[GIS] How to query for all polygons within a distance from a point

distancegeodjangospatialite

Call me stupid, but somehow I couldn't figure out how to do the following.

I've got a testing shapefile with 18 features, which I successfully imported into GeoDjango (1.3, running on Spatialite). As an practical example, I've taken a centroid of one of them (area.mpoly.centroid) and tried to find all areas within a 20 km distance (touching would be enough, needn't cover complete area).

Area.objects.filter(mpoly__dwithin=(centroid, D(km=20)))

However, this only gives me an empty list. Increasing the number to 200 km (in theory, this should cover all of my dataset) also yields nothing.

Am I doing something wrong?

Best Answer

I missed this before, you're using SpatialLite. dwithin is not supported with SpatialLite.

Old answer: 'Area' is a GeoDjango class. You probably have a name conflict. Don't name your models with the same name of any GeoDjango classes.

Also, 'centroid' should not be used as a variable name for the same reason.

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[GIS] R: distance from one point to many polygons

ok figured it out eventually myself

    centroid.ED <-SpatialPoints(coordinates(eire))
    proj4string(centroid.ED) <- eire.proj
    eire$distance <- gDistance(pt.coords,centroid.ED,byid=TRUE)

    > summary(eire$distance)
           1         
     Min.   : 18.44  
     1st Qu.: 56.78  
     Median : 99.48  
     Mean   :106.46  
     3rd Qu.:156.60  
     Max.   :223.84

[GIS] PostGIS – query maximum distance between two polygons

There is no such function, but you can create the functionality yourself (although it can be slow for large datasets) by calculating the distance from every point in the red polygon to the blue polygon. Then get the max(distance) for every red polygon and you should have your answer. As long as your red polygons have not too many vertices, this should still perform reasonably.

Query looks something like:

WITH exploded AS (
  SELECT district_code, (ST_Dumppoints(geometry)).geom FROM red
)
SELECT blue.id, blue.district_code, max(ST_Distance(exploded.geom, blue.geometry) AS max_distance
FROM exploded 
INNER JOIN blue ON exploded.district_code = blue.district_code
GROUP BY blue.id, blue.district_code;

Best Answer

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[GIS] R: distance from one point to many polygons

[GIS] PostGIS – query maximum distance between two polygons

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