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
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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.
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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.
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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.
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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.
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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
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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.
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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
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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:
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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.
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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.
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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.
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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.
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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.
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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
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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
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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.
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➢ 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.
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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
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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
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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.
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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
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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.
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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.
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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
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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.
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