Create square buffers around points in QGIS with Python?

Question

I have x, y coordinates in lat/long and I need to create 5x5 degree square cells around them, with the lat/long coordinates being the centroids.

My first option is to create a buffer around the centroids with 1 segment and a distance of 1/2(5°) * sqrt(2) (must multiply by sqrt(2) bc the tool uses centroid to the corner of the square as the buffer distance, as opposed to centroid to edge), which results in sideways squares around my points, then rotate each feature by 45 degrees. I'd prefer not to do this as the distance isn't as precise and I do not know how to rotate each individual buffer feature quickly.

My second option, which seems much simpler, is to create a buffer around the centroids with the distance I need ((1/2)*5°) and then use something like ArcMap's Feature to Envelope tool.

I see that someone has the same question here and an answer was provided, but I have no idea how to do it programmatically.

Answer 1

As you wondering in your last paragraph, to do that programmatically with PyQGIS it is not very difficult. You can try out next code. However, I used a shapefile and projected coordinates in meters (buffer has 1000 m). You only would need to do a few changes.

layer = iface.activeLayer()

feats = [ feat for feat in layer.getFeatures() ]

epsg = layer.crs().postgisSrid()

uri = "Polygon?crs=epsg:" + str(epsg) + "&field=id:integer""&index=yes"

mem_layer = QgsVectorLayer(uri,
                           'square_buffer',
                           'memory')

prov = mem_layer.dataProvider()

for i, feat in enumerate(feats):
    new_feat = QgsFeature()
    new_feat.setAttributes([i])
    tmp_feat = feat.geometry().buffer(1000, -1).boundingBox().asWktPolygon()
    new_feat.setGeometry(QgsGeometry.fromWkt(tmp_feat))
    prov.addFeatures([new_feat])

QgsMapLayerRegistry.instance().addMapLayer(mem_layer)

After running the above code at the Python Console of QGIS, I got:

It works.

Editing Note:

Next code introduces at the attributes table a column for the x-coordinate, y-coordinate, and ID number for each point.

layer = iface.activeLayer()

feats = [ feat for feat in layer.getFeatures() ]

epsg = layer.crs().postgisSrid()

uri = "Polygon?crs=epsg:" + str(epsg) + "&field=id:integer&field=x:real&field=y:real&field=point_id:integer""&index=yes"

mem_layer = QgsVectorLayer(uri,
                           'square_buffer',
                           'memory')

prov = mem_layer.dataProvider()

for i, feat in enumerate(feats):
    point = feat.geometry().asPoint()
    new_feat = QgsFeature()
    new_feat.setAttributes([i, point[0], point[1], feat.id()])
    tmp_feat = feat.geometry().buffer(1000, -1).boundingBox().asWktPolygon()
    new_feat.setGeometry(QgsGeometry.fromWkt(tmp_feat))
    prov.addFeatures([new_feat])

QgsMapLayerRegistry.instance().addMapLayer(mem_layer)

After running the new code at the Python Console of QGIS, result was:

Answer 2

QGIS 3 provides a quick and dirty alternative: - Vector --> Geoprocessing tools --> Buffer

Select your point layer as an input layer, make sure that the end cap style is set to square and the distance should be half of the intended square length (so a 1km-per-side square should have a Distance of 500m).

Answer 3

To transform points into squares you can try native:buffer processing algorithm with END_CAP_STYLE parameter set to 2 (square).

Warning! Results may vary and depend on input layer's coordinate system. In exapmle, if you use WGS 84 and set distance to 5000 this will result in a square line of 5000 degrees (not meters).

Tested with pyQGIS 3.6.1:

def buffer(input, distance, output, before_processing_reproject_to_epsg_number=None):
    params = {'INPUT': input,
              'DISTANCE': distance,
              'END_CAP_STYLE': 2 , # - 0: Round - 1: Flat - 2: Square
              'DISSOLVE': False,
              'OUTPUT': output}
    feedback = qgis.core.QgsProcessingFeedback()
    alg_name = 'native:buffer'
    # print(processing.algorithmHelp(alg_name)) # tool tips
    result = processing.run(alg_name, params, feedback=feedback)
    return  result

Usage example:

buffer_result = buffer(input=r'C:\input.shp', distance=5000, output=r'C:\output.shp')
print(buffer_result)

Answer 4

Your initial approach works if you're okay with the precision issue. Calculate the length of half the desired square's diagonal (a2+b2=c2) as this is the length the Buffer tool will use for the Distance parameter. Input 1 as the Segment parameter value. Once the tool has been run all features can be simultaneously rotated using

Vector geometry -> Rotate tool

.