Is there a way to have a 'live' buffer in QGIS

Question

I use QGIS (2.10.1) to create buffers around polygons. I often create a series of buffers, and later need to edit the underlying polygons. It is frustrating to start from scratch in re-create several buffers based on the changes when this happens.

Is there a way to create a 'live' buffer, that will automatically update to changes made to the parent polygon, or as a second best, a means to replicate with one click, the buffer settings and formatting that you originally created?

Answer 1

Creating a View in PostGIS to have a "live buffer" is of course the best choice if you are willing to setup a RDBMS. I was curious to see if you could also get some sort of live buffer without PostGIS, so I gave it a try with spatialite and it works too. The difference is just that it seems that you can't create a View with a different geometry-type in spatialite. I found this inforation here: https://groups.google.com/forum/#!topic/spatialite-users/ZZIPwYt8-uA there was written:

"there is absolutely no way to show a View geometry in spatialite-gis, if the geometry-class isn't exactly the same of the one used for the main-table"

So you have to use a buffer table and three triggers:

CREATE TABLE lines ( id INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT, name TEXT NOT NULL);


SELECT AddGeometryColumn('lines', 'geometry', 3857, 'LINESTRING', 'XY');


CREATE TABLE lines_buffer (id INTEGER PRIMARY KEY AUTOINCREMENT, gid INTEGER, name TEXT NOT NULL);

SELECT AddGeometryColumn('lines_buffer', 'geo', 3857, 'POLYGON', 'XY');

CREATE TRIGGER "lines_buffer_insert" AFTER
INSERT ON "lines"
FOR EACH ROW BEGIN
INSERT INTO lines_buffer (gid, name, geo)
SELECT new.id,
       new.name,
       ST_Buffer(new.geometry, 1000) ; 
END

CREATE TRIGGER "lines_buffer_update" AFTER
UPDATE ON "lines"
FOR EACH ROW BEGIN
UPDATE lines_buffer
SET geo=(ST_Buffer(NEW.geometry, 1000))
WHERE gid=NEW.id ;
  UPDATE lines_buffer
  SET name=NEW.name WHERE gid=NEW.id ; 
END

  CREATE TRIGGER "lines_buffer_delete" AFTER
  DELETE ON "lines"
  FOR EACH ROW BEGIN
  DELETE
  FROM lines_buffer WHERE gid=OLD.id ;
 END

I havent tested it excessively, but i created, updated and deleted several lines and the buffer was always updated (after refreshing/panning QGIS of course). So this could be an alternative where you have just one spatialite file to store the data and don't have to setup a PostGIS database...

Answer 2

Posting an answer from another question (How to add a buffer in QGIS that updates itself) as an alternative to the postgis/rdbms solutions already shared.

For a foolproof, simple visualisation, try the geometry generator.

1. Go to Layer Styling panel -> select style -> Symbol layer type: Geometry generator
2. select Geometry type: Polygon/Multipolygon
3. enter the following expression buffer($geometry,AMOUNT)
4. AMOUNT can be a fixed number in the units of your CRS, or a field in your data - in which case it would be buffer($geometry,"buffer_field")
5. buffers will appear as soon as you add or edit a point in qgis, even if you haven't saved edits

To generate polygons that you can calculate areas of and so on, try virtual layers.

1. Go to Layer -> Add Layer -> Add/Edit Virtual Layer
2. click Import and select your layer
3. enter the following query: SELECT desired_field, desired_field2, 'desired field', st_buffer(geometry,AMOUNT) AS geometry from 'Name of layer'; - desired fields are optional. Any names of layers or fields with spaces, or most non-alphanumeric characters, must be wrapped in single quotes. Click Test to check that there are no errors before clicking OK.
4. buffers are saved as a separate layer, which will update only when you press Refresh, and any edits are saved (if editing your source layer outside qgis)

Answer 3

Just do

from qgis.analysis import QgsGeometryAnalyzer

# Select your layer
layer_ref = iface.activeLayer()
# Get layer tree root
root = QgsProject.instance().layerTreeRoot()

# Prepare function that will act as slot
def save_after_edit(layer, qgsgeometrymap):
    output_filename = "output.shp"
    # Make buffer and save it in a shp
    QgsGeometryAnalyzer().buffer(layer_ref, output_filename, 500, False, False, -1)
    # If layer with a name 'buffer', remove it (only one layer 'buffer' allowed)
    if 'buffer' in [i.name() for i in QgsMapLayerRegistry.instance().mapLayers().values()]:
        QgsMapLayerRegistry.instance().removeMapLayer([i.id() for i in QgsMapLayerRegistry.instance().mapLayers().values() if i.name() == "buffer"][0])
    # Add the created buffer and add it just below the layer at the top
    if 'buffer' not in [i.name() for i in QgsMapLayerRegistry.instance().mapLayers().values()]:
        mylayer = QgsVectorLayer(output_filename, "buffer", "ogr")
        QgsMapLayerRegistry.instance().addMapLayer(mylayer, False)
        root.insertLayer(1, mylayer) # Position in the layer tree

# Use signal emitted from layer when you commit changes
# Other signals available at http://www.qgis.org/api/classQgsVectorLayer.html
layer_ref.committedGeometriesChanges.connect(save_after_edit)