GIS - Computerised Mapping
The simplest way to represent geographical data is as points on a map. Providing the data is referenced to a point-level geography, such as a postcode or a grid reference, it can be plotted on a map to see the geographical distribution.
The example above uses data on residential housing completions to plot polygons. It can be seen from the distribution of points that the majority of sites are located around Leicester and the main towns. However, we cannot determine the number of houses completed in each location, and as some points overlap others, the exact number of sites is difficult to determine. A proportional map can help overcome this.
Using a proportional map allows us to display more information on a map. The map above uses graduated circles to convey information on the number of housing completions in each location. The centre of each circle also represents the exact location of the site, so the geographical spread can also be understood at the same time as the underlying attribute data. However, even more so than the previous map, the size of the symbols and the amount of overlap means that some data is difficult to determine. Another way to display data thematically is to use shaded regions - known as a chloropleth map
In the map above, the data has been aggregated to Lower Super Output Area (LSOA) level. These are small areas created for the 2001 Census and contain approximately 1,500 people each.
Areas of higher concentrations of housing completions can be identified, but due to variation in the size of the LSOAs the larger, usually more rural areas (such as those in the eastern part of the county) dominate the map and demand the reader’s attention. This can mean that the smaller LSOAs are overlooked as they are not so noticeable on the map’ which is a problem if these are areas that have a higher concentration of the mapping attribute. Also, due to the aggregation, some positional accuracy is lost. The map below attempts to provide a solution to some of the issues of perception that are a problem when mapping to LSOA.
Another way of displaying the data is to use a cartogram which represents each level if geography (in this case LSOA) as a consistent shape (a hexagon). This means that data can be displayed without the distorting effect of varying sizes of geography in different parts of the county.
However, in order to create the cartogram, some positional accuracy is sacrificed in order to ensure that each hexagon fits together without gaps. In this case, it is best to display this above map alongside the normal LSOA map so viewers can see roughly where the corresponding hexagon is on the map and labelling of key areas of interest can also be useful. This technique is shown below:
More information on the Leicestershire cartogram can be found on LSR Online’s data visualization pages:
Page Last Updated: 28 September 2010