Create, display, and evaluate geodata with QGIS

Symbolic

QGIS offers powerful tools for displaying data in the individual layers. To make adjustments, double-click on the layer, which opens a window for modifying the geodata. The layer can be a point, line, or polygon layer.

In the Display dialog, you will find various customization options for symbols and categories of symbols. QGIS comes with libraries of various ready-made symbols, which you can switch between using Open Library. In addition to the actual symbol display, the program also lets you change the color, opacity, size, and alignment (Figure 6).

Figure 6: The Layer Properties dialog lets you modify symbols.

On saving, QGIS transfers the individual symbol to the active library. In addition, SVG vector files with symbols can be downloaded from the web, integrated into the software, and then modified like the preset symbols.

By categorizing attributes, QGIS also offers the ability to customize individual attributes in a single pass. To do this, define a column of a layer attribute table as a category in the Layer Properties settings dialog. Then assign a symbol from the existing libraries to this category so that the tool displays all data with this attribute uniformly using the selected symbol. If necessary, you can modify the symbol's look. If you want to change certain attributes depending on existing values, QGIS can also make rule-based modifications.

Dialogs

Individual input dialogs facilitate data acquisition enormously, especially for large projects that require a lot of human input. QGIS also offers many options for creating and optimizing your own input masks. You can create tabs with groups of input fields, and you can also integrate check boxes, rotary and slider controls, and date fields. The software also lets you define ranges to prevent nonsensical and thus falsifying entries. QGIS does not make you commit to the order of the attributes in the attribute table: You can modify the sequence using drag-and-drop to shift input fields into the desired order.

Objects

To edit individual objects in layers in QGIS, you have to select them by clicking on them. To select several objects simultaneously, capture them by drawing a geometric shape, such as a rectangle or a circle, around the objects you wish to select. For irregularly arranged objects, such as trees in a tree register, choose Select objects via polygon function.

If the layer contains many irregularly arranged objects with different properties and you want to select objects with a very specific property, choose the button labeled Select objects via expression. The expression editor lets you select a common characteristic; you can then add individual objects to the selection or delete them. You can even specify several characteristics in combination.

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