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Working with Vector Data in QGIS

Vector data provides a way to represent real world features within the GIS environment. A feature is anything you can see on the landscape.

QGIS can open over 60 file-based data formats (using the OGR library), including ESRI shapefile and MapInfo TAB. It can also connect to spatial database sources including PostgreSQL, SpatiaLite, Oracle, and MSSQL), to Web Feature Service (WFS) and Point Cloud data, and to Arc REST services. QGIS’s default file format is GeoPackage, a file-based database. Most QGIS functionality is available for all vector data sources, though some formats are read-only mode and need to be saved to another format for editing.

GeoPackage

GeoPackage is an OGC standard data format. It has a .gpkg file extension, and it can hold one or many layers, including non-spatial and raster data, and can also store QGIS projects.

  1. All vector data is in ...\QGIS Training\Data\vector - right-click on the Favorites icon in the Browser panel to add this location for quick access

  2. Open the project from the previous exercise

  3. Using the Data Source Manager button, add file-based vector data by clicking the Vector tab on the left and browsing to the file

  4. Add the aonb_england.gpkg GeoPackage layer

ESRI Shapefile

Shapefile is a widely-used legacy format, developed by ESRI, used on many download sites. Shapefiles comprise four or more individual files on disk - all must be available for the file to open.

  1. Use the Browser to navigate to wind_turbines_point.shp in the vector folder (it’s in D:/PhotonUser/My Files/Home Folder/QGIS Training/Data)

  2. Double-click on the file to load it into the map, then right-click > Zoom to Layer(s) to show the layer on the map

  3. Right-click on the layer in the Browser to see the other options available

MapInfo

QGIS can open both MIF and TAB files - these will appear in the browser with other file-based data. Note that MapInfo files can contain multiple geometry types, so QGIS may present a dialog with a choice of geometry types - i.e. line, polygon, point - to load.

  1. Load the aonb_wales.mif layer

Excel

QGIS will open an Excel file as vector data, and will recognise data types (for example string, integer, date). If the data includes coordinates, creating a geometry column which QGIS can use to represent the data spatially requires an extra step.

  1. Add RTC Location 2017_0.xlsx , using the Vector tab in the Data Source Manager

  2. To turn the coordinates into points, use the Search box (in the lower left corner) to find the Create points layer from table process

  3. Complete the dialogue as follows and run the process - this will create a new spatial layer on the map

Database Layers

To loading database vector layers, you first need to establish a database connection. The process is similar for all databases, but in this example we will use data in a PostgreSQL database.

  1. Create a connection to the database using Data Source Manager > PostgreSQL > New

  2. Complete the connection details as shown in the screenshot below, using the Basic tab under Authentication - note the user credentials are pgis/pgis

  3. Click Test Connection to make sure you can connect to the database - if successful, you will see a confirmation message

  4. Click OK, then Connect to open the connection

  5. Close the Data Source Manager dialog, then using the browser pane, navigate to PostGIS > QGIS 3 Training > boundaryline > county_region and click Add, then Close

Web Feature Service (WFS) Layers

Data available in OGC Web Feature Service (WFS) format can be loaded in QGIS and used in a similar way to any other vector layer, as long as you have an Internet connection is required, although the data can be saved locally once you are connected. As with database layers, first of all you need to create a connection.

  1. Create a new WFS connection using Data Source Manager > WFS > New

  2. Open Chrome in AppStream, and go to https://data.gov.uk and search for conservation areas in Carlisle, then right click on Download to copy the link, and paste to the URL field in the dialogue below  - it should be http://maps.carlisle.gov.uk/getows.ashx?mapsource=CCC/inspire&TypeName=conservation_areas&version=1.1.0&request=GetFeature&service=WFS

  3. Click OK then Connect

  4. Click on the layer you want to load, then Add and Close

  5. The layer will be added to the map

Attribute Table

Attribute tables hold the data which sits behind every layer.

  1. Open the attribute tables for some of the layers, and see how the data in the attribute tables relates to the features on the map

  2. Select an feature in the attribute table by clicking on the row number  on the left - it will be highlighted ('selected') in the table, and will also be selected in yellow on the map

  3. When you have a feature selected, use the Zoom to Selection button in the attribute table toolbar to navigate to the feature on the map

  4. Click on a column header to sort the table using that column

Joining Layer to Non-Spatial data

If you want to use data which has no spatial references in the name, you will need to join this to a corresponding spatial dataset. In this example, have Index of Multiple Deprivation (IMD) data for census areas (LLSOAs), and a polygon layer of the LLSOAs themselves, and you want to join the two in order to show the IMD figures for each areas as a label.

  1. Open IMD2019_Index_of_Multiple_Deprivation.xlsx  selecting using the IMD2019 sheet

  2. Open the llsoa_dec_2011_england_wales_ne layer from the LLSOA_Dec_2011_England_Wales GeoPackage

  3. Open the attribute tables for both IMD2019  and llsoa_dec_2011_england_wales_ne and compare the data

  4. Notice that LSOA code (2011) in in the the spreadsheet layer uses the same data as code in the spatial layer

  5. Select  llsoa_dec_2011_england_wales_ne and right-click > Properties > Joins, and press the green + button to add a join

  6. Select the target spreadsheet layer, and the correct columns to match the two layers, then click OK

  7. Open the attribute table of llsoa_dec_2011_england_wales_ne to ensure the data has been joined correctly

  8. If you have time, add the IMD Rank as a label to each LLSOA, and apply a Graduated style to the joined data using the Rank attribute, or a Categorised style using the Decile attribute - if you’re not sure how to do this, it will be covered in the next module

  9. Save the project

The join you have just created is saved in memory, and is saved in the QGIS project - it does not affect the source data, but it be updated with the current data each time the project is loaded.

Exporting Vector Data

Any of the vector formats you have been using can be exported into most other formats, including CSV and GeoPackage.

  1. Click on the layer to be exported, then right-click > Export > Save Features as

  2. In the dialog, options include:

    1. Format: save the data in a different vector format

    2. File name: browse to the location where you want the file to be saved, and give it a name

    3. Layer name: for a GeoPackage, by default this will be the same as the file name, but you can change it (GeoPackages can contain many layers)

    4. CRS: reproject the data on save to a different CRS

    5. Save only selected features: save a subset of the features based on a selection you have already made

    6. Select fields to export: save a subset of fields from the attribute table

    7. Extent: save only features visible in the current extent

  3. Use the remaining time in the exercise to try a few of these option

Note that If you export a layer with a join, the exported data will include the fields in the join - this is a way of creating a permanent new dataset from a joined layer, although from this point on it won’t be updated if the source layers change.

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