GEOMATICS

MODULE-6

GEOGRAPHIC INFORMATION SYSTEM –GIS

Syllabus

Geographical Information System - components of GIS,

GIS operations,

Map projections- methods,

Coordinate systems : Geographic and Projected coordinate systems,

Data Types- Spatial and attribute data,

Raster and vector data representation,

Data Input methods-

Geometric Transformation-RMS error,

Vector data Analysis-buffering, overlay.

References:

Introduction to GIS – Kang-tsung Chang

Remote Sensing & GIS – Basudeb Bhatta

KTUNOTES.IN

Downloaded from Ktunotes.in

GEOGRAPHIC INFORMATION SYSTEM - GIS

“Geographic Information System is a computer system for capturing, storing, checking,

integrating, manipulating, analysing and displaying data which are spatially referenced to the

earth.” These data which are spatially referenced to earth are also called geospatial data and

they describe both the locations and the characteristics of spatial features such as roads, land

parcels, water bodies, vegetation, etc. on the earth’s surface.

A Geographic Information System (GIS) is an integration of computer hardware and software

which can create, manipulate and analyse geographically referenced database to produce new

maps and tabular data. GIS includes the capabilities of CAD (Computer Aided Design) and

DBMS (Database Management System), but is more than just a combination of those

systems.

COMPONENTS OF GIS

Following are basic components of GIS:

1. Hardware: It consists of the computer system on which the GIS software will run.

The choice of hardware ranges from Personal Computers to multi-user super

computers. These computers should have essentially an efficient processor to run the

software and sufficient memory to store enough information (data). Additional

equipments may include monitors for display, digitizers and scanners for spatial data

input, GPS Receivers and mobile devices for fieldwork, and printers and plotters for

hard-copy data display.

KTUNOTES.IN

Downloaded from Ktunotes.in

2. Software: The GIS Software includes the program and the user interface for driving

the hardware. Common user interfaces in GIS are menus, graphical icons, command

lines and scripts. GIS software provides the functions and tools needed to store,

analyse and display geographic information.

3. Data: Data consist of various kinds of inputs that the system takes to produce

information. Geographic data and related tabular data (attribute data) are the backbone

of GIS. It can be collected in-house or purchased from a commercial data provider.

The digital map forms the basic data input for GIS. Tabular data related to map

objects can also be attached to digital data. A GIS will integrate spatial data with other

data resources and can even use DBMS.

4. Method: A successful GIS operates according of well-designed plan, which are the

models and operating practices unique to each task. There are various techniques used

for map creation and further usage for any project. The map creation can either be

automated raster to vector creator or it can be manually vectorised using the scanned

images. The sources of these digital maps can be either map prepared by any survey

agency or satellite imagery.

5. People: People refer to GIS professionals and users who define the purpose and

objectives, and provide the reason and justification for using GIS. GIS users range

from technical specialists who design and maintain the system to those who use it to

help them perform their everyday work. GIS operators solve real time spatial

problems. They plan, implement and operate to draw conclusions for decision

making.

GIS OPERATIONS

A GIS consists of a data base coupled with a graphics output. There are three major

types of operations in a GIS:

1. Access methods

The user will want to access data elements with certain properties. There are three

questions involved here. The first is whether the requested data is available in the

system. The second question is how long it will take to find the data. The third

question is how the user accesses the data. The conventional query language SQL is

sometimes used.

KTUNOTES.IN

Downloaded from Ktunotes.in

2. Analytical operations

Analytical operations allow the user to combine data to produce new data. The most

important operation is the overlay of different spatial data sets to produce a new data

set. For example, a biologist might want to determine what variables affect the

population of dolphins. For a given part of the ocean, there may be measurements for

water temperature, different fish populations, including predators (sharks), different

types of pollution levels, etc. The biologist could then try to find correlations. Note

that this is a multivariate visualization problem. The analytical operations can also

include computations to produce a new data set.

3. Rendering

Rendering operations show the data as some sort of map with the additional

information displayed. This is where Visualization techniques can be applied to GIS.

The following table shows a list of GIS operations.

KTUNOTES.IN

Downloaded from Ktunotes.in

COORDINATE SYSTEMS

Reference system used for locating objects in a 2D or 3D space is called coordinate system. It

can be broadly classified into

- Curvilinear system: uses angular measurements from the origin to describe one’s

position. Geographic Coordinate System is an example.

