How do you use expressions in QGIS to control the visibility and scale of your layers and symbols?

1 What are expressions in QGIS?

Expressions in QGIS are formulas or functions that can be used to calculate values, filter data, manipulate strings, perform logical tests and more. You can write expressions using the expression builder, which provides a graphical interface with a list of available functions, operators and variables, as well as a preview and a help panel. You can access the expression builder from various places in QGIS, such as the field calculator, the select by expression tool, the rule-based renderer, the data-defined override buttons and the label settings.

3 How to use expressions to control symbol scale?

Another way you can use expressions in QGIS is to control the scale of your symbols based on certain factors. For example, you may want to vary the size of your symbols depending on the scale of the map, the value of an attribute or the result of an expression. To do this, you need to enable the data-defined override option for the symbol size in the symbol properties dialog, and then write an expression that returns a numeric value for each feature. The features that have a higher value will have larger symbols, and the features that have a lower value will have smaller symbols.

4 How to use expressions to control symbol color?

You can also use expressions in QGIS to control the color of your symbols based on certain criteria. For example, you may want to assign different colors to your symbols depending on the scale of the map, the value of an attribute or the result of an expression. To do this, you need to enable the data-defined override option for the symbol color in the symbol properties dialog, and then write an expression that returns a color value for each feature. The color value can be specified as a hex code, a color name or a color function.

5 How to use expressions to control symbol rotation?

Another way you can use expressions in QGIS is to control the rotation of your symbols based on certain parameters. For example, you may want to rotate your symbols depending on the scale of the map, the value of an attribute or the result of an expression. To do this, you need to enable the data-defined override option for the symbol rotation in the symbol properties dialog, and then write an expression that returns an angle value for each feature. The angle value can be specified in degrees or radians.

6 How to use expressions to control label settings?

You can also use expressions in QGIS to control various settings of your labels, such as their visibility, position, font, color and format. For example, you may want to show or hide labels depending on the scale of the map, the value of an attribute or the result of an expression. To do this, you need to enable the data-defined override option for the label settings in the label properties dialog, and then write an expression that returns a boolean value for each feature. The features that return true will have labels, and the features that return false will not have labels.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?