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書名： QGIS Python Programming Cookbook
作者名： Joel Lawhead
本章字數： 260字
更新時間： 2021-07-23 19:48:57

Moving vector layer geometry

Sometimes, you need to change the location of a feature. You can do this by deleting and re-adding the feature, but PyQGIS provides a simple way to change the geometry.

Getting ready

You will need the New York City museums' shapefile, which you can download as a ZIP file from https://geospatialpython.googlecode.com/svn/NYC_MUSEUMS_GEO.zip.

Extract this shapefile to /qgis_data/nyc.

How to do it...

We will load the shapefile as a vector layer, validate it, define the feature ID we want to change, create the new geometry, and change the feature in the layer. To do this, perform the following steps:

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

First, load the layer and validate it:

vectorLyr = QgsVectorLayer('/qgis_data/nyc/NYC_MUSEUMS_GEO.shp', 'Museums' , "ogr")
vectorLyr.isValid()

Next, define the feature ID we are interested in changing:
```
feat_id = 22
```
Now, create the new point geometry, which will become the new location:
```
geom = QgsGeometry.fromPoint(QgsPoint(-74.20378,40.89642))
```
Finally, change the geometry and replace it with our new geometry, specifying the feature ID:
```
vectorLyr.dataProvider().changeGeometryValues({feat_id : geom})
```

How it works...

The changeGeometryValues() method makes editing a snap of the fingers. If we had to delete and then re-add the feature, we would have to go through the trouble of reading the attributes, preserving them, and then re-adding them with the new feature. You must, of course, know the feature ID of the feature you want to change. How you determine this ID depends on your application. Typically, you will query the attributes to find a specific value, or you can do a spatial operation of some sort.

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QGIS Python Programming Cookbook

Moving vector layer geometry

Getting ready

How to do it...

How it works...