- Rectangular system: uses distance measurements from origin to locate the position.

Cartesian Coordinate system is an example.

In GIS, two types of coordinate systems are used to locate the position of objects on Earth’s

surface:

• Geographic Coordinate System

• Projected Coordinate System

1. GEOGRAPHIC COORDINATE SYSTEM (GCS)

The geographic coordinate system is the location reference system for spatial features

on the Earth’s surface. It is defined by longitude and latitude. Both latitudes and

longitudes are angular measurements.

Longitude is the angle east or west of a reference meridian to another meridian that

passes through that point.

KTUNOTES.IN

Downloaded from Ktunotes.in

Latitude is the angle between the equatorial plane and the straight line that passes

through that point and through the centre of the earth.

Meridians are lines of equal longitude. The Prime Meridian passes through

Greenwich, England and has the reading of 00. Using the prime meridian as the

reference, we can measure the longitude value of a point on the Earth’s surface as 00

to 1800 East or West of Prime Meridian (1800W to 1800E). Meridians are therefore

used for measuring location in the E-W direction.

Parallels are lines of equal latitude. Using the equator as 00 latitude, we can measure

the latitude value of a point as 00 to 900 North or South of the equator (900N-900S).

Parallels are therefore used for measuring location in the N-S direction.

The prime meridian and the equator serve as the baselines of the geographic

coordinate system. The notation of geographic coordinates is therefore like plane

coordinates : longitude values are equivalent to x values and latitude values are

equivalent to y values. And it is conventional in GIS to enter longitude and latitude

values with positive or negative signs. Longitude values are positive in eastern

hemisphere and negative in the western hemisphere. Latitude values are positive if

north of equator and negative if south of the equator.

The angular measures of longitude and latitude may be expressed in degrees-minutes-

seconds (DMS), decimal degrees (DD), or radians (rad).

Shape of the earth

The earth’s surface has many undulations (geoid) and cannot be expressed by any

regular shape and hence not suitable for performing any mathematical computations.

The best approximation to the surface of earth is an ellipsoid (also called spheroid),

which is an ellipse rotated about its minor axis. The most recently developed and

widely used ellipsoidal datum is WGS 1984 (World Geodetic System). It serves as the

framework for locational measurement worldwide. A local datum aligns its spheroid

KTUNOTES.IN

Downloaded from Ktunotes.in

to closely fit the earth’s surface in a particular area. Examples of local datum are

Everest spheroids (India), NAD 1927 (North American Datum), ED 1950 (European

Datum).

2. PROJECTED COORDINATE SYSTEM

A projected coordinate system is defined as 2-Dimensional representation of the

Earth. It is based on a spheroid geographic coordinate system (GCS), but it uses linear

units for measure of coordinates. In projected coordinate system, locations are

identified by x,y coordinates on a rectangular grid with origin as the centre (similar to

Cartesian coordinates). Each position has 2 values that refer it to that central location.

One specifies its East-West position and the other, its North-South position. The two

values are called x-coordinate and y-coordinate. The coordinates at origin are x=0 and

y=0. Horizontal lines above the origin and vertical lines to right of origin have

positive values; those below or to the left have negative values. The 4 quadrants

represent the 4 possible combinations of positive and negative x and y coordinates.

The signs of x,y coordinates in a projected coordinate system

The Universal Transverse Mercator (UTM) grid system, the Universal Polar

Stereographic (UPS) grid system and State Plane Coordinate (SPC) System are some

examples of the projected coordinate system. The UTM Grid system is used

worldwide and will be discussed in detail in the following sections.

MAP PROJECTIONS

PROJECTING is the science of converting the spherical earth surface to a flat plane.

A map projection is a systematic transformation of latitudes and longitudes of

locations on the surface of the Earth into location of a plane.

KTUNOTES.IN

Downloaded from Ktunotes.in

One easy way to understand how map projections alter spatial properties is to

visualize shining a light through the earth (globe) onto a surface, called the projection

surface. Imagine the earth’s surface is clear with the graticule drawn on it. Wrap a

piece of paper around the earth. A light at the center of the earth will cast the shadows

of the graticule onto the piece of paper. You can now unwrap the paper and lay it flat.

The shape of the graticule on the flat paper is very different than on the earth. The

map projection has distorted the graticule.

