Geographical Information Systems (GIS) is defined as a system of capturing, storing, manipulating, analysing, and displaying spatial information in an efficient manner (Tsihrintzis et al., 1996). These systems are characterized by standard software packages which offer unique capabilities of automating, managing, and analysing a variety of spatial data (Jankowski, 1995). In early 1980s, GIS systems emerged as a new information processing technology, and since then, GIS has been applied in many environmental decision making situations including water resources (Seth et al., 2006).
GIS software systems principally have four major components (Malczewski, 2004)
Data input: This component collects and/or processes spatial data from a variety of sources which include manual keyboard entry, digitizers, computer scanning or the importation of existing data files.
Data storage and retrieval: This component organizes the spatial data in a form of geo-database, which facilitates quick retrieval of data for subsequent analysis, as well as allows further updates and corrections in the spatial dataset.
Data manipulation and analysis: This component has unique capability of performing an integrated analysis of spatial data and their associated attributes.
20
Spatial data are manipulated and analysed to obtain information useful for a particular application. There is an enormously wide range of analytical operations available to the GIS users with multiple available toolsets.
Data Output: The data output component of a GIS provides a way to see the data/information or the results of GIS data analysis in the form of maps, tables, diagrams, etc.
Spatial data in GIS can be represented in two types of data models: raster and vector. The selection of data model depends on the type of specific application of the study. Data in a raster model are stored in a two-dimensional matrix of uniform grid cells (pixels or rasters), usually squares, on a regular grid. Each cell in a raster model has exactly one value (land use, elevation, political division). The size of the grid can vary from meter to many kilometres, and therefore the spatial resolution of data is determined by the grid size. Generally, the grid size determination is based on the desired levels of accuracy of data. Larger cell size yields low resolution for spatial data, losing important information, and conversely small (or fine) cell size improves the data resolution considerably, preserving the data information. However, it should be noted that fine spatial resolution increases the computational time and size of data storage, thereby increasing project costs. In terms of water resource applications, the continuous variables such as elevation and rainfall can be best represented by raster data.
The spatial entities which are finite in nature can be best represented by a vector model. These entities are represented in the form of point, line, and polygon in vector format. For example, a watershed can be represented as a polygon, a river as a line and raingauge station as a point in a given vector model. A polygon of watershed can have various attributes stored in a database representing its area or hydrological information. In the vector model, relationship between the spatial objects (points, lines and polygons) is determined by the term named ‘topology’. More details on topology and associated GIS concepts can be found in Davis (2001).
There is an enormous range of GIS commercial software available, which can be used across a range of different platforms including web, computers, mobiles and supercomputers. Steiniger and Hunter (2012) identified several categories of GIS
21
software application as: (i) desktop GIS, used for data creation, editing, analysis and map generation; (ii) Spatial Database Management Systems (Spatial DBMS) that are used for storage of data; (iii) web map server for the delivery of map representations over the internet; (iv) server GIS, that are used to analyse remote spatial data; (v) web GIS clients, to display and query spatial data stored at remote locations that are only accessible via internet or intranet; (vi) mobile GIS, which are most often used for data acquisition in the field; and finally (vii) GIS libraries and extensions, which provide special functions that can enhance standard (desktop) GIS capabilities, or be used to build customized GIS applications, including web mapping applications.
Malczewski (2004) listed major commercial GIS software, on the basis of three application areas i.e. GIS data viewers, desktop GIS, and high-end GIS. This classification, along with its intended use and platform is shown in Table 2.1.
Table 2.1: Major Commercial GIS Software [Adopted from (Malczewski, 2004)]
GIS Data Viewer Desktop GIS High-End GIS
Software ArcExplorer, GeoMedia Viewer, MapInfo ProViewer ArcGIS Autodesk World, Maptitude, Idrisi, GeoMedia, MapInfo Professional ArcGIS (Advanced), GeoMedia Pro, MapInfo Professional
The GIS data viewers (such as ArcExplorer) are only used in displaying and querying a specified spatial data set, which usually cannot be further customized by users. The software belonging to this category are usually free and primarily intended for general public and non experts.
The Desktop GIS, as the name suggests, are designed to run on desktop PCs, using the Windows operating system. A full featured desktop GIS includes built-in ability to input, store, manipulate and analyse, and output spatial data. Many desktop GIS software offer a framework for implementing customizations either through a proprietary or third generation programming language such as HTML. The Desktop GIS software require basic to intermediate level of GIS knowledge.
22
The high-end GIS systems are fully functional GIS toolkits, which often require powerful database and computational facilities. These software are suitable for large commercial enterprises, in a situation where all users of an organization or enterprise have access to a central information resource consisting of spatial data. Furthermore, these software require expert knowledge of GIS systems for their operation.
The current study uses ArcGIS for Desktop (V.9.3) software, developed by ESRI, USA (Ormsby, 2004). This software is primarily selected because its popularity and easy commercial availability.