gis - sp. peters conference - oct 2008

8/2/2019 GIS - Sp. Peters Conference - Oct 2008

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Sri Rama Jayam

By

R. SUDHARSANAN, (Ph.D),M.E., A.M.I.E.

GEOGRAPHICAL INFORMATIONSYSTEMS


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Geographic Information System

Geographic implies to the surface of the earth

Information implies knowledge of or collection of data

system implies framework


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GIS DEFINITION

A set of tools for collecting, storing, retrieving at will,transforming, and displaying spatial data from the real world

for a particular set of purposes.

: Burrough (1986).

A system for capturing, storing, checking, integrating,manipulating, analyzing and displaying data which are spatially

referenced to the Earth.

: (Depart. of Environment, 1987).

Computer tool for managing geographic feature location

data and data related to those features.

: Allan B. Cox


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COMPONENTS OF GIS

Overview ComponentsData

Maps/Views/Layouts

Spatial Analysis

Physical Components

software

hardware

data

usersneed/application


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What does a GIS do?

Input data

Manage data

Manipulate data

Perform analyses

Produce output - maps, charts


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DATA

Spatial Data Represents features that have a known

location on earth.

Attribute Data The information linked to the geographicfeatures (spatial data) that describe

those features.

Data Layers Are the result of combining spatial and

attribute data. Essentially adding theattribute database to the spatial location.

Layer Types A layer type refers to the way spatial

and attribute information are connected.

There are two major layer types, vector

and raster.

Topology How geographic features are related to

one another and where they are in relation

to one another.

Metadata Data about data


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MAPS

The traditional method for storing, analyzing and presenting

spatial data is the map.

The map is of fundamental importance in GIS as a source of

data, a structure for storing data and a device for analysis and

display.

Maps are classified into topographical maps and thematic

maps.

Thematic maps show data relating to a particular theme or

topic such as soil, land use, transportation or population.

Topographic maps contain a diverse set of data on different

themes such as Survey of India Toposheets or tourist maps.


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MAP SCALE

Virtually all sources of spatial data, including maps, are smaller

than the reality they represent.

Scale can be defined as the ratio of distance on the map to the

corresponding distance on the ground (Martin, 1996).

Scale can be expressed in one of three ways: as a ratio scale, a

verbal scale or a graphical scale.

Ratio 1:5,000 1:1,00,000

Verbal 1 cm represents 50 m 1 cm represents 10 km

Graphical 0 100m 200 m 0 10 20 30 km


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SPATIAL REFERENCING AND GEO-CODING

Geographic information contains either an explicit geographicreference, such as a latitude and longitude or national grid co-

ordinate, or an implicit reference such as an address, postal

code, census tract name, forest stand identifier, or road name.

The spatial referencing can be grouped into three categories:

Geographic co-ordinate system;

Rectangular co-ordinate system; and

Non co-ordinate system.


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(i) Geographic Co-ordinate System:

The only true geographic co-ordinates are latitude andlongitude. Using lines of latitude and longitude any point on the

Earths surface can be located by a reference given in degrees and

minutes.


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(ii) Rectangular Co-ordinate System:

The lines of latitude and longitude become grid lines on

a flat map.

When small areas are being studied there will be only

minor distortions in the layout of the grid.

The rectangular co-ordinate systems are designed to

allow mapping of specific geographical regions

e.g. Universal Transverse Mercator (UTM) plane

grid system.


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(iii) Non Co-ordinate System:

Non co-ordinate systems provide spatial references using a

descriptive code rather than a co-ordinate.

Example: Postal Code.

This may be fully numeric or alphanumeric.

An automated process called geocoding is used to create

explicit geographic references (multiple locations) from

implicit references (descriptions such as addresses).

These geographic references allow us to locate features and

events on the earth's surface for analysis.


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GIS DATA BASE

It consist of

(i) The Spatial Database

(ii) The Attribute Database


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THE SPATIAL DATABASE

The geographical features are described with their location andshape, and their spatial relationship to other features.

The spatial data is obtained from maps and drawings.

The information contained in the spatial database is held in the

form of digital co-ordinates, which describe the spatial

features.

The different sets of data will be held as separate layers,known as thematic layers, which can be combined in a number

of different ways for analysis or map production.