CLASSIFICATION OF MAP PROJECTIONS

I. On the basis of projection surface:

1. Cylindrical Projection

2. Conical Pprojection

3. Azimuthal/Planar Pprojection

II. On the basis of position of view point or position of light

1. Gnomonic Projection

2. Stereographic Projection

3. Orthographic Projection

III. On the basis of preserved properties

1. Conformal projections

2. Equal area projections

3. Equidistant projections

4. True-direction projections

KTUNOTES.IN

Downloaded from Ktunotes.in

I. On the basis of projection surface

▪ Cylindrical projections result from projecting a spherical globe onto a

cylinder. Cylindrical projection equations yield projected graticules

with straight meridians and parallels that intersect at right angles.

▪ Conical projections result from projecting a globe onto a cone.

Conical projections yield straight meridians that converge toward a

single point at the poles and parallels that form concentric arcs.

▪ Azimuthal projection results from projecting a spherical surface onto

a plane. In this type of projection, a flat paper is supposed to touch the

globe at one point and project the lines of latitude and longitude on the

plane. These projections also yield meridians that are straight and

convergent, but parallels form concentric circles rather than arcs.

Aspects of projection

Aspect describes the placement of the projective surface relative to the globe. The 3

types of aspects for a cylindrical or conical projection are shown below:

KTUNOTES.IN

Downloaded from Ktunotes.in

For an azimuthal projection, the aspects can be as shown below:

II. On the basis of Viewoint or poisition of Light source

• Gnomonic Projection (viewpoint lies at the centre of the globe).

• Stereographic projection (viewpoint lies at the opposite pole).

• Orthographic projection (viewpoint lies at infinity)

III. On the basis of properties preserved

It is not an easy task to flatten an elliposoid that contains a variable surface

onto a plane. Differnt projections have different types of distortions. The

properties which are considered for classifying the map projection under this

category are area, shape, direction and distance.

1. Conformal projection: preserves shapes and local angles. Meridians

and parallels intersect at right angles in this type of projection.

2. Equal area projections: preserves the area. When a map portrays

areas over the entire map so that all mapped areas have the same

proportional relationship to the areas on the earth that they represent,

the map is an equal area map.

3. Equi-distant projection: a map is equidistant when it portrays

distances correctly from the centre of the projection to any other place

on the map. Hence it maintains consistant scale along certain lines.

KTUNOTES.IN

Downloaded from Ktunotes.in

4. True direction projection: a map preserves direction when azimuths

(or bearings) are portrayed correctly in all directions.

UTM (Universal Transverse Mercator) PROJECTION

It is a projected coordinate system used worldwide.

UTM Zones are numbered from 1 to 60.

Each zone 60 is longitude wide.

Zone 1 lies between 1800W and 1740W.

Each zone divided into horizontal band of 80 latitude wide between 840N latitude and

800S latitude.

Bands labelled from C to X (excluding I and O) from South to North (20 bands).

GIS DATA TYPES

In GIS, there are 2 types of data – spatial data and attribute (non-spatial) data. Take

the example of roads. To describe a road, we refer to its location (i.e. where it is) and

its characteristics (eg. Name, speed limit, whether one-way or two-way road, etc.).

The location, also called geometry or shape represents spatial data, whereas the

characteristics are attribute data. Thus roads, like any geospatial data, have the 2

components of spatial and attribute data.

KTUNOTES.IN

Downloaded from Ktunotes.in

Spatial Data

Spatial data describe the locations of spatial features, which may be discrete or

continuous. Discrete features are individually distinguishable features that do not

exist between observations. Discrete features include points (eg., wells), lines (eg.,

roads) and areas (eg., Landuse types). Continuous features are features that exist

spatially between observations. Examples of continuous features are elevation and

precipitation. The locations of spatial features on the Earth’s surface are based on a

geographic coordinate system with longitude and latitude values, whereas the

locations of map features are based on plane coordinate system.

The data model defines how spatial features are represented in GIS. The spatial

features can be represented in two forms-vector and raster. The vector data model

uses points and their x,y coordinates to construct spatial features of points, lines and

areas. The raster model uses a grid and grid cells to represent the spatial variation of a

feature.

KTUNOTES.IN

Downloaded from Ktunotes.in

Attribute Data

Attribute data describes the characteristics of spatial features. These characteristics

can be quantitative or qualitative in nature. Attribute data is also referred to as tabular

data. For raster data, each cell has a value that corresponds to the attribute of the

spatial feature at that location. For vector data, the amount of attribute data to be

associated with a spatial feature can vary significantly. A road segment may only have

the attributes of length and speed limit, whereas a soil polygon may have dozens of

properties, interpretations and performance data.

