My favourite software: Geographic Information Systems

one of us (P.B.) first heard about Geographic Information Systems (GIS) they were described as 'geography using computers'! Six years later, this is still the simplest description of GIS. But of course the reality is a little more complicated. A GIS is a combination of elements designed to store, retrieve, manipulate and display spatial data – information concerning location. The package has four main parts: hardware, software, data and a user. A GIS is similar to an overhead projector, with a series of transparencies laid upon it. Each transparency shows different data – one shows roads, the next hospital locations, the third patient residence, etc. Obviously, to do all this you need spatial data. Digital data are now widely available – the problem is not so much finding data as obtaining the appropriate data. Also, many of the data-sets are very expensive and this is a major hindrance to any GIS. The spatial element of data can come in one of two forms – raster or vector. Raster data are the equivalent of a continuous grid covering the surface whereby each cell in the grid represents a square on the ground. Examples include satellite images, digital terrain models (DTM) to represent altitude, and scanned maps, which can be used as backdrops to add contextual information. The vector approach attempts to represent objects as exactly and as precisely as possible by storing points, lines and areas (polygons) in a continuous coordinate space. What can a GIS do for public health? At its simplest, data can be displayed on a map. For example, mortality rates in electoral wards, or point locations of cases can be mapped, using postcodes or addresses to locate the points on a map. GIS can be used to display road networks and calculate distances or travel times between points. The overlay method (the overhead projector analogy) is very useful for linking datasets with different spatial frameworks; for example, water supply zones, ward-level Townsend deprivation scores, point locations of deaths from myocardial infarction, census population counts – all from different data sources. GIS can also be used to count the number of people living in 'buffers' (e.g. concentric circles around point sources of pollution or specified distances from main roads). This is an ideal way to calculate the rates of illness in these buffers; for example, general practitioner (GP) consultation rates for respiratory disease amongst people living within 100 …