1
INTRODUCTION
Geographic Information System
(GIS) is a computer based information system used to digitally represent and
analyze the geographic features present on the Earth' surface and the events
(non-spatial attributes linked to the geography under study) that taking place
on it. The meaning to represent digitally is to convert analog (smooth line)
into a digital form.
"Every object present on the Earth can be
geo-referenced", is the fundamental key of associating any database to
GIS. Here, term 'database' is a collection of information about things and
their relationship to each other and 'geo-referencing' refers to the location
of a layer or coverage in space defined by the co-ordinate referencing system.
1.1
What is GIS?
GIS is an acronym for:
Geographic
Information Systems (US)
Geographical
Information Systems (UK, Aust., Canada)
Geographic
Information Science (Academia) An understanding of what Geographic Information
Systems represent may be helped by considering the component parts of the term
separately.
Geographic...
This term is used because GIS tend to deal primarily with
`geographic' or `spatial' features. These objects can be referenced or related
to a specific location in space. The objects may be physical, cultural or
economic in nature. Features on a map for instance are pictorial
representations of spatial objects in the real world. Symbols, colors, lines
and styles are used to represent the
different spatial features on the two-dimensional map.
Information...
This represents the large volumes of data, which are usually
handled within a GIS. All real world objects have their own particular set of
characteristics or descriptive attributes. This non-spatial alphanumeric data
plus location information needs to be stored and managed for all spatial
features of interest.
Conventionally maintained as paper files, computer
technology has enabled much more efficient handling and management of
information within automated database management systems.
Systems...
This term is used to represent the systems approach taken by
GIS, whereby complex environments are broken down into their component parts
for ease of understanding and handling but are considered to form an integrated
whole. Computer technology has aided and even necessitated this approach so
that most information systems are now computer based.
Computer systems are becoming vital for the storage and
manipulation of the increasing volumes of data, the handling of complex spatial
algorithms and the integration of data of different scales, projections and
formats. All of which are essential to GIS.
Therefore, Geographic Information System (GIS) is a computer
based information system used to digitally represent and analyze the geographic
features present on the Earth' surface and the events (non-spatial attributes
linked to the geography under study) that taking place on it.
A GIS is typically made up of a variety of information
systems like Cartographic Display System, Map Digitizing System, Database
Management System, Geographic Analysis System, Image Processing System,
Statistical Analysis System and Decision Support System (Fig.1.2). In many
ways, learning GIS involves learning to think -learning to think about
patterns, about space, and about processes that act in space.
Defining
GIS
A GIS is an information system designed to work with data
referenced by spatial / geographical coordinates. In other words, GIS is both a
database system with specific capabilities for spatially referenced data as
well as a set of operations for working with the data. It may also be
considered as a higher order map.
GIS technology integrates common database operations such as
query and statistical analysis with the unique visualization and geographic
analysis benefits offered by maps. These abilities distinguish GIS from other
information systems and make it valuable to a wide range of public and private
enterprises for explaining events, predicting outcomes, and planning
strategies. (ESRI)
A Geographic Information System is a computer based system
which is used to digitally reproduce and analyze the feature present on earth
surface and the events that take place on it. In the light of the fact that
almost 70% of the data has geographical reference as its denominator, it
becomes imperative to underline the importance of a system which can represent
the given data geographically.
A typical GIS can be understood by the help of various
definitions given below:
A geographic information system (GIS) is a computer-based
tool for mapping and analyzing things that exist and events that happen on
Earth
Burrough in 1986 defined GIS as, "Set of tools for
collecting, storing, retrieving at will, transforming and displaying spatial
data from the real world for a particular set of purposes"
Different definitions of GIS given
by various organizations are as follows:
- A
geographic information system, commonly referred to as a GIS, is an
integrated set of hardware and software tools used for the manipulation
and management of digital spatial (geographic) and related attribute data.
- A
geographic information system (GIS) is a computer-based tool for mapping
and analyzing things that exist and events that happen on earth. GIS
technology integrates common database operations such as query and
statistical analysis with the unique visualization and geographic analysis
benefits offered by maps.
