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overview of Gis

Geographic Information System (GIS) is a computer based information system used to digitally represent and analyse 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.

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 of 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.
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.
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.
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.
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.
components of gis

GIS constitutes of five key components:

  • Hardware
  • Software
  • Data
  • People
  • Method
gis applications
Computerized mapping and spatial analysis have been developed simultaneously in several related fields. The present status would not have been achieved without close interaction between various fields such as utility networks, cadastral mapping, topographic mapping, thematic cartography, surveying and photogrammetery remote sensing, image processing, computer science, rural and urban planning, earth science, and geography.
The GIS technology is rapidly becoming a standard tool for management of natural resources. The effective use of large spatial data volumes is dependent upon the existence of an efficient geographic handling and processing system to transform this data into usable information.
The GIS technology is used to assist decision-makers by indicating various alternatives in development and conservation planning and by modelling the potential outcomes of a series of scenarios. It should be noted that any task begins and ends with the real world. Data are collected about the real world. Of necessity, the product is an abstraction; it is not possible (and not desired) to handle every last detail. After the data are analysed, information is compiled for decision-makers. Based on this information, actions are taken and plans implemented in the real world.
Fundamentals of GIS:  
(A) mapping concepts, features & properties

A map represents geographic features or other spatial phenomena by graphically conveying information about locations and attributes. Locational information describes the position of particular geographic features on the Earth's surface, as well as the spatial relationship between features, such as the shortest path from a fire station to a library, the proximity of competing businesses, and so on. Attribute information describes characteristics of the geographic features represented, such as the feature type, its name or number and quantitative information such as its area or length.
Thus the basic objective of mapping is to provide

  • Descriptions of geographic phenomenon
  • Spatial and non spatial information
  • Map features like Point, Line, & Polygon.
(b) map features
Locational information is usually represented by points for features such as wells and telephone pole locations, lines for features such as streams, pipelines and contour lines and areas for features such as lakes, counties and census tracts.
Point feature - A point feature represents as single location. It defines a map object too small to show as a line or area feature. A special symbol of label usually depicts a point location.
Line feature - A line feature is a set of connected, ordered coordinates representing the linear shape of a map object that may be too narrow to display as an area such as a road or feature with no width such as a contour line.
Area feature - An area feature is a closed figure whose boundary encloses a homogeneous area, such as a state country soil type or lake.
(c) the gis data model: implementation

Data is organized by layers, coverages or themes (synonomous concepts), with each layer representing a common feature
Layers are integrated using explicit location on the earth’s surface, thus geographic location is the organizing principal.
Layers are comprised of two data types

  • Spatial data which describes location (where)
  • Attribute data specifing what, how much,when

Layers may be represented in two ways:

  • Iin vector format as points and lines
  • In raster(or image) format as pixels

All geographic data has 4 properties:
projection, scale, accuracy and resolution

(d) map characteristics

In addition to feature locations and their attributes, the other technical characteristics that define maps and their use includes:

  • Map Scale
  • Map Accuracy
  • Map Extent and
  • Data Base Extent
(e) maps and map analysis

Maps can be broadly classified in to two groups:

  1. Topographical maps
  2. Thematic maps
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