Data models and management

8/14/2019 Data models and management

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DATA MODELS&

MANGEMENT- I


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Outlines

Introduction

Raster Data

Vector Data

Raster and Vector Structures

Raster and Vector Advantages andDisadvantages


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Introduction

Geographic Data and Information arethe heart of GIS.

Two fundamental components ofgeographic data: space (expressed asspatial data) and qualities (attributes).

Both of these are stored in database.


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Data and InformationDefinitions

Information is the primary purpose of

GIS, not just data.

Data is the input; information is theoutput.


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5

Types of data

Maps

Images

Spatial non-spatial

Postcodes/ZIP codes

Oblique photographs

Videography

Financial statements

Films

Schematic diagrams

KT1 2EE

RH8 9AA

SW1P 3AD12,000 23.45 56789

23,456 12.45 23456

45,987 29.57 87634


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Introduction

Spatial data in GIS has two primary dataformats: raster and vector.

Raster uses a grid cell structure, whereasvector is more like a drawn map.


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Spatial Data: Vector format

PointPoint- a pair of x and y coordinates(x1,y1)

LineLine - a sequence of points

PolygonPolygon - a closed set of lines

Node

vertex

Vector data are defined spatially:


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Raster and Vector Data

PointPoint

LineLine

PolygonPolygon

VectorVector RasterRaster

Raster data are described by a cell grid, one value per ce

Zone of cells


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Raster and Vector Data

Vector format has points, lines, polygons that appearnormal, much like a map.

Raster format generalizes the scene into a grid ofcells, each with a code to indicate the feature beingdepicted. The cell is the minimum mapping unit.

Raster has generalized reality: all of the features inthe cell area are reduced to a single cell identity.


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Raster and Vector DataModels

Raster: because the raster cells value or coderepresents all of the features within the grid, it doesnot maintain true size, shape, or location for

individual features. Even where nothing exists (nodata), the cells must be coded.

Vector: vectors are data elements describingposition and direction. In GIS, vector is the map-like

drawing of features, without the generalizing effectof a raster grid. Therefore, shape is better retained.Vector is much more spatially accurate than theraster format.


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Raster Data

Raster Coding

Resolution Gridding and Linear Features

Raster Precision and Accuracy


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Raster Coding

In the data entry process, maps can be digitizedor scanned at a selected cell size and each cellassigned a code or value.

The cell size can be adjusted according to thegrid structure or by ground units, also termedresolution.

There are three basic and one advanced schemefor assigning cell codes.


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Raster Coding

Presence/Absence: is the most basic method and to record afeature if some of it occurs in the cell space.

Cell Center: involves reading only the center of the cell and

assigning the code accordingly. Not good for points or lines.

Dominant Area: to assign the cell code to the feature with thelargest (dominant) share of the cell. This is suitable primarilyfor polygons.

Percent Coverage: a more advanced method. To separateeach feature for coding into individual themes and then assignvalues that show its percent cover in each cell.


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Raster Coding Problems

Raster coding produces spatialinaccuracies.


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Raster Coding Problems

One possible solution is to increase theresolution by increasing the number ofcells, making each one smaller and

therefore more sensitive to accurateclassification.


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Raster Mapping

A major problem with the raster structure is thatthe shape of features is forced into an artificial grid

cell format.

For right-angled features, such as squareagricultural fields or rectangular political districts,this may not present a major problem. However,for many features, size and shape can becomeundesirably distorted.


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Resolution

Increasing the number of cells on a data setincreases spatial resolution, which helps to

increase spatial accuracy.

One advantage to using relatively few cellsis the short processing time and ease ofanalysis.


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Gridding and LinearFeatures

Low-resolution raster results in a rathergeneralized and crude shape.

High-resolution raster shape appears morerealistic, though still a long way from thevector shape and spatial accuracy.


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Raster Precision andAccuracy

Questions of raster data precision (the exact location) andaccuracy (maximum spatial truth) are often a problem.

Because the raster cell is the maximum resolution and the

minimum mapping unit, there is no way to know exactly wheresmall feature occurs.

Smaller cells have less spatial error because the area of doubt issmaller.

Uncertainty becomes greater when measuring across cells.

Area measurement are also generalized.


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Vector Data

Vector features appear more realistic thanraster features and have better spatial

accuracy.

Vector features are defined primarily by theirshapes, more specifically by the outline oftheir shapes. In GIS, the vector system is acoordinate-based data structure.


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Vector Data

Shape points are the ends and bends that define the featuresoutline.

At the beginning and end of every line or polygon feature is a

node.