- GIS
is an integrated system of computer hardware, software, and trained
personnel linking topographic, demographic, utility, facility, image and
other resource data that is geographically referenced.
- A
geographic information system (GIS) is a computer-based information system
that enables capture, modeling, manipulation, retrieval, analysis and
presentation of geographically referenced data.
1.2
Philosophy of GIS
The proliferation of GIS is explained by its unique ability
to assimilate data from widely divergent sources, to analyze trends over time,
and to spatially evaluate impacts caused by development.
For an experienced analyst, GIS is an extension one's own analytical
thinking. The system has no in-built solutions for any spatial problems; it
depends upon the analyst.
The importance of different factors of GIS in decreasing
order is as under:
* Spatial Analysis
* Database
* Software
* Hardware GIS involves complete understanding about
patterns, space, and processes or methodology needed to approach a problem. It
is a tool acting as a means to attain certain objective quickly and
efficiently. Its applicability is realized when the user fully understands the
overall spatial concept under which a particular GIS is established and
analyses his specific application in the light of those established
parameters.
Before the GIS implementation is considered the objectives,
both immediate and long term, have to be considered. Since the effectiveness
and efficiency
(i.e. benefit against cost) of the GIS will depend largely
on the quality of initial field data captured, organizational design has to be
decided upon to maintain this data continuously. This initial data capture is
most important.
1.3
Advantages of GIS
The Geographic Information System has been an effective tool
for implementation and monitoring of municipal infrastructure. The use of GIS
has been in vogue primarily due to the advantage mentioned below:
* Planning of project
* Make better decisions
* Visual Analysis
* Improve Organizational Integration
1.3.1
Planning Of Project
Advantage of GIS is often found in detailed planning of
project having a large spatial component, where analysis of the problem is a
pre requisite at the start of the project. Thematic maps generation is possible
on one or more than one base maps, example: the generation of a land use map on
the basis of a soil composition, vegetation and topography. The unique
combination of certain features facilitates the creation of such thematic maps.
With the various modules within GIS it is possible to calculate surface,
length, width and distance.
1.3.2
Making Decisions
The adage "better information leads to better
decisions" is as true for GIS as it is for other information systems. A
GIS, however, is not an automated decision making system but a tool to query,
analyze, and map data in support of the decision making process. GIS technology
has been used to assist in tasks such as presenting information at planning
inquiries, helping resolve territorial disputes, and sitting pylons in such a
way as to minimize visual intrusion.
1.3.3
Visual Analysis
Digital Terrain Modeling (DTM) is an important utility of
GIS. Using DTM/3D modeling, landscape can be better visualized, leading to a
better understanding of certain relations in the landscape. Many relevant
calculations, such as (potential) lakes and water volumes, soil erosion volume
(Example: landslides), quantities of earth to be moved (channels, dams, roads,
embankments, land leveling) and hydrological modeling becomes easier.
Not only in the previously mentioned fields but also in the
social sciences GIS can prove extremely useful. Besides the process of
formulating scenarios for an Environmental Impact Assessment, GIS can be a
valuable tool for sociologists to analyze administrative data such as
population distribution, market localization and other related features.
1.3.4
Improving Organizational Integration
Many organizations that have implemented a GIS have found
that one of its main benefits is improved management of their own organization
and resources. Because GIS has the ability to link data sets together by
geography, it facilitates interdepartmental information sharing and
communication. By creating a shared database one department can benefit from
the work of another--data can be collected once and used many times.
As communication increases among individuals and
departments, redundancy is reduced, productivity is enhanced, and overall
organizational efficiency is improved. Thus, in a utility company the customer
and infrastructure databases can be integrated so that when there is planned
maintenance, affected people can be informed by computer-generated letters.
1.4
Need of GIS
Many professionals, such as foresters, urban planners, and
geologists, have recognized the importance of spatial dimensions in organizing
& analyzing information. Whether a discipline is concerned with the very
practical aspects of business, or is concerned with purely academic research,
geographic information system can introduce a perspective, which can provide
valuable insights as
·
70% of the information has geographic location
as it's denominator making spatial analysis an essential tool.