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THE ATTRIBUTE DATABASE

o The attribute database is of a more conventional type; it contains

data describing characteristics or qualities of the spatial features

i.e., descriptive information.

o The attribute data is obtained from either record maintained by

various organizations or by direct measurement.

o The attribute data is stored in tabular or point form. The

attribute information may be in the form of characters, numeric

or alphanumeric.

o GIS links spatial data with geographic information about aparticular feature on a map.

o The information is stored as attributes of the graphically

represented feature.


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SPATIAL ENTITIES

Traditionally, maps have used symbols to represent real-world

features. The representation of real-world features is done using point,

line and area entity.

The method chosen to represent a spatial feature will depend

on the scale used.


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Point:

The discrete location represented as a co-ordinate pair.

Points are used to represent features that are too small to be

represented as areas (e.g. Post Box, Rain gauge, etc).Line:

A set of ordered co-ordinates represented by a string of co-

ordinates. Lines are used to represent features that are linear in

nature (e.g. streams, power & pipelines, and transport routes, etc).They can also bee used to represent linear features that do not

exist in reality (e.g. administrative boundary, basin boundary, etc).

Polygon:

A closed feature whose boundary encloses a homogeneous

area represented by a closed string of co-ordinates which

encompass an area. Some of these polygons exist on the ground,

while others are imaginary. (e.g., lakes, agricultural fields,

catchment area, land use, census tracts, hospital, town boundaries,

etc).


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SPATIAL DATA STRUCTURES

Data structures provide the information that the computer requiresto reconstruct the spatial data model in digital form.

There are two major methods to input, store and visualise mapped

data in GIS.

The GIS data structure is classified into Raster Data and Vector

Data.


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Raster Data Structure

Raster GIS, which store map features in raster or grid format,

generalise the location of features to a regular matrix of cells.

Raster GIS data structures are preferred for digital elevation

modeling (DEM), statistical analysis, remotely sensed data,

simulation modeling, and natural resource applications like

sedimentation and water quality studies.


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Vector Data Structure

Vector Geographic Information Systems, which store map featuresin vector format, such as points, lines and polygons with high

accuracy.

They are preferred in urban applications where legal boundaries

and the analysis of networks are important, in net work analysis,

etc.

Applications of urban GIS include location and allocation of

critical resources such as hospitals, study of disease outbreakpatterns and crime analysis.


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MAP PROJECTIONS

The location of spatial entities, in two dimensions, is the

important task for the GIS analyst.

The method by which the world is laid flat is to use a map

projection.

Map projection transfers the spherical Earth onto a two-dimensional surface. In doing so, they approximate the true

shape of the Earth.

Based on the projection method chosen, the error may be

introduced in the spatial data. Map projections are sets of mathematical models which

transform spherical coordinates (i.e.latitude and longitude) to

planar coordinates (x and y).


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In this process, data which actually lie on a sphere are projected

onto a flat plane or a surface and converted to a planar section

without stretching.

Positions on a globe are measured by angles (i.e. longitude &latitude) rather than X, Y (i.e. Cartesian) coordinates.

The longitude is measured as the number of degrees from the

prime meridian, and the latitude is measured as the number of

degrees from the equator.

The specific point on the surface of the earth is specified by the

longitude and latitude of it.


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The map projection can be explained by means of glass sphere

marked with grid lines is kept in front of a light sources and the

way in which sphere projected outward.Three types of developable surface such as plane, cone, or

cylinder are placed outside the sphere in order to receive the

shadows.

When the surface is opened the geographic features aredisplayed a flat plane.


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TYPES OF MAP PROJECTION

The projection surfaces form the basic types of projections

namely:

(i) Conical Projection;

(ii) Cylindrical Projection;

(iii)Azimuthal Projection.


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Conical Projection

The light source is kept in a tepee analogy.

Standard parallels are where the cone touches or slices through

the globe.

The central meridian is opposite the edge where the cone is

sliced open.

Conic projections are used frequently for mapping large areas.

The scale for the most part is preserved.

The limitations of this method are the area is distorted and

distance is very much distorted towards the bottom of the

image.


