Transcript data structure

INTRODUCTION TO GIS
Geographic + Information + System
Knowledge of proper geographic location is an important
aspect in this technically advanced and competitive market.
Information is the result of processing, manipulating and
organizing data in a way that adds to the knowledge of the
person receiving it.
A collection of entities organized in a manner to cooperate
with each other in order to accomplish a common objective.
GIS or Geographical Information Systems is broadly a
computer capable of assembling, storing, manipulating and
displaying geographically referenced information.
Practitioners also regard the total GIS as including personal
and the data that go into the system.
Defining GIS is a tough task though not an impossible one.
This is more evident from the following definitions.
GIS is …
… a system for capturing, storing, checking, integrating,
manipulating, analysing and displaying data which are
specifically referenced to the earth.
(Charley 1987)
GIS is …
… an information technology which stores, analyses and
displays both spatial and non-spatial data.
(Parker 1988)
…an institutional entity, reflecting an organisational structure
that integrates technology with a database, expertise and
continuing financial support over time. (Carter 1989)
… an information system that is designed to work with data
referenced by spatial or geographic coordinates. A GIS is
thus both a database system with specific capabilities for
spatially referenced data as well as a set of operations for
working with data. (Star & Estes 1990)
… a system of computer software and procedures designed
to support the capture, management, manipulation,
analysis and display of spatially referenced data for solving
complex planning and management problems.
So.. What’s a GIS?
...a computer based system to capture, store, edit,
manipulate, analyze, synthesize and display
geographically referenced information
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GIS defined...
...a computer system capable of visualizing data
which describe places on the earth’s surface
Digital
GIS converts
data is digital
expressed
dataasinto
numbers
maps
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GIS defined...
With GIS you can do more than just view maps, you
can also query and analyze map layers simultaneously
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Need of GIS?
Many professionals, such as foresters, Army, urban planners,
and geologists, have recognized the importance of spatial
dimensions in organising & analysing information. Whether a
discipline is concerned with the very practical aspects of
business, or is concerned with purely academic research,
geographic information system can introduce a perspective,
which can provide valuable insights as 70% of the information
has geographic location as it's denominator making spatial
analysis an essential tool.
*Ability to assimilate divergent sources of data both spatial and
non-spatial (attribute data)
*Visualization Impact
*Analytical Capability
*Sharing of Information
GIS Functions
Input: adding data or creating new data
► Manipulation: updating or reformatting data
► Query: two methods - query the database (tabular
data) or query the geographic features (spatial
data)
► Analysis: combining multiple types of data over an
area to answer questions or to develop scenarios
about that area
► Visualization: creating 2D maps or 3D
representations
► Management: spatial and tabular data
►
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Questions a GIS can answer...
►
Location
Where is it?
►
Condition
What is at... ?
►
Trends
What has changed since... ?
►
Patterns
What spatial patterns exist?
►
Modeling
What if... ?
continued...
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Questions a GIS can answer...
What are the characteristics
of the area around existing
features?
►
Proximity
►
Boundary Operations What exists within a specific
region?
►
Logical Operations
What is unique about a region
or set of features?
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WHAT DOES GIS DO
1. Collect, store, organise and distribute data
2. Criteria matching: Select a suitable site for picnic that is:
on public land;
with gentle slope;
amongst shaded trees;
within 50 m of canoe-able river
A restaurant within 200 m.
3. Allows exploration of relationships among data layers:
How does population density relate to water availability
High yielding Poplar grows on which soil type
… contd.
4. Allows scenario testing:
If we raise smoke chimney to 1000 feet, will it go out of
the adjacent village
If we make the proposed sewage path, will it cut the roads
5. Serve as a data handler for other analyst:
Passing water quality and water-table data to groundwater
flow model
Passing geological and topographic data to soil erosion
modeler.
6. Aids Visualisation:
It improves understanding and pattern recognition
Facilitates public participation in alternative scenario
Coordinates group decision making
BUT THE GIS IS NOT
► East
to use
► Just a drafting (CAD) program, it needs
analytical capabilities
► Able to make decisions- That is the job of
the user/interpreter
► Free of field work
► Cheep in terms of human resources,
computer resources and data acquisition
Spatial Data handling involves different disciplines
Characteristics of Disciplines
Sample Disciplines
1.Development Of Spatial
Concepts
Geography; Linguistics
;Psychology; Cognitive Sciences.
2.Means For Capturing And
Processing Spatial Data
Remote Sensing; Surveying;
Engineering; Cartography;
Photogrammetry .
Computer Science; Expert System;
Mathematics; Statistics.