SPATIAL DATA REPRESENTATION (Spatial Data Modeling)

A spatial feature can be represented in 2 forms – raster data representation and vector

data representation.

Vector data representation

The vector data model uses the geometric objects of point, line and area to represent

a single spatial feature. The shape of a spatial feature is stored using 2-dimensional

(x,y) coordinate system.

A point feature has 0 dimension and has only the property of location. It is made of a

point or a set of separate points. Wells, benchmarks and gravel pits are examples of

point features.

A line feature is one-dimensional and has the property of length. A line has 2 end

points and points in between to mark the shape of the line. The shape of a line may be

a smooth curve or a connection of straight-line segments. A line feature is made of

lines. Roads, streams and contour lines are examples of line features.

An area is 2-dimensional and has the properties of area (size) and perimeter. Made of

connected lines, an area may be alone, or share boundaries with other areas. An area

may contain holes. Existence of holes means that the area has both external and

internal boundaries. An area is made up of polygons. Examples of area features

include timber stands, land parcels and water bodies.

KTUNOTES.IN

Downloaded from Ktunotes.in

Advantages of vector data

• Data can be represented at its original resolution and form without generalization.

• Most data, e.g. hard copy maps, is in vector form no data conversion is required.

• Accurate geographic location of data is maintained

• Compact

Disadvantages of vector data

• The location of each vertex needs to be stored explicitly.

• Complex data structure

• Continuous data, such as elevation data, is not effectively represented in vector

form.

• Spatial analysis and filtering within polygons is impossible

• Technology is expensive, particularly for the more sophisticated software and

hardware.

• Combination of several vector polygon maps through overlay creates difficulties.

Raster data representation

In raster data model, the geographic space is represented by an array of cells or pixels,

which are arranged in rows and columns. The origin of rows and columns is typically

at the upper-left corner of the raster. Rows function as y coordinates and columns as x

coordinates. Each pixel has a value that represents information. The values can be in

form of integer, floating points or alphanumeric.

Raster data represent points with single cells, lines with sequence of neighboring cells

and areas with collection of contiguous cells. Although the raster data model lacks the

vector model’s precision in representing the location of spatial features, it has the

distinct advantage of having fixed cell locations. Raster data are much easier to

manipulate, aggregate and analyse than vector data.

KTUNOTES.IN

Downloaded from Ktunotes.in

Advantages of raster data

• Simple data structures

• The geographic location of each cell is implied by its position in the cell matrix.

Accordingly, other than an origin point, no geographic coordinates are stored.

• Due to the nature of the data storage technique, data analysis is usually easy to

program and quick to perform.

• Overlay and combination of maps and remote sensed images are easy

• Technology is cheap

Disadvantages of raster data

• Crude raster maps are considerably less beautiful.

• It is especially difficult to adequately represent linear features depending on the cell

resolution.

• Processing of associated attribute data may be cumbersome if large amounts of data

exist.

• Since most input data is in vector form, data must undergo vector-to-raster

conversion.

• Most output maps from grid-cell systems do not conform to high-quality

cartographic needs

DATA INPUT METHODS

There are several methods for entering the spatial data into GIS. They are:

1. DIGITISING

Digitising is the process of converting data from analog (maps, aerial photos, etc.)

to digital format. Digitising is of 2 types:

a. Using Digitising Tables:

A digitizing table has a built-in electronic mesh, which can sense the position

of the cursor. To transmit the x,y coordinates of a point to the connected

KTUNOTES.IN

Downloaded from Ktunotes.in

computer, the operator simply clicks on a button on the cursor after lining up

the cursor’s cross hair with the point. Digitizing point features is simple: each

point is clicked once to record its location. Digitising a line feature can follow

either point mode or stream mode. In point mode, the operator selects the

certain points in a line to digitize. In stream mode, lines are digitized at a

preset time or distance interval. Point mode is preferred if features to be

digitized have many straight-line segments. Because the vector data model

treats a polygon as a series of lines, digitizing polygon features is the same as

digitizing line features.

b. On-screen digitizing:

On-screen digitizing is manual digitizing on the computer monitor using data

sources such as a satellite image or scanned map or aerial photographs as

background.

