I have two vector layers: one point and one polygon and I want to calculate the minimum distance of each point from the polygons (ie the distance from the closest point of the closest polygon).
Is there any available plugin in QGIS for doing this?
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1you mean polygon vertices, the centroid or something else? – lynxlynxlynx Jun 22 '12 at 20:35
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This task would be better handled by postgis. – Pablo Jun 23 '12 at 00:03
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I mean the distance from the borders of the polygon (actually it's the distance of observed seabirds in the sea from the coastline that I want to calulate) – Sylvia Jun 23 '12 at 10:49
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Pablo, unfortunately I don't know how to use PostGIS and I can't write code... – Sylvia Jun 23 '12 at 11:05
4 Answers
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Converting your polygon to points first is a bad idea. You will not be calculating the distance to your polygon, you will be calculating the distance to the closest node of your polygons (not the edges which may be closer). You can use the NNJoin Plugin to get accurate measurements to your polygon. Make sure your layers are in the same projection before doing analysis.
gistutor
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1The QGIS NNJoin Plugin will take care of coordinate system transformation. But you should make sure that you use a projected coordinate system for the join dataset (the input dataset will be transformed to the coordinate system of the join dataset if their coordinate systems differ). – Håvard Tveite Apr 18 '15 at 23:41
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If you want to use QGIS, the GRASS v.distance function in the GRASS plugin should do the job. The NNJoin plugin should work for small datasets, and is less cumbersome to use.
The NNJoin plugin uses the QgsGeometry.distance() function (which uses the GEOSdistance) to calculate the distances.
Håvard Tveite
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First convert the polygon to points:
Vector -> Geometry Tools -> Extract Nodes
Add this new layer, then calculate a distance matrix between your points layer and the new polygon-points layer you just created
Vector -> Analysis Tools -> Distance Matrix
Be sure to choose "Use only the nearest (k) target points" as 1
Brendan Nee
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This is a good answer to the original question, but the question has since been modified in a comment. Extract nodes and Distance Matrix may not find the closest place on the boundary (the closest place may be between the points of the boundary polyline). GRASS v.distance or the NNJoin plugin will. – Håvard Tveite Mar 18 '18 at 23:28
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2Hub distance did the job, but for the record, I first converted the polygon into a point layer and calculated the minimum point-to-point distances. With the polygons it was calculating the minimum distance from the centroids or the middle of the coastal boundary – Sylvia Jun 23 '12 at 13:58