Classified Quantitative Symbology for Vector Features
This tutorial demonstrates different symbology approaches for visualizing classified quantities. For this, we use polygon features to create a choropleth map with graduated colors and point features to apply graduated symbols to features.
Included in this tutorial:
Accessing classified quantitative symbology options
Graduated colors symbology, with a polygon layer example
Graduated symbols symbology, with a point layer example
Software version in examples: QGIS-LTR 3.40.5-Bratislava
Tutorial Data: The tutorial includes demonstration with sample data available here.
Credits: Sally Kaye and L. Meisterlin (2025)
NOTE: This tutorial focuses on the interface options for accessing and applying a classified quantitative symbology. It does not cover specific classification methods, which are reviewed in Classifying Features by Quantitative Attribute Values.
Accessing classified quantitative symbology options
Below, we demonstrate opening the symbology window by right-clicking the layer name in the Layers panel and navigating through Properties (window) > Symbology. The options can also be accessed by double-clicking a layer’s symbol swatch in the Layers Panel.
In the Symbology Properties tab, click to open the symbology option dropdown menu and select Graduated. This is the dropdown at the very top of the Symbology options (the default option is Single Symbol).
accessing the Symbology window by right-clicking on the layer’s name in the Layers panel
Graduated Colors
We can apply graduated colors to classified quantities based on values in a layer’s attribute table. This means that values are grouped into classes and each class is assigned a color, generally along a color ramp that corresponds to the ascending or descending values per class. Features with values within the same class will be symbolized with the same color.
The software interactions demonstrated here are the same for applying graduated colors to polyline or point feature classes.
After accessing Graduated symbology, the demonstration (below) shows the following:
Choose the numeric field from the layer’s attribute table containing the values we want to classify and symbolize, from the Value dropdown list. In this example, we choose the “Int_lrg” field. Notice that only numeric fields are listed for quantitative symbology approaches.
To normalize or otherwise modify values, you can do so with the Expression Builder. Select the main value in the Value dropdown, then click the sigma (ε) symbol to the right of the menu to open the Expression Builder dialogue box.
Choose a color ramp from the preset dropdown menu of choices.
Choose the classification mode and number of classes. (These options are both toward the bottom of the window.) In this example we will map these polygons’ attribute values in quartiles: four classes each containing the same number of features. For more on classification methods, see Classifying Features by Quantitative Attribute Values.
You may have to click classify to display the classification scheme and corresponding symbols in the legend box.
Click Apply to apply the classified symbology to the map canvas.
Legend Formatting
Individual labels for your legend are editable. Here, we demonstrate changing the first label to say “bottom 25%.” When Apply is clicked, the legend in the layers panel updates to include changes
You can change the default legend formatting for all symbols using the Legend Format options (toward the top of the window), including typing directly into the text box. In our example, however, we forego this step in favor of demonstrating how to individually label a legend item..
Symbol Formatting
We change the formatting for all symbols by clicking the Symbol bar at the top of the window; here, we demonstrate changing the outline weight for all symbols.
You can also change the formatting for individual symbols by double-clicking on the symbol’s swatch in the legend box.
applying graduated colors to symbolize polygon features
Graduated Symbols
Like graduated colors, we can also apply graduated symbols to classified quantities based on values in a layer’s attribute table. Rather than assigning different colors to distinguish values, the features grouped into each class are assigned a size. Usually, these sizes increase or decrease with the range represented per class.
In the demonstration below, we apply graduated symbols to point features. The software interactions are similar for polyline geometry, resulting in features symbolized with a graduated range of line weights. In QGIS, graduated symbols cannot be applied to polygon features.
After accessing Graduated symbology, the demonstration (below) shows the following:
Choose the numeric field from the layer’s attribute table containing the values we want to classify and symbolize, from the Value dropdown list.. In this example, we choose the “Int_lrg” field. Notice that only numeric fields are listed for quantitative symbology approaches.
Choose the symbology method. The default method is “Color,” demonstrated above. Here, we change the method to Size, which changes the symbology options from color ramp to size range.
Choose the classification mode and number of classes. In this example we will map these polygons’ attribute values in quintiles: five classes each containing the same number of features. For more on classification methods, see Classifying Features by Quantitative Attribute Values.
You may have to click classify to display the classification scheme and corresponding symbols in the legend box.
Click Apply to apply the classified symbology to the map canvas.
Then, we walk through several of the options to customize the symbols:
Changing the minimum and maximum size of the symbol (the symbols applied per class are proportionately rendered between these sizes by default, but can also be individually adjusted);
Adjusting the template swatch applied to all symbols; and
Adjusting the appearance of only one symbol.
applying graduated symbols to point features