Lecture 8

Geographic Information System (GIS)

Geographic information system (GIS) is defined as a powerful set of computer-based tools for collecting storing, retrieving at will, and transforming and-displaying spatial him from the real world for a particular set of purposes. GIS consists of

(i) An extensive database of geographic information involving both positional data about land features and descriptive / non-locational data about these features at different points of time; and

(ii) Sets of programmes of applications, which enable the data to be input, assessed, manipulated, analyzed and reported.

          The GIS history dates back 1960 where computer based GIS have been used and their manual procedures were in life 100 years earlier or so. On introduction of and spread of personal computers in 1980’s, exponential growth of GIS technology took place. However, potentiality of GIS is realized in the recent past and now it has become popular among many users for variety of applications. GIS is all about analysis of spatial data for extracting information, thereby it is called Geographic Information and is done through an integrated system comprised of four components: (i) hardware comprising of computer as processing unit, input (keyboard, digitizer, mouse, scanner) and output devices (monitor or display unit, plotter), (ii) designated software, (iii) data both spatial and non-spatial, and (iv) live ware i.e. people responsible as interface between software and data using specific methods and protocols for required output.

Objectives of GIS are:

·        Maximizing the efficiency of planning and decision making

·        Providing efficient means for data distribution and handling

·        Elimination of redundant data base minimize duplication

·        Capacity to integrate information from many sources, and

·        Complex analysis/query involving geographical referenced data to generate new information.

For any application there are five generic questions a (GIS) can answer: 

·        Location what exists at a particular location?

·        Condition Identify locations where certain conditions exist.

·        Trends what has changed since?

·        Patterns what spatial pattern exists?

·        Modeling What if........... ?

GIS differs from digital map due to features such as (i) it makes map dynamic, (ii) displays map information interactively, (iv) builds spatial relationship between features, and (V) analyze to answer real world problem.

Data in GIS

           Most important part of GIS is the data. GIS stores information about the world as a Collection of layers on various themes that can be used together a layer can be anything that contains similar features such as customers, buildings streets, lakes, or postal codes. This could be either an explicit geographic reference, such as a latitude and longitude called spatial data, or an implicit reference such as an address, postal code, census tract name, forest stand identifier, or road name called non-spatial data or attributes. Spatial data show where the feature is and attribute, provide information about the feature. These are linked by the software. Both spatial and attribute data in GIS allow the digital database to be exploited in many more ways than a conventional database making the maps dynamic and interactive. Therefore, GIS is more than all the functionalities of the DBMS and adds spatial functionality
 Spatial Data: Spatial data represent the location, size and shape of an object on planet Earth such au building lake, Mountain or township. Spatial data may also include attribute that provide more information about entity that is being represented. Geographic Information Systems (GIS) or other specialized software applications are to be used to access, Visualize, manipulate and analyze these data. Spatial data typically include various kinds of maps, ground survey data and remotely sensed imagery and can be represented by points, lines or polygons

Attribute Data: Attribute data refers to various types of administrative records, census, field sample records and collection of historical records. Attributes are either the qualitative characteristics of the spatial data or are descriptive information about the geographical location. Attributes are stored in the form of tables where each column of the table describes one attribute and each row of the table corresponds to a feature.

In raster type of representation of the geographical data, a set of cells located by coordinate is used; each cell is independently addressed with the value of an attribute. Each cell contains a single value and every location corresponds to a cell. One set of cell and associated value is a layer. Raster models are simple and spatial analysis is easier and faster. But it is voluminous. Vector data model uses line segments or points represented by their explicit x, y coordinates to identify locations. Discrete objects are formed by connecting line segments and its area is defined by set of line segments. Compared to raster, vector data models require less storage space, and are precise in estimating area/perimeter. Editing is faster and convenient. Spatial analysis is difficult. The vector model is extremely useful for describing discrete features, but less useful for describing continuously varying features such as soil type or accessibility costs for hospitals. The raster model has evolved to model such continuous features. A raster image comprises a collection of grid cells rather like a scanned map or picture. Both the vector and raster models, storing geographic data, have unique advantages and disadvantages. Modern GIS packages are able to handle both these models.GIS Layers and Coverage

The common requirement to access data on the basis of one or more classes has resulted in several GIS employing organizational schemes in which all data of a particular level of classification, such as roads, rivers or vegetation types are grouped into so-called layers or coverages. The concept of layers is to be found in both vector and raster models. The layers can be combined with each other in various ways to create new layers that are a function of the individual ones. The characteristic of each layer within a layer-based GIS is that all locations with each layer may be said to belong to a single arial region or cell, whether it be a polygon bounded by lines in vector system, or a grid cell in a raster system. But it is possible for each region to have multiple attributes.