2. SCANNING

Scanning is a digitizing method that converts an analog map into a scanned file

(scanned files will be in raster format), which is then converted back to vector

format through tracing. A scanner converts an analog map into a scanned image

file in raster format. To complete the digitizing process, a scanned file must be

vectorised. Vectorisation turns raster lines into vector lines in a process called

tracing. Tracing or vectorisation involves 3 basic elements: line thinning, line

extraction and topological reconstruction.

3. COORDINATE GEOMETRY

A third method for spatial data input involves the calculation and entry (using

keyboard) of the coordinates of the spatial feature using coordinate geometry

(COGO) procedures. This involves entering, from survey data, the explicit

KTUNOTES.IN

Downloaded from Ktunotes.in

measurement of features from some known monument. This input technique is

obviously very costly and labour intensive.

4. CONVERSION OF EXISTING DIGITAL DATA

This method is becoming increasingly popular for data input. A variety of spatial

data, including digital maps are openly available from a wide range of government

and private sources. The most common digital data used in GIS is the data from

CAD systems. A number of data conversion program exist, to transform data from

CAD formats to a raster or vector GIS format.

GEOMETRIC TRANSFORMATION

Geometric transformation is the process of using a set of control points and

transformation equations to register a digitized map, a satellite image or an aerial

photograph onto a projected coordinate system

A newly-digitised map, either manually digitized or traced from a scanned

file, is based on digitizer units. Digitizer units can be in inches or dots per inch.

Geometric transformation converts the newly digitized map into projected coordinates

in a process often called map-to-map transformation.

Image-to-map transformation applies to remotely sensed data. The term

suggests that the transformation changes the rows and columns of a satellite image

into projected coordinates. Another term describing this kind of transformation is

georeferencing. A georeferenced image can register spatially with other feature or

raster layers in a GIS database, as long as the coordinate system is the same. Whether

map-to-map or image-to-map, a geometric transformation uses a set of control points

to establish a mathematical model that relates the map coordinates of one system to

another or image coordinates to map coordinates.

Transformation Methods

Different methods have been proposed for transformation from one coordinate system

to another. Each method if distinguished by the geometric properties it can preserve

and by the changes it follows. The effect of transformation varies from changes of

position and direction, to a uniform change of scale, to changes in shape and size. The

following summarises these transformation methods and their effect on a rectangular

object.

KTUNOTES.IN

Downloaded from Ktunotes.in

➢ Equiarea transformation allows rotation of the rectangle and preserves its

shape and size.

➢ Similarity transformation allows rotation of the rectangle and preserves its

shape but not size.

➢ Affine transformation allows angular distortion but preserves the parallelism

of lines (i.e. parallel lines remain as parallel lines).

➢ Projective transformation allows both angular and length distortions, thus

allowing the rectangle to be transformed into an irregular quadrilateral.

Different types of Geometric Transformation

Affine Transformation

Affine transformation allows rotation, translation, skew and differential scaling on a

rectangular object while preserving line parallelism. Rotation rotates the object’s x

and y axes from the origin. Translation shifts its origin to a new location. Skew

allows a non-perpendicularity (or affinity) between the axes, thus changing its shape

to a parallelogram with a slanted direction. And differential scaling changes the scale

by expanding or reducing in the x and/or y direction. The figure below shows these 4

transformations graphically.

KTUNOTES.IN

Downloaded from Ktunotes.in

Root Mean Square Error

The root mean square (RMS) error is a quantitative measure that can

determine the quality of geometric transformation. It measures the displacement

between the actual and estimated locations of the control point. If the RMS error is

acceptable, then a mathematical model derived from the control points can be used for

transforming the entire map or image.

Each individual difference between the actual and estimated locations is called

residual (e). The RMS Error aggregates the residuals into a single measure. The RMS

Error is derived by squaring the differences between the actual and estimated values,

adding these residuals together, dividing that by the total number of values, and taking

the square root of the result.

RMS Error =√𝑒1

2+𝑒22+𝑒3

2+⋯..+𝑒𝑛2

𝑛 = √

∑ 𝑒𝑖2𝑛

𝑖=1

𝑛

Where e1 is the residual of the first control point, e2 is the residual of second

control point, en is the residual of nth control point and n is the number of control

points. Here residual is the difference between the actual and estimated locations.

