Geo4a & Geo4b Selecting techniques to present data

1. Geo4a & Geo4bSelecting techniques to present data

2. Choosing map or graph techniques On a number of past papers candidates have been asked to choose a technique to display data – using either a map or a graph. Such questions have often been done poorly. Justifications have often been rather trite and have missed the main reasons for choosing particular methods of presentation.

3. Choosing maps With data that show information about places given /sq.km, etc. ... use a choropleth map... ...because data will show an average figure for the whole of each patch of the map under consideration.

4. Choosing maps With data that show information given as totals for each place... ...it is generally best to use located symbols (circles, bars, etc.)... ...because they will show the actual values in each area. A choropleth map would be unsuitable as it would seem to exaggerate the population of large areas by showing their high totals spread over the whole area.

5. Choosing maps So when drawing symbols which which type should be used? Located bars are the easiest to draw and probably the easiest to interpret. However, if there is a big range of values to be shown it might be more useful to use proportional circles.

6. Choosing maps When the data show information given as totals for each place... ...a dot map could be used, with each dot representing a certain number... ...in which case the dots should be spread evenly in each patch on the map (which is difficult)... ...unless there is evidence to allow them to be drawn at a greater density in some areas than others.

7. Choosing maps When the data is for points spread across an area, and not for patches... ...an isoline map should be used... ...as long as there is data for enough points and/or enough data to interpolate accurately in between the known points.

8. Choosing maps If the data is about the movement of people or goods into or around an area... ...flow line, trip line or desire line maps can be used. Flow line maps are used when the data gives the size of flows along a route or routes and how the flows increase or decrease along the route(s).

9. Choosing maps A desire line map is used to show how many people or goods approach a central point from a number of different directions. Lines of proportional thicknesses are used to represent the different numbers. A trip line map is used to show the starting points of a number of people or goods moving to a central place, and when the map is of sufficient scale to show a line for each trip.

10. Choosing graphs Why sometimes bar and sometimes line graph? Bar graphs are used to show totals (e.g. monthly rainfall totals). A line graph is used to show average, mid-point figures. e.g. a temperature graph shows the average temp, which is expected in the middle of each month – so if two values are joined with a line you can interpolate the value for any day in the month.

11. Choosing graphs • What are compound graphs? How are they read? • These are used when a number of different parts contribute to a total. They often show both parts and total changing over time. • They are often used to show the energy mix of a country changing over time. • They could be used in the following example...

12. Choosing graphs What are compound graphs? How are they read? A baker bakes: • 10 pies in the morning • 8 pies in the afternoon • 6 pies in the evening Show that on a simple graph.

13. Choosing graphs She also bakes some buns: • 10 in the morning • 10 in the afternoon • 10 in the evening Add those onto the baker’s production graph. Use a different shading.... ....and add a key.

14. Choosing graphs She also bakes some doughnuts: • 5 in the morning • 10 in the afternoon • 20 in the evening Add those to the graph.... ...and show your shading in the key.

15. Choosing graphs Now, using only the graph, work out her total production in each period. Then, using only the graph, work out her production of buns + doughnuts in each period. Describe any trends shown on the graph.

16. Choosing graphs So what is a comparative graph? Used when two (or more) independent sets of data can be shown on one graph. For instance, they are used to compare changes in the BR and the DR of a country over a period of 50 years... ...or to compare total production of wheat and of barley each year of a period.

17. Choosing graphs When are triangular graphs used? There must be three sets of data, for each of a number of different places, or different times. The three sets must add up to 100% in each case. The triangular graph shows grouping of the data.

18. Choosing graphs What about radial diagrams? These can only be used for sets of data that end where they began....or form a continuous, repeating series, e.g.: • the hours in a day • the months in a year • the points on a compass

19. Choosing graphs What about radial diagrams? When the data are for months or hours they are usually used to show average values (e.g. for temperature). When the data are for degrees of the compass they can show totals (e.g. wind rose, till orientation).

20. Choosing graphs The special case of kite diagrams Kite diagrams are very useful for displaying data collected on vegetation transects. In our spec they are most often used to present and analyse the results of sand dune transects. They show how different species grow in different parts of the dune ecosystem, changing with distance inland. They allow the viewer to see how different communities occupy different micro-environments, influenced by slope, drainage, micro-climate, soil, etc.

21. Choosing graphs What is the point of logarithmic graphs? These are hard – hard to draw and hard to understand! However: • they allow a very large range of data to be shown on one graph (Each cycle of the graph represents a ten-fold increase in values.) • they are good for showing the rate of change rather than absolute change (A steeper line indicates a faster rate of change.)

22. Choosing graphs Why use a scatter graph rather than a Spearman correlation? They are both use to research the relationship between two sets of data, where each set relates to the same group of places.

23. Choosing graphs Why use a scatter graph rather than a Spearman correlation? A scatter graph: • shows the direction of any relationship (positive or negative) • gives a visual impression of the strength of the relationship • allows anomalous results to be seen

24. Choosing graphs Why use a scatter graph rather than a Spearman correlation? A Spearman correlation test shows: • the direction • the strength • and the statistical reliability of the relationship when significance tables are used

25. Choosing graphs Why use a scatter graph rather than a Spearman correlation? So, in general, a scatter graph is used to give a quick analysis of the results. If there seems to be a relationship then it is worth following up with the S test. But, of course, the scatter graph can provide a very useful visual illustration, whereas the S test is less easily understood by non-specialists.

26. Choosing graphs When do you use dispersion diagrams? Dispersion graphs display the distribution of points around the median. They allow you to see: • the total range of the data • how spread out the data are, or how bunched • the position of each piece of information within the spread

27. Choosing graphs When do you use dispersion diagrams? They are very useful for comparing two sets of data. For instance two samples of bed material from different parts of the same river had the same mean size... ...but when they were plotted on dispersion diagrams it became obvious that one sample was clustered around the median, but the other had very dispersed values... ...because it was very close to a glacial moraine, which was providing much of the load at that point.

28. Choosing graphs When do you use dispersion diagrams? Note that a dispersion diagram can then be used to calculate: • the range • the upper and lower quartiles • the inter-quartile range all of which are useful measures of central tendency.