22nd Sep, 2017
University of Warsaw
Question
Asked 3rd Mar, 2016
How to calculate the intersected area from a layer with overlapping buffers in qgis?
I need to calculate the area of landusetypes in 100m around various sampling point. For this i have two shp: Landuse file and sampling point file. I made a buffer with 100m diameter arround each point. My problem is, the buffers around the points are overlapping each over in some places. I tried to intersect/clip the layer (e.g. landuse) with the layer "buffer", but the many overlapping parts are combine in one area, only counted once. This overestimate area for some small part of intersection. My purpose is to have each intersection area individually. Is there any tool in qgis which might help me? Or is there any other way to get this output? Thanks for your help.
Please find attached how it looks like: Yellow is landuse data and circles are buffer arround sampling points(red dot)
Most recent answer
It is not in QGIS but in ArcGIS tool Count overlapping polygons can be useful.
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All Answers (7)
Create a shapefile for each point buffer. Intersect one buffer with another individually. Use ArcGIS if you have it. I hope I am understanding your issue.
Cheers!
Thank you Eric, this is a possibility but I have many points (434) and make buffer for each point seem to be impossible (huge).
Thank you!
What about doing the buffer again for all the points, but this time, you have to specify that you will dissolve the buffers into one? This means that buffers are dissolved together into a single feature, removing any overlap. Once you have this single buffer feature, intersect this file to your landuse file.
Cheers!
Run the follwing Python script in QGIS Python console to create a memory layer with samples points and the intersection area in map units. You can join this layer on id with the original points.
samples = QgsMapLayerRegistry.instance().mapLayersByName('Sample Points')[0]
landuse = QgsMapLayerRegistry.instance().mapLayersByName('Landuse')[0]
result = QgsVectorLayer('Point?crs=EPSG:4326&field=id:integer&field=area:double', 'Result', 'memory')
prov = result.dataProvider()
fields = prov.fields()
feats = []
for sample in samples.getFeatures():
for poly in landuse.getFeatures():
# 100 units buffer with a 25 segments boundary
part = sample.geometry().buffer(100, 25).intersection(poly.geometry())
feat = QgsFeature(fields)
feat.setGeometry(sample.geometry())
feat['id'] = sample.id()
feat['area'] = part.area()
feats.append(feat)
prov.addFeatures(feats)
QgsMapLayerRegistry.instance().addMapLayer(result)
1 Recommendation
I think one problem, here, is the size of the buffers you have created. Measure distance between the two most distant proximate points, half that, try using that as the radius of your buffer. This will give you overlaps where the points are concentrated but cut some of the confusion. Then, once the buffers are created you'll identify the intersects as a set (in QGIS or ArcGIS), either extract them or create a new set of buffers based on their center. We used this in a study of food poisoning, building "super buffers" from overlapping buffer intersects.
1 Recommendation
Thank you for the nice contribution. It works now and I could estimated the impact of landuse.
Many thanks.
All the best!
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