The foundations of geographical analysis
How mapping was first used to display and analyse data
We use our Geographic Information System (GIS) products to analyse and present data in a geographical context. Displaying data on maps makes it easier to understand, as well as giving a new perspective on a problem. Using a GIS to prepare and present data has become increasingly popular over the last 20 years, and improvements in computer processing power and internet speeds make it easier than ever to put data in a spatial context. But graphical displays of data on maps were around long before computers. Geographers, physicians, philanthropists and historians have been using maps to analyse and present data for 100’s of years.
Mapping population density
Population maps for France in 1830 were produced by a Franciscan Friar, Armand Joseph Frère de Montizon, he produced a dot distribution map where each dot represented 10,000 people. The dots were arranged on a grid where the distance between each dot decreased as total population increased, so the dots for a particular département (the administrative areas of France) all fit within that boundary. The map at a glance shows total population as well as population density.
Saving lives with location analysis
In 1854 Soho in London was ravaged by cholera. A York born physician, John Snow, was investigating the causes and spread of these outbreaks. Snow recorded the location of each cholera case on a map and interviewed the households concerned. He recognised that the majority of cases were clustered around a local water pump. Snow used geographical and statistical analysis to link the cholera incidents to the water supply. Further investigation showed that the water from the infected pump was being contaminated by an old cesspit.
On proceeding to the spot, I found that nearly all the deaths had taken place within a short distance of the [Broad Street] pump. There were only ten deaths in houses situated decidedly nearer to another street-pump. In five of these cases the families of the deceased persons informed me that they always sent to the pump in Broad Street, as they preferred the water to that of the pumps which were nearer. In three other cases, the deceased were children who went to school near the pump in Broad Street…
With regard to the deaths occurring in the locality belonging to the pump, there were 61 instances in which I was informed that the deceased persons used to drink the pump water from Broad Street, either constantly or occasionally…
The result of the inquiry, then, is, that there has been no particular outbreak or prevalence of cholera in this part of London except among the persons who were in the habit of drinking the water of the above-mentioned pump well.
I had an interview with the Board of Guardians of St James's parish, on the evening of the 7th inst [7 September], and represented the above circumstances to them. In consequence of what I said, the handle of the pump was removed on the following day.
John Snow, letter to the editor of the Medical Times and Gazette
Mapping social trends
Baron Pierre Charles François Dupin was a mathematician, engineer, economist and politician. His work on thematic and statistical mapping in 1826 used colour to display data over a map. The first "cartes teintées" (coloured map in French) showed the distribution of illiteracy in France.
The term "choropleth map" was introduced in 1938 by the geographer John Kirtland Wright. These maps colour regions and areas within geographic boundaries according to the underlying data. The term is often used interchangeably with “heat map” which also uses colour to display information. The main difference is that a “choropleth map” works to geographical boundaries but heat maps can be more abstract.
Adding an extra dimension with contour lines
Maps that show lines linking things of equal value have been produced for centuries. In 1584 a map showing river depths was produced by Dutchman Pieter Bruinsz, this map was the earliest known isobath (same depth). Depth measuring charts with contour intervals of 1 or 10 fathoms were produced by the great seafaring nations of the enlightenment. Beginning in the middle of the 18th century, maps with lines showing the surface of the land were produced across Europe. Most commonly used to describe topographic and hydrographic features, maps showing lines of equal value are used for pressure (isobar), temperature (isotherm) and time (isochrone). Using a contour line displays 3 dimensions of data over 2 dimensions.
Using maps to display data in a geographical context has long been a useful tool. The earliest examples were hand drawn on maps. With the advent of GIS you can take data that has a geographical component and display it in an easily understandable form.
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