书名：Mastering Geospatial Development with QGIS 3.x
作者名：Shammunul Islam Simon Miles GISP Menke GISP Smith Jr. Luigi Pirelli GISP Van Hoesen
本章字数：427字
更新时间：2025-04-04 14:40:31

Styling vector data

When you load spatial data layers into QGIS Desktop, they are styled with a random single symbol rendering. To change this, right-mouse click on the layer and choose properties, and, in the context menu, click on Symbology.

There are several rendering choices available from the menu in the top-left corner, which are as follows:

Single Symbol: This is the default rendering in which one symbol is applied to all the features in a layer.
Categorized: This allows you to choose a categorical attribute field to style the layer. Choose the field and click on Classify and QGIS will apply a different symbol to each unique value in the field. You can also use the Set column expression button to enhance the styling with a SQL expression.
Graduated: This allows you to classify the data by a numeric field attribute into discrete categories. You can specify the parameters of the classification (classification type and number of classes) and use the Set column expression button to enhance the styling with a SQL expression.
Rule-based: This is used to create custom, rule-based styling. Rules will be based on SQL expressions.
Point Displacement: If you have a point layer with stacked points, this option can be used to displace the points so that they are all visible.
Point Cluster: This option groups clusters of points together at a specified distance. This will be covered more in later chapters.
Inverted Polygons: This is a new renderer that allows a feature polygon to be converted into a mask. For example, a city boundary polygon that is used with this renderer would become a mask around the city. It also allows the use of Categorized, Graduated, and Rule-based renderers and SQL expressions.
Heatmap: This allows you to create a heat map rendering of point data based on an attribute.
2.5D: This allows you to extrude data into a 2.5D space. A common use case is to extrude building footprints upward, creating an almost 3D rendering.

The following screenshot shows the symbology properties available for a vector line layer:

For a given symbol, you can work with the first level, which gives you the ability to change the transparency and color. You can also click on the second level, which gives you control over parameters such as fill, border, fill style, border style, join style, border width, and X/Y offsets. These parameters change depending on the geometry of your layer. You can also use this hierarchy to build symbol layers, which are styles built from several symbols that are combined vertically.