3.Formal And Theoretical
Foundation
4.Applications
5.Supports
Archaeology; Architecture;
Forestry; Geo-Sciences; Regional
& Urban Planning; Surveying .
Legal Sciences; Economy
Components of a GIS
Data and
Information
Results and
Applications
Modeling the
Real World
Hardware/
Software
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ELEMENTS OF GIS
Elements of GIS
Details
Hardware
Model Personnel Computers; High
Performance Workstations; Minicomputers;
Mainframe Computers.
Input Devices: Scanners, Digitizers, Tape
drivers, CD, Keyboard, Monitor
Output Devices :Plotters, Printers
Software
Input Modules ; Editing; MRP
Manipulation/Analysis Modules; Modelling
Capability
Data
Attribute Data; Remote Sensing Data; Spatial
Data; Global Database
Live Ware
People responsible for digitizing,
implementing using GIS.
There are three basic perceptions prevalent in the GIS
community
Cartographic Perception :
Which focuses on the map aspects of GIS. Supporters
of this view see GIS as a map processing and display system
where each map is represented as a layer, mostly in raster
format, and is manipulated by a set of map arithmetic
functions.
•Database Perception :
emphasizing the importance of a well designed and
implemented database. A sophisticated database management
system (DBMS) is an integral part of such systems. This view
is predominant amongst the GIS community who have
computer science background. Application around querying,
transaction, processing, large area active database
maintenance, map production etc. are suited to this approach.
Spatial Integration Perception :
emphasizing the spatial analysis capabilities of GIS. This
view focuses on integrated analysis and modeling and seen more
as a spatial information science than a technology. It is towards
this perception that GIS of present day are developing and is
likely to become most widely accepted by the GIS community.
DATA IN GIS
Broadly classified into following categories :
1. Spatial data
2. Non-spatial data
Spatial data :
Spatial data has physical dimensions and geographic
locations on the surface of earth. Maps are used
to graphically represent geographic features of the
earth. A map conveys :
-
Positional information about the location on the
surface of earth
-
Spatial relationship such as adjacent to located
within etc.
-
Measurable quantities such as length, area,
latitude, longitude etc.
Non-Spatial data :
Data that qualifies the spatial data. It is
quantitative data expressed as names, numbers,
dates, values etc.
Data Models
and Feature Types
►Raster
►Vector
 Points, Lines
and Areas
 cell matrix
Y
X
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Types of Data Models - how do you
represent the real world in a computer?
Raster Data Model
Cell or Grid Matrix
Continuous Data
Vector Data Model
Points, Lines, Polygons
Discrete Data
Real World
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DATA MODELS
Raster Data Structure
(Cartographic Map Model )
The simplest raster data structures consist of an
array of grid cells (sometimes termed pixels). Each
grid cell is referenced by a row and column number
and it contains a number representing the type or
value of the attribute being mapped. In raster
structures a point is represented by a single grid
cell; a line by a number of neighboring cells strung
out in a given direction and an area by an
agglomeration of neighboring cells.
Raster:
A set of cells located by coordinate is used, each
cell is independently addressed with the value of
an attribute.
•One set of cell and associated value is a LAYER.
•It is simple with which spatial analysis is easier
and faster.
•Requires a huge volume of data to be stored,
fitness of data limited by cell size .
Vector representation (Georelational Model)
Vector representation of an object is an attempt to
represent it as exactly as possible.
Advantages:
Good representation of phenomenological data structure
Compact data structure
Topology can be completely described with network
linkages.
Accurate graphic
Retrieval, updating and generalization of graphic and
attributes are possible.
Vector
•Uses line segments or points represented by their
explicit (X,Y) coordinates to identify locations
•Discrete objects are formed by connecting line
segments which area is defined by set of line
segments.
•It requires less storage space, outputs are
appreciable, estimation of area/perimeter is accurate
and editing is faster and convenient.
•Spatial analysis is difficult with respect to writing the
software program.
CONCEPTS IN GIS
Three basic spatial features can be identified :
Points
Lines
Polygons
Points:
Point locations are used in geography in two ways. The first
way is shorthand reference for an area so that a map might
show the locations of towns as a scatter of points. The
second is as a precise locational reference. For example, the
site at which certain measurements are made.
Lines:
Individual points can be joined together to form a second type
of spatial feature, a line. Minimum number of points needed
to define a line over a short distance is two, in which case the
line is necessarily straight. However if more than two points
are used, line may be curved and the path of the line between
individual points area called segments. Start and end points of
a line and the point at which three or more lines join are
called nodes.