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Cylindrical Projection The light in a circular room analogy is adopted. In this

method, the surface of the earth is projected onto a cylinder

which encompasses the globe.

This is very much suitable for making maps of an area which

have only a small extent in longitude.

The most common cylindrical projection is the Mercatorprojection, which is the basis of the UTM (Universal

Transverse Mercator) system.

It gives continuous picture of the earth, countries near the

equator in the true relative positions and most part of the areais preserved.

At the same time distance increases between countries located

towards top and bottom of image and the view of poles are

much distorted.


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Azimuthal Projection

This method adopts light in a square room with flat walls

analogy.

This method preserves most part of distance.

But only a part of the earths surface is visible.

The view will be of half the globe and distortion will occur at

all four edges.


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Conical Projection Azimuthal Projection

Cylindrical Projection


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Polyconic Projection

A multiple number of cones draped over the mode of the earth

is known as polyconic projection.

Each of these cones is tangent to its corresponding latitude,

thereby making each parallel a standard parallel.

Half of these cones have their apexes over the North Pole while

the other halves have apexes over South Pole.

This projection minimizes all distortions.

The scale of the map will be true along the central meridian

and along each parallel.

Survey of Indias topographical maps produced using

polyconic projection.


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A particular subset of the transverse Mercator is the Universal

Transverse Mercator (UTM).

In the UTM system, the globe is divided into 60 zones

between 84 S and 84 N, most of which are 6 wide.

Each UTM zone has its own central meridian and spans 3

west and 3 east from the center of the zone.

Note that the position of the cylinder

developable surface is positioned at

a different place around the globe foreach zone.

X- and Y-coordinates are in meters by

convention.

Universal Transverse Mercator (UTM)


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UTM Zone locations and grid designations


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METHODS OF DATA INPUT

Data, in analogue or digital form need to be encoded tobe compatible with the GIS being used.

All data in analogue form need to be converted to

digital from before they can be input into GIS.

Reformatting or conversion may also be required after

analogue data have been converted to digital form.

Four methods are widely used:

Keyboard entry

Manual digitizingAutomatic digitizing

Scanning


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(i) Keyboard Entry

Keyboard entry, often referred to as key coding, is the entry of

data into a file at a computer terminal.

This technique is used for attribute data that are only available onpaper.


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(ii)Manual Digitizing

The most common method of encoding spatial feature from

paper maps is manual digitizing.

It is an appropriate technique when a selection of features is

required from a paper map.

Manual digitizing requires a table digitizer that is linked to a

computer workstation.

(a) Point mode

The user begins digitizing each line segment with a start node,

records each change in direction of the line with a digitized

point and finishes the segment with an end node. The user must choose a sensible number of points to represent

the curve.

Some digitizing packages allow the user to record smooth

curves as mathematically defined splines.


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(b) Stream Mode:

The digitizer is set up to record points according to a stated time

interval or on a distance basis.

The speed at which the cursor is moved along the line

determines the number of points recorded.

The line is more complex and the cursor needs to be moved

more slowly and with more care, a greater number of pointswill be recorded.

Stream mode digitizing requires more skill, more points, and

larger files.

The accuracy of data generated by this method of encoding isdependent on many factors, including the scale and resolution

of the source map, and the quality of the equipment and

software being used.


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(iii) Automatic Digitizing & Scanning:

Scanning is an appropriate method of data encoding when raster

data are required.

A scanning is a piece of hardware for converting an analogue

source document into digital raster format transmitted or

reflected light.

Flat-bed scanners, Rotating drum scanners.The selection of appropriate scanning tolerance to ensure

important data is encoded, and background data ignored.

The format of files produced and the input of data to GIS

software.


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The accuracy of scanned output data depends on the quality of

the scanner, the quality of the image-processing software used to

process the scanned data, and the quality (and complexity) of

the source document.

Resolution affects the quality, and quantity, of output data. The

higher the resolution, the larger the volumes of data produced.

Automatic line follower this encoding method might be

appropriate where digital versions of clear, distinctive lines on a

map are required.

The method reduces manual digitizing and uses a laser- and

light- sensitive device to follow the lines on the map.


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DATA EDITINGData may include errors derived from the original source. When

the spatial data is obtained from the other sources the followingquestions are to be asked. They are:

What data are available?