·
Ability to assimilate divergent sources of data
both spatial and non-spatial (attribute data).
·
Visualization Impact
·
Analytical Capability
·
Sharing of Information
1.5
Factors Aiding the Rise of GIS.
·
Revolution in Information Technology. Computer
Technology, Remote Sensing, Global Positioning System
·
Communication Technology.
·
Rapidly declining cost of Computer Hardware, and
at the same time, exponential growth of operational speed of computers.
·
Enhanced functionality of software and their
user-friendliness.
·
Visualizing impact of GIS corroborating the
Chinese proverb "a picture is worth a thousand words."
·
Geographical feature and data describing it are
part of our everyday lives & most of our everyday decisions are influenced
by some facet of Geography.
1.6
History of GIS
Work on GIS began in late 1950s, but first GIS software came
only in late 1970s from the lab of the ESRI. Canada was the pioneer in the
development of GIS as a result of innovations dating back to early 1960s. Much
of the credit for the early development of GIS goes to Roger Tomlinson.
Evolution of GIS has transformed and revolutionized the ways in which planners,
engineers, managers etc. conduct the database management and analysis.
1960’s – Canada Geographic Information System (CGIS) –
developed by Roger Tomlinson.
-
In USA a similar system developed for processing
natural resources data
1970’s – Main developments took place in universities in the
US, Canada and UK
-
Commercial agencies like ESRI and Intergraph
staring to develop and offer
-
Growing awareness of the need for sound and
stable structure to store and analyze map data becoming a dominant trend
1980’s – Marked with the widespread availability of PCs,
tremendous progress on research on spatial data structures, indexing methods
and spatial databases
-
In 1988 with the formation of NCGIA major
contributions towards the progress in GIS research.
1990’s – Breaking through of object orientation in system
and database design.
-
Geo-informatics in professional recognition,
spatial information theory – theoretical basis for GIS
-
GIS enters medium and small businesses and new
domains such as geo-marketing
-
Evolution of Desktop GIS, Internet GIS
1.7
GIS
Applications
1.7.1
Government
Applications
·
Economic development
·
Transportation and Service Routing
·
Housing
·
Infrastructure
·
Health
·
Tax Maps
·
Human Services
·
Law Enforcement
·
Land use planning
·
Parks and Recreation
·
Environmental Monitoring
·
Emergency Management
·
Geodemographics
1.7.2
Business Applications
·
Delivery of goods and services
·
Retail store placement
·
Distribution of competitors
·
Location of potential customers
·
Traffic flow
·
Parking locations
1.7.3
Environmental Applications
·
Land cover and land use analysis
·
Modeling watersheds
·
Soil Erosion
·
Forest management
·
Conservation and Protected Areas
1.7.4
Computer
Cartography
The growth of
computer-assisted-cartography (CAC) has been largely dependent on the
development of vector-based GIS. With the help of GIS, cartographic tasks such
as thematic overlays of information, map projections, and map sheet layouts can
be performed much more conveniently.
Continually updated geographic
databases provide an easy way to produce new map editions. Automated mapmaking
and virtual map images have replaced traditional paper maps in many
applications. Web-based maps have made general-purpose navigation far more
accessible to the public.
However, manually digitized paper
maps remain the primary form of data input in an automated cartography GIS.
Scanned maps are also often used.
1.7.5
Land
Information
GIS has aided management of land
information by enabling easy creation and maintenance of data for land records,
land planning and land use. In particular, a flourishing number of municipal
governments have started to implement GIS to help manage their land
information. GIS makes input, updates, and retrieval of data such as tax
records, land-use plan, and zoning codes much easier then during the paper-map
era.
Typical uses of GIS in land
information management include managing land registry for recording titles to
land holdings, preparing land-use plan and zoning maps, cadastral mapping etc.
Input of data into a land information GIS includes: political and
administrative boundaries, transportation, and soil cover.
1.8
Spatial Operation
Spatial operations are functions that form important
components of an underlying model that takes input data, performs analysis on
it, and assimilates the data to produce output information.
