Thursday, 26 January 2012

Data on Maps

Dear All

As some revision for Unit 4B, here are a selection of different ways to represent data on maps! You should be able to comment on how to present data, and the benefits and disadvantages of using each method. Remember that you could be asked to demonstrate your chosen method in the exam, and that you have already been given the full data range for the different SOA's. Hence, it would be a very good idea to look at the groups of data (Population Density, Employment, Housing) and to assess how you would represent each on the map you have been given.

1. Chloropleths

Description:This is a type of map where different regions are coloured or shaded according to their value in a given data set. The range of categories should not be too high (preferably no more than 5/6) and the colour choice needs to be appropriate, often it is best to use a graduated colour change option, rather than several completely different colours, which make it hard to interpret

Method: Assess the range of data for your given statistic, common examples would be population density. Divide this range into 5 or 6 categories, and assign a colour to each. Locate the correct area on the map and colour in according the value for that region. Ensure that you include a scale, title and a key for the colours.

Benefits: This is a very clear way of showing geographical variations in values, looking at a chloropleth allows you to identify patterns easily, and see if there is a geographical factor that is affecting the value, in the example shown, there is a significantly higher percentage of people employed in higher managerial professions living in the coastal areas of poole. This is also a relatively rapid way to transfer data onto a map, that does not require further calculations, specialist kit or programmes, or significant knowledge of statistical tecnhiques.

Disadvantages: It only effectively represents one piece of data, it cannot be used to display more complex data that covers a range of variable such as different emplyment groups, or types of housing.

Best used for: A value with only one variable, such as population density, where you think there could be a geographical pattern.

2. Locational Bar Charts

Description: Bar charts drawn for several categories of information (in this case three) that are drawn directly onto the area in which the measurements were taken. The one below is for percentage of people living in Owner Occupied, Private rented or council rented accommodation in each area. The bar charts need a scale and a key.

Method: Convert all values to percentages, decide on an appropriate scale for the bar charts, the scale needs to show the variation within the categories, but not be so large as to obscure the map. Locate a suitable point within each region to draw the bar chart, and try to ensure that the entire bar chart can be located within the region.

Benefits: Allows for multiple pieces of data to be clearly represented on the map, the result is clear and easy to read, highly visual and not too complicated. The graph does not require any special equipment, and the only maths involved is to convert to percentages, which is essential for comparison.

Disadvantages: Can only be used for a few pieces of data, too many and it becomes hard to read. Deciding on a scale can be difficult if there is a considerable amount of variation across the regions, as some bars will be very small. It can be quite time consuming, as each region will need a different graph.

Best used for: Data with several variables, but can be used for any type of data that is converted to a percentage. In the case of this AIB, the percentage of people in each type of housing, or the percentage in each type of employment would be suitable.

3. Stacked Proportional Bar Charts

Description: A percentage representation of a set of data located on a map with colours assigned to each category. The bar chart scale needs to be chosen carefully, as it needs to not obscure too much of the region, and large enough for the percentages in each category to be seen. 

Method: Convert all values to percentages of the total, decide on a scale that will allow clear data representation, and not too large for the map. Draw on the bar chart, shade in the areas, making sure you measure from the top of the last upwards, not the base of the bar chart. 

Benefits: Clear, allows multiple data values to be displayed, shows a clear pattern if one is there, only requires a conversion to percentages. 

Disadvantages: Time consuming, can overshadow the regions and is often difficult to locate all the bar charts on the map, some often have to be drawn off to one side which makes interpretation more difficult. 

Best used for: Data sets with a large range of values that need to be displayed on the map. 

5. Locational Pie Charts

Description: Pie charts drawn on the graph in or around the region and coloured according to category. 

Method: Find a suitable scale for the pie charts, draw the circles onto an appropriate area of the graph, trying to contain the chart within the region, and then complete each chart in turn with the appropriate values. 

Benefits: Very visual, allows multiple categories of data to be represented. 

Disadvantages: The pie charts can end up being quite small, hence the categories can be difficult to read, and the pie charts will obscure the region they are meant to represent. It is a very time consuming graph to draw, as Pie charts do require protractors and compasses. 

Best used for: Larger maps, with a large amount of data needs to be represented, suitable for any type of data, whether in percentage or not. 

6. Proportional Area Representations (Ok, I made that one up)

Description: The areas are shaded to a proportion of the area they represent, this is a more imaginative way to represent data, and could be valid as long as you justify why you have chosen it and explain your method clearly, but it is not part of the syllabus.

Method: Convert values to percentages, find the central point of each region, and draw a pie chart only extending out to the edges of the region. Colour according to value.

Benefits: Very visual. marks for creativity.

Disadvantages: Quite time consuming, and you have to work out a way to find the central point, then the area of the region could distort the true representation of the value, as the shape of the region changes through 360 degrees. 

Best used for: Possibly best not used in the exam, unless you are asked to come up with an imaginative new way to represent data. 

The key to doing well on map questions, is that you need to pick the right method to display the data they ask for. Some methods are inappropriate for multiple values, some are too time consuming to use in an exam, and some are too confusing to read, make sure you think about it, spend this weekend planning how you would plot all the data you have, AND any you could gather from fieldwork (I am thinking of an isoline map of traffic congestion before and after the bridge opens - but I may have spent too much time reading about Poole!). 

Also, bear in mind that the exam is next week, and the long delayed christmas movie will bring up sharply back into the study of plate tectonics and hazard management. Please feel free to bring popcorn to your first triple after the exam. 


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