At each bend (change of direction) is a vertex (plural: vertices).

Node are end points and vertices are between, defining the

shape.

Point features are standalone nodes.


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Vector Data

Chains connect the shape points to draw the features outline.

Chains are vectors or data structure paths that are not part ofthe actual stored data elements; they are not real lines, but

define and present the connection between shape points.

Vector system data files store only the coordinate of each nodeand vertex; the hardware draws the connecting chainsegments. It is virtual component.

The vector data structure is also known as an arc-node modelbecause it uses chains (arcs) and end points (nodes).


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Raster and VectorStructures

Raster and vector structure have differentmethods of storing and displaying spatial

data.

Raster cells are stored and displayed ascells, but in the vector format only the nodesand vertices are stored. This results inconsiderable data storage differences.


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A point in a raster system is a single cell, but in a vectorsystem it is only a node represented by a symbol with itscoordinate position noted.

A simple line in a raster system consists of a sequence ofcells. In a vector system, a simple line consists of two nodesand a chain that connects them.

A more complex raster line consists of connected cells,

sometimes in stair-step fashion when they are diagonal.Complex lines in the vector format have vertices to markchanges in direction, with nodes at each end.


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Raster polygons are filled with cells. Forsingle polygons, the vector format usuallyhas a single node and several vertices tomark the boundary direction changes.

Connected polygons are simply two blocks

of cells in the raster format, but in vectorthey share a common border and somecommon nodes.


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Raster to Vector Conversion

There are at least four basic reasons to convert fromraster to vector:

(1) better visual appearance of vector features;

(2) some plotter work only on vector data;

(3) comparison with vector data is best when bothdata files have identical formats;

(4) some GIS systems have vectors as the centraloperating data structure.

Rasterization of vector data is often called gridding.


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Raster Advantages

A relatively simple data structure;

The simple grid structure makes analysis easier.

The computer platform can be low tech and inexpensive.

Remote sensing imagery is typically obtained in raster format.

Modeling is the creation of a generalized data file or a set ofuniversal procedures to accomplish a certain GIS task.


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Raster Disadvantages

Spatial inaccuracies

Because each cell tends to generalize a landscape, the result

is relatively low resolution compared to the vector format.

Because of spatial inaccuracies caused by datageneralization, a raster format cannot tell precisely whatexists at a given location.

Each cell must have a code, even where nothing exists.


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Vector Advantages

In general, vector data is more map-like.

Is very high resolution.

The high resolution supports high spatial accuracy.

Vector formats have storage advantages.

The general public usually understands what is shown on

vector maps.

Vector data can be topological.


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Vector Disadvantages

May be more difficult to manage than raster formats.

Require more powerful, high-tech machines.

The use of better computers, increased management needs,and other considerations often make the vector format moreexpensive.

Learning the technical aspects of vector system is moredifficult than understanding the simplicity of the rasterformat, particularly when topology is introduced.


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GIS Data Characteristics

Location, or position, is a major staring point ofspatial measurement. Location can be descriptive,or uses a Lat-Lon system.

Size characteristics: Polygon: area and perimeter;Lines: length.

Shape: an important descriptive element used inmap and image interpretation. The shape of afeature often indicates its identity and role on thelandscape.


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Point features have no real shape or spatialdimension, only the position of objects oroccurrences. They are represented by symbols,such as dots, geometric shapes, or icons.

A line feature has length from beginning to end.

Polygon features have a wide variety of shapes,from easily interpreted circles and squares tocomplicated shapes that defy description.

GIS DataCharacteristics


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Spatial Data Relationships

Spatial relationships are how featuresrelate to each other in space.

It includes distance, distribution,density, and pattern.


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Distance from one feature to another is anelementary but important relationship. It is

available through simple measurement.

Distribution is the collective location of features;the geographic dispersal or range. There are two

basic ways of perceiving distribution: featuresamong themselves and their spatial relationshipwith other features.



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Density is the number of items per unitarea; how close features are to each other.

Pattern is the consistent arrangement offeatures, similar to (and can include)

distribution and density.


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The Data ModelThe Data Model

Geographical variation in the real world isinfinitely complex. Therefore, we require a setof rules (the data model) to convert realgeographical variation into discrete objects.

A set of guidelines for the representation ofthe logical organisation of the data in adatabase (consisting) of named logical unitsof data and the relationships between them.

