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書名： QGIS Python Programming Cookbook
作者名： Joel Lawhead
本章字?jǐn)?shù)： 289字
更新時(shí)間： 2021-07-23 19:48:56

Adding a line feature to a vector layer

Adding a line to a vector layer in QGIS is identical to adding a single point, but here you just have to add more points to the QgsGeometry object.

Getting ready

For this recipe, you will need to download a zipped line shapefile that contains two line features from https://geospatialpython.googlecode.com/svn/paths.zip.

Extract the ZIP file to a directory named paths in your /qgis_data directory.

How to do it...

In this recipe, we will load the line layer from the shapefile, build a list of points, create a new geometry object, and add the points as a line. We will also create a new feature, set the geometry, and add it to the layer's data provider. Finally, we will update the extent of the layer to make sure that the bounding box of the layer encapsulates the new feature:

Start QGIS.
From the Plugins menu, select Python Console.

First, load the line layer and ensure that it is valid:

vectorLyr = QgsVectorLayer('/qgis_data/paths/paths.shp', 'Paths' , "ogr")
vectorLyr.isValid()

Next, access the layer's data provider:
```
vpr = vectorLyr.dataProvider()
```

Now, build our list of points for a new line:

points = []
points.append(QgsPoint(430841,5589485))
points.append(QgsPoint(432438,5575114))
points.append(QgsPoint(447252,5567663))

Then, create a geometry object from the line:
```
line = QgsGeometry.fromPolyline(points)
```
Create a feature and set its geometry to the line:
```
f = QgsFeature()
f.setGeometry(line)
```
Finally, add the feature to the layer data provider and update the extent:
```
vpr.addFeatures([f])
vectorLyr.updateExtents()
```

How it works...

As with all the geometry in QGIS, we use the four-step process of building points, geometry, feature, and data provider to add the line. Interestingly, the QgsGeometry object accepts Python lists for the collection of points instead of creating a formal object, as is done with the QgsPoint object.

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QGIS Python Programming Cookbook

Adding a line feature to a vector layer

Getting ready

How to do it...

How it works...