Changing CRS of layer using PyQGIS without saving file and iterating every feature

Question

So I have one layerA with CRS:4326 and I can change CRS saving it:

# This layer has crs:4346
layerA = QgsVectorLayer("layerA.sqlite", "layerA", "ogr")

#New crs
crs = QgsCoordinateReferenceSystem("epsg:32629")

#saving layerA to layerB with new crs:32629
error = QgsVectorFileWriter.writeAsVectorFormat(layerA, "layerB.sqlite", "UTF-8", crs, "SpatiaLite")

With this code I can get a new LayerB with another CRS

I have read this: http://docs.qgis.org/testing/en/docs/pyqgis_developer_cookbook/crs

But I think this solution need to iterate over every feature to reproject it.

Is there any solution to reproject a layer with only one instruction (like when saving it) but without need to create a new layer?

Something like layerA.reproject(newCRS) ?

Answer 1

I don't think you can reproject a layer and overwrite its data at the same time it is being accessed. One option is to:

1. Create a memory layer
2. Transfer all data from the original layer to the memory layer
3. Remove any dependencies on the original layer
4. Overwrite original layer with the new CRS

---

# Get original shapefile and its features
layer = QgsVectorLayer("path/to/shapefile", "polygon", "ogr")
feats = [feat for feat in layer.getFeatures()]

# Create memory layer and copy over all attributes and geometries
mem_layer = QgsVectorLayer("Polygon?crs=epsg:4326", "memory_layer", "memory")
mem_layer_data = mem_layer.dataProvider()
attr = layer.dataProvider().fields().toList()
mem_layer_data.addAttributes(attr)
mem_layer.updateFields()
mem_layer_data.addFeatures(feats)
QgsMapLayerRegistry.instance().addMapLayer(mem_layer)

# Get original shapefile path
layer_source = layer.source()
# Remove dependence on shapefile
del layer
#New crs
crs = QgsCoordinateReferenceSystem("EPSG:32629")
# Overwrite original shapefile
error = QgsVectorFileWriter.writeAsVectorFormat(mem_layer, layer_source, "utf-8", crs, "ESRI Shapefile")
# Add result to canvas
QgsMapLayerRegistry.instance().addMapLayer(QgsVectorLayer(layer_source, "Result", "ogr"))

Answer 2

Yes, you can reproject a layer in use without having to create a new one.

Unfortunately, no there is (still) no such short and simple method. It would shorten up things, but basically if there would be such a method, it would also have to iterate over the geometries to reproject them. Since the code can be really short I dont see much of an issue in doing it like:

layer = iface.activeLayer()
source_crs = QgsCoordinateReferenceSystem(layer.crs().authid())
target_crs = QgsCoordinateReferenceSystem("EPSG:25832")
with edit(layer):
    for feat in layer.getFeatures():
        # you need to store the geometry in a variable to reproject it; you can not reproject without doing so
        geom = feat.geometry() 
        # transform the geometry
        geom.transform(QgsCoordinateTransform(source_crs, target_crs, QgsProject.instance()))
        # now update the features geometry
        feat.setGeometry(geom)
        # dont forget to update the feature, otherwise the changes will have no effect
        layer.updateFeature(feat)
    # finally simply set the layers crs to the new one
    layer.setCrs(target_crs)

Also see:

• https://qgis.org/pyqgis/3.16/core/QgsGeometry.html#qgis.core.QgsGeometry.transform

Answer 3

Since QgsMapLayerRegistry has been removed, the QgsMapLayerRegistry in the @Joseph answer must be replaced by QgsProject.

Now, answering the question:

from qgis.core import *

iface.mainWindow().blockSignals(True)
vlayer = QgsVectorLayer(dfx_file, 'layer_test', 'ogr') 
crs = vlayer.crs()
crs.createFromId(5347) # Replace '5347' with whatever CRS id you want
vlayer.setCrs(crs)
QgsProject.instance().addMapLayer(vlayer)
iface.mainWindow().blockSignals(False)