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Fri Oct 31 10:25:08 2014

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QGIS Planet

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.


Topology in QGIS

Introduction

Topology rules define the permissible relationships of features within a given GIS layer or between features in two different GIS layers. An example is that features in a road dataset must be connected to other roads at both ends, unless the road is specified as a dead end street.

Advantage of topology over queries

A lot of the checks that topology rules carry out could be achieved using spatial queries. You may have to use queries if the GIS software you’re using doesn’t have a topology feature.

Topology rules have the advantage that they only need be created once and then they can check your work as you go.

Queries would need to be re-created each time they are run. They can be saved, depending on the GIS being used, but this is still more time consuming and it is a task that must be carried out separately at the end of a work session.

Rules

QGIS 2.2 topology tool has the following rules pre-defined:-

  • End points must be covered by (e.g. a railway line usually begins and ends at a station)
  • Must contain (e.g. a building polygon must contain at least one address point seed)
  • Must not have dangles (a line must begin and end at another line)
  • Must not have duplicates (each feature should be unique, e.g. postcode areas)
  • Must not have gaps (e.g. administrative area polygons cannot have gaps)
  • Must not have invalid geometries
  • Must not have multi-part geometries (each feature should be a separate entry)
  • Must not overlap (e.g. administrative area polygons cannot overlap each other)
  • Must not overlap with (a feature from layer must not overlap with another layer)

Example 1 – Roads must not have dangles

The following example uses the “Must not have dangles” rule to identify polylines from a roads dataset that are not snapped to other lines. Roads usually begin and end at a junction with another road, so this is a useful rule to identify where lines were not correctly snapped together.

To create and validate a Topology Rule

  • Open the Topology Panel, by selecting Vector menu, Topology Checker, Topology Checker
  • The Topology Panel appears in the lower right corner of the QGIS desktop window

Image

  • Press the Configure button to open the Topology Rule Settings dialog
  • The top of the box will have 2 or 3 pull down boxes depending on the layer and rule that is chosen. Use these to build the rule and then press the Add Rule button.

Image

  • Press OK when done, the dialog box closes and the window returns to the QGIS Desktop.
  • Press either the Validate All or Validate extent, depending on whether you wish to validate the entire dataset or just the current view extent.
  • The errors will be listed. Double click on a row will make the map window zoom and pan to the error.

Image

 


Joining Layers in QGIS

Joining tables allows a layer with the attributes of both source layers to be created. Joins can be based on common attributes or common location (spatial join).

The joined fields are displayed as a new view rather than actually added to the layer. However the join is saved with the project and it will be reloaded each time the project is opened. To permanently add the joined fields to the layer, it must be saved as a new file. To do this, right click on the layer in the Layer Panel and select Save As.

This is because databases are generally designed to be more permanent data repositories than spreadsheets. Therefore their structure shouldn’t be changed for short term projects. Queries can be run and saved without affecting the underlying data.

Join on Common Attributes:-

This method is similar to ArcGIS’ Joins and Relates function. This is done by creating a Vector Join in the layer’s Properties box:-

  1. Right click on the layer in the Layer Panel
  2. Select Properties to open the Properties dialogue box
  3. Select the Join tab
  4. Press the + button to create a new Vector Join:-
Add Vector Join in QGIS

Add Vector Join in QGIS

The Add Vector Join dialogue box opens:-

  • Join Layer – the layer that the join will be to
  • Join Field – the layer in the Join Layer that contains the common attribute
  • Target Field – the field in the currently selected layer that contains the common attribute

Join By Location (Spatial Join)

This function creates a new shapefile containing the geometry of the target layer and the attributes of both layers. The join is based on the Joining layer overlapping or intersecting the target layer:-

  1. Use the menus to select Vector, Data Management Tools; Join Attributes by Location
  2. The Join Attributes by Location box opens, select:-
  • Select the Target vector Layer (the layer to update)
  • Select Join Vector Layer (the layer that will provide the new data)
  • Attribute Summary. This determines the action when a target feature is overlapped by multiple join features:-Take the attributes of the first located feature.
  • Take a summary of intersecting features
  • Output Shapefile – the file that will contain the new data.
Join By Location (Spatial Join) in QGIS

Join By Location (Spatial Join) in QGIS


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

Editing Vector Features in QGIS

Editing Features

A lot of of GIS work involves editing polygons and polylines as well as calculating the size of any overlapping features. For example when I provided GIS support to a local authority’s planning team, I was regularly asked to calculate what proportion of proposed developments overlapped constraints such as flood zones.

In this tutorial, I first look at the re-shaping features on the Advanced Digitising Toolbar. Then I will go through geo-processing tools such as buffer, union, intersect etc.

Advanced Digitising Tools

QGIS Advanced Digitising Toolbar

QGIS Advanced Digitising Toolbar


From left to right, the tools are:

  • Undo
  • Redo
  • Simplify feature
  • Add ring
  • Add part
  • Delete ring
  • Delete part
  • Reshape features
  • Merge selected features
  • Merge attributes of selected features

To use any of the tools,

  • Begin an edit session, by right click on the layer’s name in the Layer Panel and select toggle editing.
  • Select the feature that you wish to edit
  • Select the tool

This is the polygon at the start of the edit session, I will show a screen shot of each result:

Polygon in QGIS

Polygon in QGIS

Add Ring

is used to subtract a new internal region from the existing polygon. Select the tool and then draw the desired internal polygon:-

Add Ring in QGIS

Add Ring in QGIS

Delete Ring

deletes the selected internal region. Select the tool, and then click on the internal region’s nodes.

Add Part

adds a new external region to the existing region. Select the tool, then draw the new region, ensure it snaps to the existing region:-

Add Part in QGIS

Add Part in QGIS

Delete Part

deletes the selected region. Select the tool, and then click on the region’s nodes.

Reshape Features

draws a temporary region which is then subtracted from the existing region:

Reshape Features in QGIS

Reshape Features in QGIS

Reshape Features in QGIS

Reshape Features in QGIS

Split Features

draws a line through the region. The region is then split along the line

Split Features in QGIS

Split Features in QGIS

Merge Features – merges two or more selected features. Note Ctrl+click is used to select multiple features

Merge Features in QGIS

Merge Features in QGIS

A dialogue box allows you to choose which attributes to keep, discard. It is also possible to carry out mathematical operations such as sum, maximum on numeric fields and to concatenate text fields:-

Attributes in QGIS

Attributes in QGIS


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