Introduction to Spatial Data Types - University …Introduction to Spatial Data Types Daniel...

OverviewData Models

Summary

Introduction to Spatial Data Types

Daniel McInerney

Urban Institute Ireland,University College Dublin,

Richview Campus,Clonskeagh Drive,

Dublin 14.

16th June 2009

Daniel McInerney UII Summer School - 15.06.2009

OverviewData Models

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Presentation Outline

1 OverviewObjectivesBasicsSynthesising Reality

2 Data ModelsSpatial Data ModelVector Data ModelRaster Data Model

3 SummaryVector vs. Raster ModelsExamples of DataConclusion

OverviewData Models

Summary

OverviewData Models

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OverviewData Models

Summary

ObjectivesBasicsSynthesising Reality

Objectives of lecture

To provide some theory and background to GIS, datamodels and spatial data

Provide some examples of spatial data available within andoutside of UCD.

OverviewData Models

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What is a GIS?

A Geographic Information System (GIS) has been definedas a system for organised collection of computer hardware,software, geographic data and personnel designed toefficiently capture, store, update, manipulate, analyse anddisplay all forms of geo-referenced information

Defined by Burroughs and McDonnell as a means ofchecking, manipulating and analysing data, which arespatially referenced to the Earth.

At its core is a spatial database that optimally stores bothspatial and attribute data

OverviewData Models

Summary

What is a GIS?

A Geographic Information System (GIS) has been definedas a system for organised collection of computer hardware,software, geographic data and personnel designed toefficiently capture, store, update, manipulate, analyse anddisplay all forms of geo-referenced information

Defined by Burroughs and McDonnell as a means ofchecking, manipulating and analysing data, which arespatially referenced to the Earth.

At its core is a spatial database that optimally stores bothspatial and attribute data

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Conceptual Overview of a GIS

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Representing Reality

REALITY - what actually exists

DATA MODEL - Abstraction of the real world incorporatingonly properties relevant to application

DATA STRUCTURE - A representation of the data model,expressed using arrays and programming structures thatare incorporated in computer programs

FILE STRUCTURE - The representation of the data instorage hardware in terms of bits and bytes on disk sectors

OverviewData Models

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OverviewData Models

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OverviewData Models

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OverviewData Models

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Spatial Data ModelVector Data ModelRaster Data Model

The Data Model

Critical decision is the choice of the data model, which isthe basis of a GIS

Different data models are required for spatial and attributedata

They are a set of constructs for describing andrepresenting selected aspects of the real world in acomputer

Models need to cope with: maintaining data, modellingtasks, analysis tasks and presentation.

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The Data Model

OverviewData Models

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The Data Model

OverviewData Models

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The Data Model

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Spatial Data Model

Two fundamental data models exist: Vector & Raster

... but also Vector representation of linked lines

and Delauney Triangulation (TINs) irregular tesselations offields.

OverviewData Models

Summary

Spatial Data Model

Two fundamental data models exist: Vector & Raster

... but also Vector representation of linked lines

and Delauney Triangulation (TINs) irregular tesselations offields.

OverviewData Models

Summary

What are Spatial Data?

A geographical entity is defined in terms of:

Location (spatial reference)

Dimensions

Attribute

It is common in spatial analysis to refer to places as (spatial)objects.

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Spatial Reference

Latitude Longitude

National Grid Reference

An address

A postcode

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Coordinate Reference Systems

A flat map distorts geometry because the Earth is curved.

If you plot lat long coordinates of a Cartesian system − astraight line will appear bent and areas will be distorted.

3D points are projected from the Earth’s surface onto the2D map surface.

Despite these projections, this process can lead todistortions over large areas.

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Map Projections

Many types of map projections exist, perhaps the mostcommonly used one is:

Universal Transverse Mercator Coordinate System

It divides the Earth into zones that are six degrees inlongitude & from 80 ◦ south latitude to 84◦north latitude

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Map Projections

Bear in mind that map projections distort five geographicrelationships:

Angles

Gross Shapes

Distances

Directions

Some projections will preserve local angles but not areasand vice versa!

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Irish National Grid

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Zero Dimensional Object Types

Point - as pairs of coordinates in lat/long or some otherreference system

A point feature is a zero-dimensional cartographic object.

It specifies the geometric location and no other meaningfulmeasurement

The size of the point may vary, but the area of thosesymbols is meaningless

Four types of points exist: entity point, label point, areapoint and node

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One Dimensional Object Types

Line - ordered sequence of points connected by straight lines

Line features are one dimensional features, despiteoccupying two-dimensional space.