What will the data cost?

On what media will the data be supplied?

What format will the data be in?

During the encoding process there may be errors in co-ordinate

data as well as inaccuracies and uncertainty in attribute data.

The process is known as data editing or clearing.

Data editing can be likened to the filter in the fuel tank.Three topics are covered here: detection and correction of errors;

re-projection, transformation and generalization; and edge

matching and rubber sheeting.


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DETECTING AND CORRECTING ERRORS

Errors in the source data; errors introduced during encoding;

and errors propagated during data transfer and conversion.

Errors in source data may be difficult to identify.

During data transfer, conversion of data between different

formats required by different packages may lead to a loss of

data.

Errors in attribute data are relatively easy to spot and may beidentified using manual comparison with the original data.


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METHODS OF ATTRIBUTE DATA CHECKING

Impossible valuesExtreme values

Internal consistency

Scatter-diagrams

Trend surface

Errors in spatial data are often more difficult to identify and

correct than errors in attribute data.

Depending on the data model being used (vector or raster) and the

method of capture.


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Common errors in spatial data

Error Description

Missing entities Missing points, lines or boundary segments

Duplicate entities Points, lines or boundary segments that have been

digitized twice

Mislocatedentities

Points, lines or boundary segments digitized in thewrong place

Missing labels Unidentified polygons

Duplicate labels Two or more identification labels for the same polygon

Artifacts of

digitizing

Undershoots, overshoots, wrongly placed nodes, loops

and spikes

Noise Irrelevant data entered during digitizing, scanning or

data transfer


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Re-projection, Transformation and Generalization:

Data derived from maps drawn on different projections are

converted into a common projection system before they can be

combined or analysed is known as Re-projection.

Data derived from various sources with different spatial

referencing are transformed to a common grid system is known asTransformation.

If source maps of widely differing scales are converted into a

scale, which is comparable with, the data derived from smallerscale maps is known as Generalization. This will save

processing time and disc space by avoiding the storage of

unnecessary details.

Edge Matching and Rubber Sheeting


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Edge Matching and Rubber SheetingEdge matching is simply the procedure to adjust the position of

features that extend across typical map sheet boundaries.

Theoretically data from adjacent map sheets should meet preciselyat map edges. However, in practice this rarely occurs.

Misalignment of features can be caused by several factors

including digitizing error, paper shrinkage of source maps, and

errors in the original mapping.

Edge matching always requires some interactive editing.

Accordingly, GIS software differs considerably in the degree of

automation provided.

Spatial database editing software that attempts to correct errors by

stretching a map to fit known control points or monuments is

called Rubber Sheeting.

It uses Mathematical method to stretch or warp images to match

existing vector data.


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SPATIAL ANALYSIS AND GIS FUNCTIONS

Spatial Analysis not just a map.

With GIS, users can turn data into information, ask questions

and interact with the system.

GIS provides both simple point-and-click query capabilities and

sophisticated analysis tools to provide timely information to

managers and analysts alike.GIS technology really comes into its own when used to analyse

geographic data to look forpatterns and trends and to undertake

"what if" scenarios.

Modern GIS have many powerful analytical tools, but two areespecially important:proximity analysis and overlay analysis.


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Overlay Analysis:

The integration of different data layers involves a process called

overlay.

At its simplest, this could be a visual operation, but analytical

operations require one or more data layers to be joined physically.

This overlay, or spatial join, can for example link land-use and

environmental data to population and disease data.

Analysis requires data linkage, within the same dataset and/or in a

second dataset.

GIS uses geography, or space, as the common key element

between datasets.

Information is linked only if it relates to the same geographic area.


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Spatio-Temporal Analysis:

By adding a temporal (time) dimension to spatial data and

analysis, changes that might occur regarding some

variable/condition within the same location with time were

tracked.

Also the variable/condition we are studying might changelocations with time, or extend beyond the original location to

involve additional ones.


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Visualisation:

For many types of geographic operation the end result is best

visualised as a map or graph.

Maps are very efficient at storing and communicating

geographic information. Map displays are integrated with

reports, three-dimensional views, photographic images, andother output such as multimedia.


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Sri Rama Jayam


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gis - sp. peters conference - oct 2008

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