at a stop


• Cover-ID– Unique user-id for each center

• Route-ID

Network - Center

– Feature-id of the route created

• Max-Impedance – Serving area (distance, time)

• Supply– The maximum total demand

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An Example of the Allocation Problem

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Typical Applications

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Best Route Closest Facility

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Typical Applications

Service Area

Origin-Destination Cost Matrix

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• Further readings– D. Buckey, Vector and Raster - Advantages and Disadvantages

(http://bgis.sanbi.org/gis-primer/page_19.htm)– Finding Your Way with ArcGIS Network Analyst

Review

(http://proceedings.esri.com/library/userconf/devsummit06/papers/network_analyst.pdf)

• Summarization of the main ideas presented in this lecture:

• Questions?

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dr. bo wu · lsgi 521: principles of gis lecture 4: data modelling 1. learning outcomes 2. vector...

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