introduction
definition of GIS
GIS applications
geospatial data
data for GIS applications
digital representation of geospatial data

introduction

definition of GIS

Like the field of geography, the term Geographic Information System (GIS) is hard to define. It represents the integration of many subject areas. Accordingly there us no absolutely agreed upon definition of a GIS (deMers, 1997). A broadly accepted definition of GIS is the one provided by the National Centre of Geographic Information and Analysis:

a GIS is a system of hardware, software and procedures to facilitate the management, manipulation, analysis, modelling, representation and display of georeferenced data to solve complex problems regarding planning and management of resources
(NCGIA, 1990)

Geographic information systems have emerged in the last decade as an essential tool for urban and resource planning and management. Their capacity to store, retrieve, analyse, model and map large areas with huge volumes of spatial data has led to an extraordinary proliferation of applications. Geographic information systems are now used for land use planning, utilities management, ecosystems modelling, landscape assessment and planning, transportation and infrastructure planning, market analysis, visual impact analysis, facilities management, tax assessment, real estate analysis and many other applications.

Functions of GIS include:
data entry
data display
data management
information retrieval and analysis

Figure. 1. The concept of layers (ESRI)

GIS applications

mapping locations
GIS can be used to map locations. GIS allows the creation of maps through automated mapping, data capture, and surveying analysis tools.

mapping quantities
People map quantities, like where the most and least are, to find places that meet their criteria and take action, or to see the relationships between places. This gives an additional level of information beyond simply mapping the locations of features.

mapping densities
While you can see concentrations by simply mapping the locations of features, in areas with many features it may be difficult to see which areas have a higher concentration than others. A density map lets you measure the number of features using a uniform areal unit, such as acres or square miles, so you can clearly see the distribution.

finding distances
GIS can be used to find out what's occurring within a set distance of a feature.

mapping and monitoring change
GIS can be used to map the change in an area to anticipate future conditions, decide on a course of action, or to evaluate the results of an action or policy.

geospatial data

Geospatial data has both spatial and thematic components.

Conceptually, geographic data can be broken up in two elements:
observation or entity and attribute or variable.
GIS have to be able to manage both elements.

Spatial component: The observations have two aspects in its localisation: absolute localisation based in a coordinates system and topological relationship referred to other observations. Example: The Department of Geomatics is located at the particular coordinate X,Y, or, The Department is located between Grattan Street and Old Engineering Building. A GIS is able to manage both while computer assisted cartography packages only manage the absolute one.

Thematic component: The variables or attributes can be studied considering the thematic aspect (statistics), the locational aspect (spatial analysis) or both (GIS).

data for GIS applications

data for GIS applications includes:
digitised and scanned data
databases
GPS field sampling of attributes
remote sensing and aerial photography

digital representation of geospatial data
The advantages of digital versus analogue data are outlined in the table below: