QGIS create a new merged layer from selection on multiple layers

Question

I have a big map with several layers. I want to extract a small area in the map and merge the layers.

I tried to merge the layers and than to select the zone I am interested, but the merging is impossible due to the size of the original map (the algorithm started and after 20 minutes it was at 1%)

Is it possible to select the same area from multiple layers and create a new merged layer (without extracting zone from the original layers one-by-one) ?

Answer 1

If you are using QGIS and the area you are wanting to select can drawn as a polygon, you can use the graphical modeler to batch process clipping each individual layer then merging all the smaller layers together.

Using the graphical modeler allows you to automate the process and reduce the number of intermediary layer being saved so you will be left with just the clipped and merge area.

Essentially this is what you tried but backwards (merge-clip -> clip-merge). But because the layers are pre-clipped when you merge you are using smaller files and so the process should be faster overall.

Answer 2

You could use a bit of Python to get the selected features from multiple layers and save them to a new shapefile:

1. First select your features from your layers.

2. In the Python Console, run the following code to define a function which iterates through each layer and saves the selected features into a memory layer which is finally saved as a shapefile (edit the code to change the path of the output):

   def run(name, crs):
       # To create polygon memory layer, use "Polygon"; "Point" for point layer; "LineString" for line layer
       mem_layer = QgsVectorLayer("Polygon?crs=epsg:" + str(crs), "Layer_Name", "memory")
       mem_layer.startEditing()
       for layer in QgsMapLayerRegistry.instance().mapLayers().values():
           attr = layer.dataProvider().fields().toList()
           mem_layer.dataProvider().addAttributes(attr)
           mem_layer.updateFields()
           feats = [ feat for feat in layer.selectedFeatures() ]           
           mem_layer.dataProvider().addFeatures(feats)
       mem_layer.commitChanges()
       path = r"path/to/directory/" + name
       new_layer = QgsVectorFileWriter.writeAsVectorFormat(mem_layer, path, "utf-8", None, "ESRI Shapefile")
       del mem_layer
       QgsMapLayerRegistry.instance().addMapLayer(QgsVectorLayer(path + ".shp", name, "ogr"))
   

3. Then run the following to execute the function where, for example, the output shapefile name is "merged" and its CRS set at ESPG:27700:

   run('merged', 27700)