Data Attributes

Attributes turn map features into useful information. A basic map shows where things are; attributes tell you what they are. For example, a building footprint can store its height (a number), its construction date (a date), and its current use (text). Each attribute has a data type that determines what values it can hold.

In QGIS, every point, line, or polygon stores attributes in its Attribute Table. You can filter features by their characteristics, run targeted analysis, or style maps based on attribute values.

This section covers viewing, filtering, and editing attributes to answer real-world questions about spatial data.

Note

For these steps, you’ll need to download the madrid_premises.gpkg and street_network.gpkg datasets linked on the datasets page. Add these to your QGIS project.

Attribute Table

Right-click any layer in the Layers Panel and select “Open Attribute Table” (or click the icon in the toolbar). This opens a spreadsheet-like interface showing all data associated with your spatial features. Each row represents one feature (a street, a premises location), and each column represents a different attribute.

This works like a spreadsheet, except every row links to a geographic feature on your map. When you select a row in the Attribute Table, QGIS highlights the corresponding feature on the map, and vice versa.

Let’s open an Attribute Table for the streets data and look at its key components:

  • The menu bar at the top provides tools for selecting, filtering, and editing your data
  • The field names (column headers) show what type of information is stored
  • The row numbers on the left correspond to unique features
  • The cell contents store the actual values for each attribute

QGIS Attribute Table

The Attribute Table in QGIS offers several useful tools for organising and navigating your data.

  • You can sort rows by clicking any column header - click once to sort in ascending order, and again to sort in descending order. This is particularly useful when you want to quickly find the highest or lowest values in a numeric field, or organise names alphabetically.
  • To select features, click on a row in the table (or select multiple if you wish). The selected features will highlight both in the table and on your map.
  • If you want to see where your selected features are located, click the Zoom map to the selected rows button (the magnifying glass icon) to centre the map on those features.
  • You can also do the reverse - selecting features on the map will highlight the corresponding rows in your Attribute Table. If it doesn’t work as intended, check that you are using the Select Features rather than Identify Features button, since these do different things!
  • Click the Move selection to top button to display selected features at the top of the panel.
  • To clear your selection, click Deselect all features.

The Attribute Table also includes handy filtering options in the bottom left - you can choose to show “All Features,” “Selected Features,” or “Filtered Features,” making it easier to focus on just the data you’re interested in.

Tip

Docking the attribute table is a useful feature when working with limited screen space, such as on a laptop. You can set this as the default behaviour by navigating to Settings → Options....

Under Data Sources, tick Open attribute table as docked window

You can also toggle this setting using the right-most button at the top of the attribute table.

ImportantExpression Syntax

Throughout this section, you’ll write expressions to query and calculate data. The syntax matters:

  • Double quotes reference column names: "street_name", "population"
  • Single quotes denote text values: 'residential', 'Madrid'
  • No quotes for numbers and built-in functions: 100, mean(), $length
  • $ prefix indicates built-in geometry properties: $length, $area, $perimeter

Example: "division_desc" = 'food_bev' finds rows where the division_desc column equals the text value food_bev.

Getting this wrong is a common source of errors. "food_bev" looks for a column named food_bev. 'food_bev' is a literal text value.

Modifying data

The attribute table in QGIS allows you to modify existing data and add new columns to store additional information.

  • Toggle Editing Mode by clicking the Toggle Editing Mode button (pencil icon).

In this mode, you can edit the contents directly, for example, you can edit the street names if you wish. In this case, let’s instead add a new field called length and we’ll assign the street lengths to this new attribute:

  • Click on the New Field button in the toolbar
  • Set the Name as length and the type as Decimal number, then OK.
  • Check that the “Only update selected” option is not selected.
  • Open the Field Calculator by clicking its icon (abacus symbol) in the toolbar.
  • Select Update existing field and select the length column.
  • In the Expression pane, enter $length, then press the blue forward button below to preview the results. This expression extracts the length of each LineString geometry.
  • Press OK and check that your length column has been updated. The above expression will have been applied to each row individually, saving the measured length of the LineString geometries into the new length column.
  • Press the Save edits button to save your calculation.
  • Turn off editing mode.
Tip

The field calculator is a window that you will keep coming back to over and over again. Take some time to understand the following parts:

  • Only update X selected feature(s)
  • Create a new field / Update existing field
  • Expression box
  • Function list / helper
  • Preview

Remember to navigate this window from left to right, top to bottom. Certain fields will only be enabled according to previous choices.

Field Calculator
NoteReading

Selecting and Filtering

Expressions let you query your data using attribute values. Instead of manually scrolling through thousands of rows, you can select exactly what you need. Let’s filter the premises data by landuse types.

  • Open the premises data Attribute Table.
  • Explore the landuse types, pay attention to the division_desc and epigraph_desc columns. For this case, we’ll select all features with a division_desc value of food_bev.
  • Open the Select features using expression panel (yellow icon with Epsilon symbol).
  • Type in "division_desc" = 'food_bev' – here we are referencing the column name and we’re filtering all rows where column values equal the string value of 'food_bev'. Notice the use of double quotes for column names and single quotes to denote a string.
  • Click Select Features – you should now see all relevant rows highlighted.
  • Export the premises to a new GeoPackage called prems_food_bev.gpkg, making sure that Save only selected features is selected.

You now have a separate layer containing only food and beverage locations. Remove the original madrid_premises layer from the map.

NoteReading

Manipulating Columns

There is a sneaky issue in the file, which we’ll fix: The epigraph_desc column name actually contains a trailing space after the visible text characters. This will cause confusion because it will trigger errors when used in expressions by someone possibly unaware of the space in the column name: QGIS expressions won’t be able to find the column if typed without the trailing space.