Location in fact is represented by 2 values (x and y coordinates). So to accommodate

these two coordinates to find residuals, the equation can be rewritten as:

RMS Error = √⌊∑ (𝑥𝑎𝑐𝑡,𝑖 − 𝑥𝑒𝑠𝑡,𝑖)2

+ ∑ (𝑦𝑎𝑐𝑡,𝑖 − 𝑦𝑒𝑠𝑡,𝑖)2𝑛

𝑖=1𝑛𝑖=1 ⌋ /𝑛

To ensure the accuracy of geometric transformation, the RMS error should be

within a tolerance value. If the RMS error is within the acceptable range, then the

assumption is that this same level of accuracy based on control points can also apply

KTUNOTES.IN

Downloaded from Ktunotes.in

to the entire map or image. If the RMS error exceeds the established tolerance, then

the control points need to be adjusted.

VECTOR DATA ANALYSIS

Vector data can be analysed by different methods and tools. They include:

• Buffering

• Overlay

• Distance measurement

• Pattern analysis

• Map manipulation

1. BUFFERING

Buffer can be defined as an influenced area/zone of an object in GIS. Based on the

concept of proximity, buffering creates 2 areas: one area that is within a specified

distance of select features and the other area that is beyond. The area that is within the

specified distance is called buffer zone.

The spatial features for buffering may be points, lines or areas. Buffering around

points creates circular buffer zones. Buffering around lines creates a series of

elongated buffer zones. Buffering around polygons creates buffer zones that extend

outward from the polygon boundaries.

Variations in Buffering

There are several variations in buffering from those of the 3 above mentioned cases.

The buffer distance or buffer size does not have to be constant; it can vary according

to the values of a given field. For example, the width of the riparian buffer can vary

Buffering around lines Buffering around

areas

KTUNOTES.IN

Downloaded from Ktunotes.in

depending on its expected functions and the intensity of adjacent land use.

A feature may have more than 1 buffer zones. As an example,a nuclear power plant

may be buffered with distances of 5, 10, 15 and 20 miles, thus forming multiple rings

around the plant.

Buffering around line features does not have to be on both sides of line, it can be

either on the left side or right side of the line feature.

Buffer zones around polygons can be extended either outward or inward from the

polygon boundaries.

KTUNOTES.IN

Downloaded from Ktunotes.in

Boundaries of buffer zones may remain intact so that each buffer zone is a separate

polygon. Or these boundaries may be dissolved so that there are no overlapped areas

between buffer zones.

Even the ends of buffer can be either rounded or flat.

Applications of Buffering

A buffer zone is often treated as a protection zone and is used for planning or

regulatory purposes:

• A city control authority may specify that no liquor stores shall be within

1000feet of a school or church.

• Government regulations may set 2-mile buffer zones along streams to

minimize sedimentation from logging operations.

• A national forest may restrict oil and gas well drilling within 500feet of roads

or highways.

• A planning agency may set aside land along the edges of streams to reduce the

effects of nutrient, sediment, and pesticide runoff; to maintain shade to prevent

the rise of stream temperature; and to provide shelter for wildlife and aquatic

life.

• A resource agency may establish stream buffers or vegetated filter strips to

protect aquatic resources from adjacent agricultural land-use practices.

2. OVERLAY

An overlay operation combines the geometries and attributes of 2 feature layers to

create the output. The geometry of output represents the geometric intersection of

features from input layers. The figure below shows an overlay operation with 2

polygon layers. Each feature on the output contains a combination of attributes from

KTUNOTES.IN

Downloaded from Ktunotes.in

the input layers and this combination differs from its neighbors. Feature layers to be

overlaid must be spatially registered (georeferenced) and based on the same

coordinate system.

Feature Type and Overlay

In practice, the first consideration for overlay is feature type. There are two types of

overlay operations. The first group uses two polygon layers as inputs. The second

group uses one polygon layer and another layer, which may contain points or lines.

Overlay operations can therefore be classified by feature type into point-in-polygon,

line-in-polygon and polygon-on-polygon. To distinguish layers in the following

discussion, the layer that may be a point, line, or polygon layer is called input layer

and the layer that is a polygon layer is called the overlay layer.

In a point-in-polygon overlay operation, the same point features in the input layer are

included in the output but each point is assigned with attributes of the polygon within

which it falls. For example, a point-in-polygon overlay can find association between

wildlife locations (point features) and vegetation types (polygon).

