Central and Western Europe are freezing these days. Such widespread frost and snow at this time of the year is quite unusual and creates the appearently inevitable chaos on road, rail and in the air. Since even Great Britain is hit hard by this early winter the guardian’s Datablog came up with a list of temperatures measured throughout Britain on December 20th. The temperatures are quite impressing – -18.7°C in England is quite something.
The plot of the measuring stations on google maps on the article’s side to me is nothing but playing around. What we want here is a decent map that visualizes the distribution of the temperature. One comment below the article linked to an application called bime. The “Heat Map” seems to me like a visualisation of the density of the measuring stations. At least one can use a slider on the right side to limit the range of temperatures displayed, but a comprehensive overview is not given. When switching the rendering mode to graduated circles the visualisation does not make sense at all.
Another google mashup that enables querying of the data is also linked: http://homepage.ntlworld.com/keir.clarke/web/cold.htm. I am not really satisfied with that either. The shown visualisations don’t represent the theme temperature as the continous phenomenon that it is.
The authors encourage their readers to download the data as spreadsheet and let them know what they did with it. So here it comes:
My suggested solution is the following:
- download the data as spreadsheet
- import it in Quantum GIS by using “Add Delimited Text Layer”
- add a shapefile with outlines of Britain; for some reason my outlines do not exactly match with the plotted stations, since some of them are clearly placed in the ocean although they probably represent coastal towns. Since this is a rather quick hack I did not fix that.
- use the plugin function “contours” to interpolate the temperature values; take the Layer with the stations as input, select “min temp air temp overnight” as Data Field; select filled contours; I used 8 classes to classify the range of temperatures occuring
- now one should give meaningful colours to the contours; in the newly created layers with the temperature polygons select properties and go to symbology; select graduated symbol. The result should be a colour range of cold colors of different shades of blue, however at the freezing point mark there should be a transition to another colour, prefereably green to visually differentiate between freezing and non-freezing areas.
- I decided to label some of the measuring stations with their corresponding temperature. In the properties of the layer I selected label and ticked “Label only selected features”. In the main view I then selected some stations with some distinct values and made sure there is no large area without a label.
Here is the result – a rather quick hack. It is not a nice finished map yet but shows in which direction it should go. I feel that the values sometimes were not matched to the classes perfectly. The plus 1.2° in Cornwall for instance should be in the green class according to my classification. So there is potential to play with it further.