Introduction to Unit 4: Geovisualisation of Health Data
This is the fourth unit in the module ‘GIS for the Analysis of Health’. This unit describes cartographic techniques that can be used to produce maps from health data. Maps of health data provide a means of exploring spatial patterns in disease, assessing data quality and are a means of communicating information about disease risks to policy-makers, health professionals, and the general public. Whilst maps are a useful way of identifying spatial patterns in health data, visual interpretation of disease maps can be misleading. Any apparent spatial variation in disease risk identified through a map should be investigated further using statistical analysis. The unit assumes that you already have a basic knowledge of cartographic principles and focuses on how these may be used in health applications of GIS.
There are 7 subsections in this unit:
- Point mapping
- Choropleth mapping
- Small number problems and smoothing
- Cartograms
- Dynamic mapping
- Exploratory spatial data analysis
- Assignment for Unit 4
This introductory subsection sets out the structure for the rest of the unit. ‘Point mapping’ introduces the point (or dot) map as a means of visualising health data in the form of points. ‘Choropleth mapping’ is perhaps the most widely used means of visualising spatial data. It involves classifying a set of polygons according to some attribute and then shading the polygons according to attribute class. ‘Small numbers problems and smoothing’ investigates difficulties arising from mapping disease rates in areas with small populations – the so-called ‘small numbers’ problem. This subsection also describes one possible solution to this problem, which involves smoothing disease rates by calculating a local average using data from the area surrounding a given geographic unit. The next subsection introduces the use of cartograms in health mapping. Cartograms are a means of displaying disease rates for geographic units which can adjust for spatial variation in the units’ attributes, such as the population at risk of a disease. ‘Dynamic mapping’ investigates the technology by which map presentations can be changed either as a result of user interaction or through changes in the spatial database on which they are based. The sixth subsection introduces the concept of Exploratory Spatial Data Analysis (ESDA), which uses graphical, cartographic and statistical methods to better understand the characteristics of a health data set.
The seventh subsection is the assignment for this unit, which involves critically reviewing the cartographic output you produced for the ‘point mapping’ and ‘choropleth mapping’ subsections and suggesting possible methods for improvement.
Expect to spend about 1 week working through these materials. These activities can be carried out in any order but the assignment should be left as the final activity, since it builds on work in earlier activities.
Activity
In preparation for studying this unit, read Chapter 4 of the course textbook Cromley and McLafferty. Whilst reading the chapter, think of an example of a particular disease and for your own record, make some notes about the possible advantages to be gained from producing maps of the disease’s distribution.
References
If you are particularly interested in cartography for health applications of GIS, you may also wish to take a look at Tom Koch’s book ‘Cartographies of Disease: Maps, Mapping and Medicine’ from ESRI Press, Redlands California. This book describes the history of mapping and medicine and only focuses on GIS in the last few chapters. It does give a good description of John Snow and his work on cholera in 19th century London.
Note that reading this book is optional and for those very interested in this subject only – you are not required to read it.