A line segment is the direct connection between two points

A line feature is typically represented as a sequence ofvectors

An Arc is the location of points that are defined by amathematical function to form a curve

Link or edge is the connection between two nodes

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Two Dimensional Object Types

Areas as ordered rings of points connected by straight lines toform polygons

Area is a two dimensional, bounded and continuous object

Interior area is an area not including its boundary

Simple polygon consists of an interior area and an outerring. The boundary does not intersect itself

Typically refers to vector polygons, but also relates to pixels andgrid cells.

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Attribute Data

Attribute (Aspatial) information is the label / name /categorisation / descriptiong associated with a spatialobject

The attributes can be as important as the spatial datathemselves

May be more complex than the spatial data

May be a simple text label (e.g. town name, river name,county population)

Attributes usually stored in some form of database (i.e.DBF, ORACLE table ...)

Relational or object oriented DBMS used to spatial objectsto complex attribute tables

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Attribute Data

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Attribute Data

OverviewData Models

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Attribute Data

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Attribute Data

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Attribute Data

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Common types of attribute data

Nominal Attribute that distinguishes between locationswithout any ranking or potential for arithmetic

Ordinal Attribute that implies a ranking, but arithmeticcalculations are nonsensical

Interval Attributes where differences make sense

Ratio Attributes where it makes sense to divide onemeasurement by another

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Raster Data

Popular means of referencing continuous fields

Designed for rapid response to queries e.g.

What is at a given location?

Where does attribute X occur?

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Types of continuous fields

Elevation

Air Temperature

Land ownership

Ice thickness

Timber Volume or Biomass

Population density

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What is a raster?

...A raster is a geographic data set in which values areassigned to a rectangular array of objects (Goodchild,1997)

...A regular grid of cells with a fixed resolution completelycovering an area (Burrough, 1986)

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What is a raster?

...A raster is a geographic data set in which values areassigned to a rectangular array of objects (Goodchild,1997)

...A regular grid of cells with a fixed resolution completelycovering an area (Burrough, 1986)

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Basic raster definition

Space is tesselated at a fixed resolution, the cell size

Cell values are stored in a fixed order (e.g. row by row fromtop left to bottom right)

No need for coordinates of each cell as the scheme isordered and regular

Store the coordinates of one cell and cell size to registerthe raster to an external coorindate system

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Vector vs. Raster ModelsExamples of DataConclusion

Advantages

Raster AdvantagesSimple data structure

Simple overlay operations

High spatial variability is efficiently represented

Satellite & other formats already in this format

Vector AdvantagesVector representation is more compact

Provides efficient encoding of topology, i.e. better fornetwork operations

Better suited to produce maps with crisp line-work

OverviewData Models

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Advantages

Raster AdvantagesSimple data structure

Simple overlay operations

High spatial variability is efficiently represented

Satellite & other formats already in this format

Vector AdvantagesVector representation is more compact

Provides efficient encoding of topology, i.e. better fornetwork operations

Better suited to produce maps with crisp line-work

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Disadvantages

Raster DisadvantagesRaster data are less compact

Topological relationships difficult to represent

Poorer graphical output (step-like effect / blockyappearance)

Vector DisadvantagesMore complex structure than a simple raster

Overlay operations are more difficult to implement

Representing high spatial variability is inefficient

Handling image data is not possible

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Disadvantages

Raster DisadvantagesRaster data are less compact

Topological relationships difficult to represent

Poorer graphical output (step-like effect / blockyappearance)

Vector DisadvantagesMore complex structure than a simple raster

Overlay operations are more difficult to implement

Representing high spatial variability is inefficient

Handling image data is not possible

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Demonstration of different types of data

Frequently, access to data is the biggest hinderence tocarrying out spatial analysis

Multitude of data exist, some proprietary others free...

OverviewData Models

Summary

Demonstration of different types of data

Frequently, access to data is the biggest hinderence tocarrying out spatial analysis

Multitude of data exist, some proprietary others free...

OverviewData Models

Summary

Concluding remarksHopefully this lecture has provided you with somebackground and theory to GIS

An understanding of data models and data types willfacilitate your spatial analysis

Type of data model are dependent on your needs andanalysis

Questions, comments?

OverviewData Models

Summary

Concluding remarksHopefully this lecture has provided you with somebackground and theory to GIS

An understanding of data models and data types willfacilitate your spatial analysis

Type of data model are dependent on your needs andanalysis

Questions, comments?

Introduction to Spatial Data Types - University …Introduction to Spatial Data Types Daniel...

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