Polygons:
When a series of lines form an enclosing boundary they create
the third type of spatial feature – an area or a polygon. In
this case, the number of pairs of x and y coordinates are
variable since it depends on the number of lines and number of
segments.
RASTER vs VECTOR
Raster representation:
Set of cells located by coordinates, each cell is
independently addressed with the value of an
attribute.
Advantages:
-Simple data structures.
-The overlay and combination of mapped data with
remotely sensed data is easy.
-Various kind of spatial analysis is easy.
-Simulation is easy because each spatial unit has the
same size and shape.
Disadvantages:
Volumes of graphic data.
The vector model is extremely useful for describing Discrete features,
but less useful for describing continuously varying features such as soil
type or accessibility costs for hospitals.
The raster model has evolved to model such continuous features.
A raster image comprises a collection of grid cells rather like a scanned
map or picture.
Both the vector and raster models for storing geographic data have
unique advantages and disadvantages.
Modern GIS packages are able to handle both models.
Comparing Vector and Raster Models
Vector
Raster
• Primary Focus
Geographic Feature
(Thing)
• Question asked
“what do I know about
“what phenomenon
this geographic feature?” occur at this location?”
• Representation
better for precise or
discrete kinds of features
Location
( Place )
better for continuous data
or change in values along
a gradient
• Accuracy
accurate portrayal of
feature shape
feature shape is
generalized
• Modeling capability
requires complicated
geometric algorithms
simple mathematic
combination of cells
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MAP OVERLAYS
Because each cell in a two dimensional array can only hold one
number, different geographical attributes must be represented
by separate sets of Cartesian arrays, known as ‘overlays’.
The overlay idea for separating data is not restricted to
computer cartography, having been used by cartographers for
preparing printed maps and by landscape planners. The
overlays concept is realized by stacking two dimensional array
in raster data structures.
Vector data (the streets) laid on top of raster data (the mountains
and valley floor).
Organizing Spatial Features
in Common Themes
Creating data “themes”
is basically the process
of abstracting or
simplifying the real
world into different
components that can be
input into a computer.
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The GIS Data Model:
Geographic Integration of Information
Raster Imagery
• Data are organized by layers, coverages or themes (synonymous
concepts), with each theme representing a common feature.
• Layers are integrated using explicit location on the earth’s
surface, thus geographical location is the organizing principal.
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LAYERS AND COVERAGES:
All data of a particular level
of classification, such as
roads, rivers or vegetation
types are grouped into so
called layers or coverage.
In this picture, Customers,
buildings and streets have
been divided into different
layers.
This gives their exact
locational information which
proves to be very useful.
DATA STRUCTURE
These are used to organize the data inside the
information system.
The choice of data structure affects both; Data storage
volume and processing efficiency.
Types :
Relational
Hierarchical
Network
Relational
Data organized in 2-dimensional table
Row-> Record
Column is Attribute
Different tables are related through use of a common identifier called
KEY.
Relation extracts the information, which are defined by query.
RELATIONAL
Hierarchical
It stores data in a way that a hierarchy is maintained
among the data items.
Each node can be divided into one or more additional
node.
Stored data gets more and more detailed as one
branches further out on tree
Hierarchical
NETWORK
Similar to hierarchy structure except that a node may
have more than one parent.
Each node can be divided into one or more additional
nodes.
It is limited since pointers must be updated every time a
change is made to database.
NETWORK
APPLICATIONS
The potential of Geoinformatics is huge:
►Agricultural
development
• Land evaluation analysis
• Change detection of vegetated areas
• Analysis of deforestation and associated
environmental hazards
• Monitoring vegetation health
• Mapping percentage vegetation cover for the
management of land
• Degradation
• Crop acreage and production estimation
• Wasteland mapping
• Soil resources mapping
• Groundwater potential mapping
• Geological and mineral exploration
• Snow-melt run-off forecasting
• Monitoring forest fire
• Monitoring ocean productivity etc.
Summary
► GIS
is a tool for storage, manipulation and
output of spatial data
► Remote sensing is a primary source of data for
GIS
► GIS is a useful tool for processing remotely
sensed data
► To use GIS effectively you must understand the
characteristics of spatial data (e.g. data
structures, projections, implications of math
algebra, etc.)
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Brief HISTORY
1960's
1970's
1980's
Initial development in North
America and use of Computers
Introduction of topology in GIS by
stable data structures
Relational database technology
became standard, Researches on
spatial data structures
Recognition of Geoinformatics as a
professional activity,
1990's
Spatial information theory as the
theoretical basis for GIS.