In a line-in-polygon overlay operation, the output contains the same line features as

in the input layer but each line feature is dissected by the polygon boundaries on the

overlay layer. Thus, the output has more line segments than does the input layer. Each

line segment on the output combines attributes from the input layer and the

underlying polygon. For example, a line-in-polygon overlay can find soil data for a

proposed road. The input layer includes the proposed road. The overlay layer contains

soil polygons. And the output shows a dissected proposed road, each road segment

having a different set of soil data from its adjacent segments.

KTUNOTES.IN

Downloaded from Ktunotes.in

The most common overlay operation is polygon-on-polygon overlay, involving 2

polygon layers. The output combines the polygon boundaries from the input and

overlay layers to create a new set of polygons. Each new polygon carries attributes

from both layers and these attributes differ from those of adjacent polygons. For

example, a polygon-on-polygon overlay can analyse the association between elevation

zones (polygon feature) and vegetation types (polygon feature).

Overlay Methods

All overlay methods are based on Boolean connectors- AND, OR and XOR.

Union: An overlay operation is called Union if it uses the OR Connector. Union

preserves all features from the inputs. The area extend of the output combines the area

extend of both input layers. Union requires that both input layers be polygon layers.

KTUNOTES.IN

Downloaded from Ktunotes.in

Intersect: An overlay operation is called Intersect if it uses AND connector. Intersect

preserves only those features that fall within the area extent common to the inputs.

Symmetrical Difference: an overlay operation that uses XOR connector is

symmetrical difference. It preserves features that fall within the area extent that is

common to only one of the inputs. In other words, symmetrical difference is opposite

to Intersect in terms of the output’s area extent.

Identity: an overlay operation is called Identity if it uses the following expression:

[ (input layer) AND (identity layer) ] OR (input layer)

Identity preserves only features that fall within the area extent of the layer defined as

the input layer. The other layer is called the identity layer.

3. MAP MANIPULATION METHODS

APPLICATIONS OF GIS

1. Disaster management: Today, well-developed GIS Systems are used to protect

the environment. It has become an integrated, well developed and successful tool

in disaster management and mitigation. GIS can help with risk management and

analysis by displaying which areas are likely to be prone to natural or man-made

disasters. When such disasters are identified, preventive measures can be

developed.

2. Crime statistics: GIS is now vital to law enforcement and planning in terms of

crime statistics. Though most police forces in the USA have used them for a long

time, automated and digital mapping of reported crimes has made the process

much easier, especially when looking at different types of crimes from different

KTUNOTES.IN

Downloaded from Ktunotes.in

departments in larger cities. The ability to share maps and look for correlations

between different types of crimes can give police a much better idea of an overall

picture of a wider region.

3. Archeology: GIS is now critical to many elements of archaeology as it takes on

more elements and characteristic of an environmental science. There are many

applications in the field of historical research but none has been more beneficial

than the prediction of historic site location. Several US universities recently

plotted an area to the south of Caucasus to identify pre-historic sites and areas that

may have potential for future on-the-ground research, most notably of the

migration route out of Africa in antiquity. The project successfully identified a

number of potential new sites for future investigation.

4. Civic Planning: GIS has been a superb tool for rural and urban planning for the

last few decades, working out local tax rates, planning desirability and mapping

social deprivation, where new roads could go or which should be prioritized for

repair. It is now a vital part of our green future too. Cascade in Montana is a prime

site for wind farms and there is a website that uses GIS data to plot wind speeds

over the course of a year in order to best site the wind farms.

5. Health/medical resource management: GIS is vital to the proper planning and

analysis of the provision of cancer services for the UK socialized healthcare

system, the NHS (National Health Service). The package is used to plan and

examine a number of issues including catchment areas for GP surgeries. A study

recently found that there was greater provision for cancer treatment in the

midlands than the actual population. Such maps are used to better manage

resources of the NHS.

6. Transport: GIS can be used in managing transportation and logistical problems.

If the transport department is planning for a new railway or a road route, then this

can be performed by adding environmental and topographical data into the GIS

platform. This will easily output the best route for the transportation based on the

criteria like flattest route, least damage to habitats and least disturbance from local

people. GIS can also help in monitoring rail systems and road conditions.

KTUNOTES.IN

Downloaded from Ktunotes.in

KTUNOTES.IN

Downloaded from Ktunotes.in

KTUNOTES.IN

Downloaded from Ktunotes.in