Map symbol (Symbol) is the language of the map. It is the simplification, generalization and abstraction of the spatial shape of geographic elements. The essence of the map symbol system is to express and transmit information. Therefore, the design of the symbol system not only intuitively reflects the “color of the map”. Value” and affects the performance ability of spatial information.
Source: https://www.flickr.com/photos/95856760@N05/46228237415/in/pool-qgis/
In addition to being free and open source, QGIS is recognized by the majority of GISers for its excellent symbolic expression effects. QGIS can not only quickly create maps through simple symbolization settings, but also provides flexible symbol design capabilities, giving more possibilities for map symbolization.
This article is aimed at students with zero foundation in QGIS, starting from understanding the layer symbolization setting interface, and taking the vector layer symbol setting as an example to demonstrate the rapid layer symbolization process.
01 Download demo data
This article uses QGIS sample data, download link:
https://qgis.org/downloads/data/qgis_sample_data.zip。
Or download from the network disk, the address is:
Link: https://pan.baidu.com/s/1zJJYoZiIbUlhc7UA5y_A9Q
Extraction code: ovms
02 Layer symbolization setting interface
In QGIS, there are three ways to call up the layer symbolization setting interface:
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Layer properties
Open the layer properties dialog box and switch to the [Symbolization] tab page.
The Layer Properties dialog box provides the most comprehensive symbolization setting function. However, because the dialog box is a modal window, as long as this window is open, other functions of QGIS cannot be used, resulting in poor user experience.
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Layer style panel
Click the 
[Open Layer Style Panel] button above the [Layer] panel , and open the [Layer Style] docking panel on the right.
[Layer Style] The panel separates some layer attribute setting functions, and provides quick access on the dockable panel, such as symbolization settings, label settings, etc. The biggest advantage of this panel lies in its linkage with the map window. After checking the [real-time update] checkbox below, change the symbol on the panel, and the map window will be redrawn immediately and the changed content will be displayed. In addition, the panel will not block the QGIS interface, and other operations can still be performed while setting symbols.
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Layer right-click menu
Right-click the layer in the [Layer] panel and select [Style] from the pop-up menu.
In the [Style] menu, you can quickly change the color of the symbol by selecting the Color Wheel, or you can easily share the symbol through the copy and paste menu. If you need to further change the symbol style, click the [Edit Symbol...] menu to open [ Symbol selector] dialog box for setting.
03 Vector layer symbolization
According to the spatial dimension of the geographical phenomenon expressed, vector layer symbols are generally divided into three categories: point symbols, linear symbols, and surface symbols.
Let's start with the dot symbol setting.
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Dot symbol
First click the 
[Open Data Source Manager] button on the toolbar to open the Data Source Manager dialog box. Switch to the [Vector] tab, select [Source] as "File", browse to "\qgis_sample_data\climate", and open the "climate.shp" file.
As you can see from the map window, "climate.shp" is a point layer. QGIS assigns circular symbols and random colors to it by default, which is difficult to meet aesthetic requirements. Right-click the "climate" layer to open the layer properties dialog box.
In the layer properties dialog box, switch to the "Symbolization" tab. The upper drop-down box can select the rendering type of the symbol, the default is "single symbol".
The symbol layers are displayed in a tree structure in the middle, the root node is the final effect of the symbol, that is, the way it is displayed on the map, and the child nodes are the symbol layers. The left side is the symbol preview, and the right side provides operation buttons for the symbol layer.
Click on the symbol layer. With different layer types, the symbol setting area below provides different options. For example, the following figure shows the point symbol option. You can set the symbol unit, transparency, symbol color, symbol size, and symbol rotation angle.
At the bottom is a preview of the saved symbols. The saved symbols are organized by tags, and the symbols under the "Favorites" tag are displayed by default. Click the drop-down arrow on the right to see the labels that have been set for this project. A symbol can be classified under multiple labels. Click the label name to view the symbols under that category. Click "All Symbols" to display all symbols of this project.
To illustrate the symbolization effect, we choose an exaggerated symbol for the layer. Click to select the 
effect drop shadow, and you can see that the symbol is composed of two "simple mark" symbol layers.
Click the [OK] button to return to the QGIS map window, and the symbolization effect is as follows:
In order to learn more about the symbol layer, right-click the "climate" layer, open the layer properties dialog box again, and observe how the two symbol layers are combined into a star symbol with a shadow effect.
Click the first "simple mark" sub-node of the symbol layer tree, the key parameters of the symbol are as follows:
Size = 7.2mm
Color and stroke = red
Rotation angle=34°
Anchor point = vertical center, horizontal center
Symbol = five-pointed star
Select the second "simple symbol" sub-node of the symbol layer tree, and compare the first "simple symbol" sub-node, you can see that the following parameters have changed:
Symbol size: 6.6 mm
Fill and stroke color: dark red
Rotation angle: 0°
Note that the symbol layer has checked the [Drawing Effect] checkbox below, and click the 
[Custom Effect] button to pop up the Effect Properties dialog box, and you can see that the "Source" and "Shadow" effects are enabled.
The changes in the size, color, rotation angle and drawing effect of the two symbol layers make the star symbol appear full and layered.
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Line symbol
The setting process of linear symbols is basically the same as that of point symbols. It should be noted that, compared with point symbols, the layer types and corresponding setting options of linear symbols are different.
Click the button on the toolbar to open the Data Source Manager dialog box. Browse to "qgis_sample_data\ shapefiles\rivers.shp" and click [Add] to add the river layer to the map window.
Right-click "rivers" in the [Layer] panel to open the layer properties dialog box.
Switch to the [Symbolization] tab page. As you can see, the linear symbol also provides a symbol layer tree. The symbol preview is on the left. Select the symbol layer. Below you can set the symbol's unit, transparency, color, width, and line type parameters. The saved line types are stored as labels, and the line types of "Favorites" are displayed by default.
Select "simple blue line" for the "rivers" layer, set the width to 0, click [OK] to return to the QGIS map window, and you can see the effect of the symbol.
Select a more complex line type, such as "pattern circles", and observe the structure of the symbol layer tree and the setting parameters of each layer. Compared with point symbol layers, in terms of symbol layer types, linear symbols provide five types: arrow, geometry generator, hash line, mark line and simple line. The parameters that can be set for each type are also different. , Friends who are interested can study in depth.
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Faceted symbol
The setting process of the area symbol is the same as the setting operation process of the point and line symbols, and will not be repeated one by one.
04 Summary
The selection and design of map symbols involves many fields such as geometry, semiotics and psychology. It is a very challenging task. QGIS has a rich symbol library, flexible combination of symbol layers and rich customization options. The design of objective and beautiful map symbols laid the foundation.
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