The
following operations can be used to create new data from input data.
1.8.1
Buffering
geometry
1.8.2
Difference of geometries
The ST_Difference function returns the portion of the
primary geometry that is not intersected by the secondary geometry—the logical
AND NOT of space. Smaller figures are primary input and bigger figures are
secondary input.
1.8.3
Intersection of geometries
The ST_Intersection function
returns the intersection set of two geometries.
1.8.4
Symmetric difference of geometries
The ST_SymmetricDiff function returns the symmetric
difference of two geometries-the logical XOR of space. The source geometries
must have the same dimension. If the geometries are equal, the ST_SymmetricDiff
function returns an empty geometry; otherwise, the function returns the result
as a collection.
1.8.5
Union of geometries
The ST_Union function returns the union set of two
geometries—the Boolean logical OR of space. The source geometries must have the
same dimension. ST_Union always returns
the result as a collection.
1.8.6
Minimum distance
1.8.7
Aggregates
Aggregate operations return a single geometry as a result of
analysis performed on a collection of geometries.
1.9
Spatial Objects
·
the objects in a spatial database are representations of
real-world entities with associated attributes
·
the power of a GIS comes from its ability to look at entities
in their geographical context and examine relationships between entities
·
thus a GIS database is much more than a collection of objects
and attributes
·
a spatial database can be assembled from simple objects
Ø e.g.
how are lines
linked together to form complex hydrologic or transportation networks
Ø e.g. how can points,
lines or areas be used to represent more complex entities like surfaces?
Types
of Spatial objects.
1.9.1
POINT DATA
·
the simplest type of spatial object
·
choice of entities which will be represented as points
depends on the scale of the map/study
Ø e.g.
on a large scale map - encode building structures as point locations
Ø e.g.
on a small scale map - encode cities as point locations
·
the coordinates of each point can be stored as two additional
attributes
·
information on a set of points can be viewed as an extended
attribute table
Ø each
row is a point - all information about the point is contained in the row
Ø each
column is an attribute
Ø two
of the columns are the coordinates
overhead
- Point data attribute table
Ø here
northing and easting represent y and x coordinates
- each point is independent of every other point,
represented as a separate row in the database model
1.9.2
LINE DATA
·
infrastructure networks
Ø transportation
networks - highways and railroads
Ø utility
networks - gas, electric, telephone, water
Ø airline
networks - hubs and routes
·
natural networks
Ø river
channels
·
a network is composed of:
Ø nodes
- junctions, ends of dangling lines
Ø links
- chains in the database model
diagram
·
valency of a node is the number of links at the node
Ø ends
of dangling lines are "1-valent"
Ø 4-valent
nodes are most common in street networks
Ø 3-valent
nodes are most common in hydrology
·
a tree network has only one path between any pair of nodes,
no loops or circuits are possible
Ø most
river networks are trees
·
examples of link attributes:
Ø direction
of traffic, volume of traffic, length, number of lanes, time to travel along
link
Ø diameter
of pipe, direction of gas flow
Ø voltage
of electrical transmission line, height of towers
Ø number
of tracks, number of trains, gradient, width of most narrow tunnel, load
bearing capacity of weakest bridge
·
examples of node attributes:
Ø presence
of traffic lights, presence of overpass, names of intersecting streets
Ø presence
of shutoff valves, transformers
·
note that some attributes (e.g. names of intersecting
streets) link one type of entity to another (nodes to links)
·
some attributes are associated with parts of network links
Ø e.g.
part of a railroad link between two junctions may be inside a tunnel
Ø e.g.
part of a highway link between two junctions may need pavement maintenance
·
many GIS systems require such attributes to be attached to
the network by splitting existing links and creating new nodes
Ø e.g.
split a street link at the house and attach the attributes of the house to the
new (2-valent) node
Ø e.g.
create a new link for the stretch of railroad which lies inside the tunnel,
plus 2 new nodes
·
this requirement can lead to impossibly large numbers of
links and 2-valent nodes
Ø e.g.
at a scale of 1:100,000, the US rail network has about 300,000 links
Ø the
number of links would increase by orders of magnitude if new nodes had to be
defined in order to locate bridges on links
1.9.3
AREA DATA
·
is represented on area class maps.