Let’s create a new column name without spaces. We’ll create a copy of the original data and we’ll then delete the original column:

  • Open the Attribute Table and enter edit mode.
  • Click the Open field calculator button.
  • Select Create a new field
  • Use epigraph_desc_fix for the name and Text (string) for the type.
  • Use "epigraph_desc " for the expression. Notice the double quotes (denoting a column name) and the intentional trailing space. Without that space, QGIS would look for a column called epigraph_desc which doesn’t exist.
  • This expression tells QGIS to simply reuse the values from the existing "epigraph_desc " column without further modification.
  • Click OK.

You’ll now see a new column called epigraph_desc_fix added to your Attribute Table, with values matching the original. We can now delete the original column:

  • Click the Delete field button.
  • Select the column with the trailing space (it will appear as epigraph_desc followed by a space) and click OK.
  • Toggle editing mode and save.

Filtering Map Views

Expressions are also useful for filtering the visibility of features in the map view. For example, using the new prems_food_bev layer:

  • Right click the layer in the Layer Panel and select Filter...
  • Type an expression using the columns and available functions, for example: "epigraph_desc_fix" = 'BAR RESTAURANTE'
  • Click to Test or Clear your expressions.

If all you want is to work with a subset of the data without creating a separate file you can use the Filter... option. However, if editing, remember that the entire file is still stored locally and you are only seeing/editing a subset.

Lab Work

Challenge: Deleting Features

Select all features where epigraph_desc_fix has a value of BAR CON COCINA and delete them. Note that there is a Delete selected features button in the toolbar.

Warning

Deleting features is permanent once you save. If you make a mistake, toggle off editing mode without saving to discard changes.

  • Open the Attribute Table and enter edit mode.
  • Select by expression using "epigraph_desc_fix" = 'BAR CON COCINA', and close the panel
  • Click Delete selected features (trash can icon).
  • Toggle editing mode and save.

Challenge: Select and Export

From the prems_food_bev layer, select all CAFETERIA features and save to a separate new GeoPackage called prems_cafeteria.

  • Open the Attribute Table.
  • Select by expression using "epigraph_desc_fix" = 'CAFETERIA', and close the panel.
  • Export the premises to a new GeoPackage called prems_cafeteria.gpkg, making sure that Save only selected features is selected.

Challenge: Categorising Street Lengths

Switch to the street_network layer. Add a street_cat column to classify streets by length. Set the value to long where the street is greater than average length for Madrid, and short otherwise.

Tip
  • Your expression will need to use a greater-than > symbol and a built-in mean() function (“mean” is the statistical term for “average”).
  • Note the Toggle multi edit mode button which enables edits of all selected features at once.
  • Note the Invert selection button.
  • Open the Attribute Table and toggle edit mode.
  • Click on the New Field button in the toolbar
  • Set the Name as street_cat and the type as Text (string), then OK.
  • Select by expression using $length > mean($length), and close the panel.
  • Click the Toggle multi edit mode button, type long in the street_cat field and apply changes. This will update the street_cat column for all currently selected features.
  • Click the Invert selection button, type short in the street_cat field and apply changes.
  • Toggle multi edit and edit modes and save changes.

Alternatively, there are more advanced expression options involving if-else logic. In this case, the Field Calculator can be used to update the fields directly:

  • Open the Field Calculator by clicking its icon (abacus symbol) in the toolbar.
  • Select Create a new field and set the Name as street_cat and the type as Text (string). This assumes you’ve not already created the column.
  • Use the expression if ($length > mean($length), 'long', 'short'). This will set the value to long if greater than average length, else short.

Note that you can search for functions such as if() or mean() in the searchbox (in the Field Calculator) and that clicking on the relevant result shows documentation explaining how the function can be used, along with some examples.

Challenge: Create (and Save) a Style

Using the newly created street_cat column, visualize the streets with a categorical color set. Save this style under the name Above/below avg length.

  • Open the Layer Styling panel (View → Panels → Layer Styling)
  • Change the style type from Single Symbol to Categorized
  • Set the Value dropdown to street_cat
  • Click Classify to generate categories for long and short
  • Adjust colours as desired
  • Right-click the layer → Styles → Rename Current
  • Enter Above/below avg length

Bonus Advanced Challenge

For more than two categories, you’ll need a CASE expression. This works like a series of if-then checks:

CASE
    WHEN condition1 THEN 'result1'
    WHEN condition2 THEN 'result2'
    ELSE 'default'
END

QGIS evaluates conditions in order and uses the first match.

Add a new street length category called street_5cats and prepare an associated style based on the following 5 categories:

  • shortest (less than 50% of the average length)
  • short
  • about average (within +- 5% of the average length)
  • long
  • longest (greater than 150% of the average length)

If you already know some statistics, you can adapt the above categorisation to use quartiles (q1, median, q3) or standard deviations (stdev) instead of percentage thresholds.

  • Open the Field calculator
  • Create a new field street_5cats of type Text (string)
  • Use the expression:
CASE
    WHEN $length < 0.5 * mean($length) THEN 'shortest'
    WHEN $length < 0.95 * mean($length) THEN 'short'
    WHEN $length <= 1.05 * mean($length) THEN 'about average'
    WHEN $length <= 1.5 * mean($length) THEN 'long'
    ELSE 'longest'
END

Since CASE evaluates in order and stops at the first match, you don’t need to repeat the lower bounds. If a street reaches line 2, it’s already ≥ 0.5 × mean (otherwise line 1 would have matched).

  • Add a new style named 5 categories.
  • Use a divergent colour range (eg. Spectral) to denote your categorical distribution.