The GIS Model: example


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The GIS Model: example

roads

hydrology

topography

Here we have three layers or themes:

--roads,

--hydrology (water),

--topography (land elevation)

They can be related because precise geographic

coordinates are recorded for each theme.

longitude

latitu

de

longitude

longitude

latitu

de

latit

ud

e

Layers are comprised of two data types

Spatial data which describes location (where)Attribute data specifing what, how much,when

Layers may be represented in two ways:in vectorformat as points and linesin raster(or image) format as pixels

All geographic data has 4 properties:

projection, scale, accuracy and resolution


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Types of data model

The Raster Model

Equivalent of a

continuous grid coveringthe surface, wherebyeach cell in the gridrepresents a square onthe ground.

The Vector Model

Attempts to represent

objects as exactly andprecisely as possible bystoring points, lines(arcs) and polygons(areas) in a continuous

co-ordinate space


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Raster-Vector Data Model

Raster

Vector

Real World

R ti D t ith R t d


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Representing Data with RasterandVectorModels

Raster Model area is covered by grid with (usually) equal-

sized, square cells

attributes are recorded by assigning each cell asingle value based on the majority feature(attribute) in the cell, such as land use type.

Image data is a special case of raster data inwhich the attribute is a reflectance value

from the geomagnetic spectrum cells in image data often calledpixels

(picture elements)

R ti D t ith R t d


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Vector Model

The fundamental concept of vector GIS is that allgeographic features in the real work can berepresented either as:

points or dots (nodes): trees, poles, fireplugs, airports, cities

lines (arcs): streams, streets, sewers,

areas (polygons): land parcels, cities,counties, forest, rock type

Representing Data with RasterandVectorModels

Vector and Raster Models in


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Vector and Raster Models inGIS

Representation of

Lines

Raster

Vector

TOPOLOGY (for vector


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TOPOLOGY(for vectordata) What is topology? Why is important? Three types of topological models in GIS Spatial operations of topology

Contiguity Connectivity

Trade-offs of topological structure Application model

Triangular Irregular Network (TIN):Vector-basedGIS

S ti l f t d ti l


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Spatial features and spatialrelationships

Spatial features in maps Points, lines and polygons

Human being interprets additional

information from maps about thespatial relationships betweenfeatures A route trace from an airport to a house

Land contiguity adjacent to streets alongwhich the lands are located


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The definition of Topology

The spatial relationships can be interpreted identification of connecting lines along a path definition of the areas enclosed within these

lines

identification of contiguous areas In digital maps, these relationships are

depicted using Topology Topology =A mathematical procedure for

explicitly defining spatial relationship Topology is the description of how the spatial

objects are related with spatial meaning


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Topological data models

Three types of topological concepts Arc, Node and polygon topologies

Arc Arcs have directions and left and right polygons

(=contiguity) Node

Nodes link arcs with start and end nodes(=connectivity)

Polygon Arcs that connect to surround an area define a

polygon (=area definition)


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Terms and concepts

To

Node

Left

Polygon

Right

Polygon

From

Node

Connectivity - from and to nodes

Contiguity - Polygon Enclosure

Adjacency - from Direction

Ar

c

Spatial operations of


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Spatial operations oftopology Connectivity and contiguity (Aronoff, 1989)

A basic, but core spatial analysis operations in GIS Contiguity

A biologist might be interested in the habitats thatoccur next to each other

A city planner might be interested in zoning conflictssuch as industrial zones bordering recreation areas

Connectivity Transportation network, telecommunication systems,

river systems

To find optimum routings or most efficient deliveryroutes or the fastest travel route

To predict loading at critical points in a river channel To estimate water flow at a bridge crossing that will

result from heavy flood


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Trade-offs of topology

Advantages Spatial data is stored more efficiently Analysis process faster and efficient for large

data sets

By topological relationships, we can performspatial analysis functions, Modelling flow through the connection of lines in

a network (i.e. buffering) Combining adjacent polygons with similar

characteristics (i.e. spatial merge) Overlaying geographical features (i.e. spatial

overlay)


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Disadvantages

Extra cost and time creating topological structure does impose a

cost Topology should be always updated when a new

map or existing map is updated

Additional batch job working To avoid the extra efforts, GIS systems need to

run a batch job (i.e. a process that can be runwithout user interactions); 70% of total GIS

costs Autoexec.bat in DOS Macro languages such as AML (Arc/Info), Avenue

(ArcView), MapBasic (MapInfo) and etc


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Conclusions of topology

When topology is created, we canidentify

Know its positions of spatial featuresKnowwhat is around it

Understandits geographicalcharacteristics by virtue of recognising

its surroundings

Know how to get from A to B


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Thank You

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