In simple terms a Geographical Information System (typically shortened to GIS) is a way of presenting information that has geographical references in a computerised system. The interface between the information and the user is a map displayed on a computer screen.
Information can be simple, such as the location of health posts or water sources. Various types of information can be combined to develop sophisticated maps displaying the relationship between the different sources. In this way the map may show the relationship of how reported incidences of diarrhoea relate to local water sources. If there is a high level of diarrhoea related to a water source, action can be targeted. In more sophisticated systems, parameters of soil type, gradient, pH, rainfall, temperature and so forth can be related to find areas best suited to different crops.
Raster and Cadastra Maps
The starting point for any GIS is a map. There are two types of map depending on the sophistication of the system required and the resources available. The more sophisticated type of map is a digitised representation of geographical features, called a Cadastra map. All features are represented on the map as points, lines or polygons. This type of map is used for detailed land registry, land ownership and taxation.
The other type is a Raster map, which is more common. In its basic form it can be an aerial photo, satellite image or a map scanned into the computer. Depending on the required level of detail the most relevant type of map can then be chosen. So, for example, if a basic Global Positioning System is used to locate the position of structures, the accuracy of the equipment is plus or minus 100 metres. Therefore, having extremely accurate maps has no advantage.
Information collection and database management are at the core of Information Systems and GIS is no exception. Information or data must be collected in a systematic and standardised format and entered in to the database. All the data is geographically referenced and this information can be referenced in different ways. For example, a water well would be a point on the map, a river would be a line and a soil type would be an area (polygon). Each type of data would be mapped out on a separate layer of the same geographical area or map1.
Layers can then be overlaid on each other to give details. If we are looking at the incidence of reported diarrhoea in relation to water sources, placing coloured pins on a map on the wall could do this. Where a cluster of pins is seen, an investigation of the water source and hygiene habits can be undertaken. However, when other factors are added the single layer on the wall is insufficient. With regard to water, other factors might be quantity, quality, risk assessment and economic factors, such as household income. When all these factors are related some surprising results may emerge, or existing assumptions may be confirmed.
The above looks at existing physical data, but when this is related to predicted data, GIS can be used in many different ways. For example, if the physical data for a water catchment and known water run-off rates are related to predicted rain fall, flooding can be predicted and flood alleviation measures taken.