·
boundaries may be defined by natural phenomena, e.g. lake, or
by man, e.g. forest stands, census zones
·
there are several types of areas that can be represented
·
examples include
Ø land
cover data - forests, wetlands, urban
Ø geological
data - rock types
Ø forestry
data - forest "stands", "compartments"
Ø soil
data - soil types
·
boundaries are defined by the phenomenon itself
Ø e.g.
changes of soil type
·
almost all junctions are 3-valent
·
includes census tracts, ZIP codes, etc.
·
boundaries defined independently of the phenomenon, then
attribute values are enumerated
·
boundaries may be culturally defined, e.g. neighborhoods
Ø land
parcel boundaries, land use, land ownership, tax information
overhead - Areal coverage
1.
entities are isolated areas, possibly overlapping
Ø any
place can be within any number of entities, or none
Ø e.g.
areas burned by forest fires
Ø areas
do not exhaust the space
2. any place is within exactly one entity
Ø areas
exhaust the space
Ø every
boundary line separates exactly two areas, except for the outer boundary of the
mapped area
Ø areas
may not overlap
·
any layer of the first type can be converted to one of the
second type
Ø each
area may now have any number of fire attributes, depending on how many times it
has been burned - unburned areas will have none
1.9.4
REPRESENTATION
OF CONTINUOUS SURFACES
·
examples of continuous surfaces are:
Ø elevation
(as part of topographic data)
Ø rainfall,
pressure, temperature
Ø population
density
·
potential must exist for sampling observations everywhere on
an interval/ratio level
·
critical points
Ø peaks
and pits - highest and lowest points
Ø ridge
lines, valley bottoms - lines across which slope reverses suddenly
Ø passes
- convergence of 2 ridges and 2 valleys
·
faults - sharp discontinuities of elevation - cliffs
·
fronts - sharp discontinuities of slope
·
slopes and aspects can be derived from elevations
·
traditional data models do not have a method for representing
surfaces
Ø therefore,
surfaces are represented by the use of points, lines or areas
·
note: the following series of three overheads on Tiefort
Mountains all represent the same area
1.
points - grid of elevations overhead - Elevation represented as points
Ø DEM
or Digital Elevation Model
Ø based
on sampling the elevation surface at regular intervals
Ø result
is a matrix of points
Ø much
digital elevation data available in this form
2.
lines - digitized contours overhead - Elevation represented as lines
·
from DLG hypsography layer, identical to those on the printed
map, plotted directly from stereo photography
Ø based
on string object type
Ø a
line connecting sampled points of equal elevation
Ø elevation
is attribute
Ø could
be done for rainfall, barometric pressure etc.
3.
areas - TIN (Triangulated irregular network) overhead - Triangulation of a
terrain surface
overhead
- Elevation represented as areas
·
note: perspective diagram is developed from the triangulated
surface (TIN created by M.P. Kumler, USGS)
Ø sample
points often located at peaks, pits, along ridges and valleys
Ø sampling
can be varied depending on ruggedness of the surface
Ø a
very efficient way of representing topography
Ø result
is TIN composed of nodes, lines and triangular faces
1.10 Spatial interpolation
- frequently
when using continuous data we wish to estimate values at specific
locations which are not part of the point, line or area dataset
Ø these
values must be determined from the surrounding values using techniques of
spatial interpolation (see Units 40 and 41)
Ø e.g.
to interpolate contours, a regular grid is often interpolated from an irregular
scatter of points or densified from a sparse grid
1.11 The FOUR M’S of GIS
There are four key activities in a Geographic Information
System. These are measurement, mapping, monitoring and modeling.
The scientists, engineers, resource managers and urban
planners observe and manage these four key parameters, and develop maps which
portray characteristics of the earth. The above groups are monitoring the
changes in our surroundings in space and time. They model alternatives of
action and process operation in the environment.
No comments:
Post a Comment