Page 1 of 1 (3 posts)

  • talks about »
  • data analysis

Tags

Last update:
Fri Jul 11 04:35:08 2014

A Django site.

QGIS Planet

QGIS – Mapping Election Results, pt 2: Adding and overlaying the data in QGIS

Continuing on from the previous tutorial:-

Return to QGIS. Add the westminster_const_region.shp file if necessary

  1. Press the Add Delimitated Text file button, and select the .csv export of the cleansed electoral data
  2. The two options I changed from the default settings are:-
  • First record contains field names
  • No geometry (attribute only table)
QGIS - Create layer from text file

QGIS – Create layer from text file

Step 3 – Joining the data

Joining the polygons in westminster_const_region.shp to the data imported from the Results_Cleansed spreadsheet will allow the data to be presented in a spatial and visual format which will be much easier to interpret, allow for spatial analysis and also give the viewer an idea of the geographic spread. Using QGIS’ Join function will hopefully save a lot of copying and pasting!

Right click on westminster_const_region.shp and select Properties to open the Properties dialog

  • Select the Joins button from the left panel
  • Join Layer – the layer that you want to join to
  • Join Field – the field that you want to join to
  • Target Field – the field in this layer that contains the matching data
QGIS - Add vector layer

QGIS – Add vector layer

The join will now appear in the layer’s Joins list:-

QGIS layer properties

QGIS layer properties

The attribute table will now show the combined  data for both layers:-

QGIS attribute table

QGIS attribute table

This data can now be used to create a thematic map that colours each constituency according to party that won the seat in 2010.

I won’t go through all the steps of creating a thematic map as an earlier tutorial does this.

I’ve used the same colours that the different parties in the UK use:-

QGIS Layer properties

QGIS Layer properties

The thematic map shows the results across the entire UK. It is easy to identify patterns in the result, for example

  • The Liberal Democrats mostly won seats in Scotland, the North East, Wales and South West.
  • There is strong Labour support in South West Scotland, North West England, West Midlands, South Wales, London, Liverpool and Manchester.
  • The Conservative support covers much of the rest of England, especially South East England, excluding London.
2010 election results map

2010 election results map


Combining skills – Mapping Election Results

I would like to show you how to use QGIS to combine different skills to

  • Import data from Excel or other spreadsheets
  • Analyse the data
  • Present the results as a thematic map
  • Use a feature subset to hide superfluous data

There is a year to go to the next general election. Under the British electoral system, the country is divided into 660 constituencies. The MP for each constituency is elected using the First Past the Post system, where the candidate with the most votes is chosen as MP for that constituency. The party that has the most MPs elected wins the election, and the right to form the nest government.

Parties concentrate their resources on the constituencies that they are most likely to win or lose. These are usually the ones where majority in the previous election was closest.

This project will use QGIS to join fields between OS Boundary line data and the 2010 election results to identify and map these constituencies.

I downloaded the following data sets:-

OS Boundaryline:  https://www.ordnancesurvey.co.uk/opendatadownload/products.html

Election results: http://www.electoralcommission.org.uk/__data/assets/excel_doc/0020/105725/GE2010-constituency-results-website.xls

Step 1 – Examine and prepare the data

I need both data sets to use exactly the same name and formatting in order to In order to add the 2010 election results to the OS Boundary line polygons.

  1. Start QGIS and set the map projection to OSGB.
  2. Use the Add Vector Layer button to add the file westminster_const_region.shp from the OS Boundary line data
  3. Open the attribute table to check the data structure and contents:-
Image

Constituency attribute table

Now to check the GE2010-constituency-results-website.xls

 

Image

Election results Excel screenshot

Unfortunately the two datasets don’t use exactly the same constituency names! It is fairly easy, but time consuming to match the record from the Election Results dataset to the OS Boundary Line record using Excel or Libre Office.

To export the OS Boundary line polygon names to Excel:-

  1. Right click on the westminster_const_region.shp in the Layers Panel
  2. Select Save As

I prefer to use the .dbf format when exchanging data between GIS and Excel as it is quicker to import than using .csv format.

 

Image

Both name fields are needed. The first, constituency _name will be used to link to the constituency polygons once the table is imported into QGIS. The second, Results_Table_Name is used by the VLOOKUP query that adds the five columns from the results data.

Image

Excel screenshot showing the cleansed election results matched to constituency names

Save the data as a .csv file when this stage is complete.


Updating Attribute Data

Browser Window:

QGIS Browser Window

QGIS Browser Window

Buttons on the bottom from left to right are:-

  • Unselect All
  • Move Selection to Top
  • Invert Selection
  • Copy Selected Rows to Clipboard
  • Zoom Map to the Selected Rows
  • Pan Map to the Selected Rows
  • Toggle Editing
  • Save Edits
  • Delete Selected Features (rows)
  • New Column
  • Delete Column
  • Open Field Calculator
  • Look For (enter search terms) in (use the pull down to select the column to search)

The attribute table has several duplicate rows. To delete them

  1. Press the Toggle Editing to begin an Edit Session
  2. Press the Delete Columns button
  3. Select the column(s) and press OK

To add new columns:

  1. Press the New Column button
  2. Enter the details for the new column
Add column dialog

Add column dialog

Update Columns with the Field Calculator

I want to update the three new columns I’ve added. To update columns:

  1. Right click on the layer in the Layer Panel
  2. Select View Attribute Table
  3. Press the Toggle Editing button to begin an Edit Session
  4. Press the Field Calculator
QGIS Field Calculator

QGIS Field Calculator

  1. Select to either create a new field or to update an existing field. If you’re updating an existing field then use the pull down to select the field
  2. Complete the Expression builder by either double clicking functions or typing the expression.
  3. Functions include:
  • Operators e.g.+-*/ etc
  • Maths e.g. cos, sin etc
  • Conversions e.g. to integer, to string, to decimal
  • Geometry e.g. feature’s length, area, perimeter, x, y
  • Record – Row number or ID
  • Fields and Values

  • Page 1 of 1 ( 3 posts )
  • data analysis

Back to Top

Sponsors