CODEWITHSUNDEEP
postgisgeodjangogeos
Jake
Jake

Reputation: 13151

PostGIS - certain MultiPolygons cause "BOOM! Could not generate outside point!"

I'm trying to represent a rectangular area which crosses 180 degrees longitude. For more background see In PostGIS a polygon bigger than half the world is treated as it's opposite

Here's my test case:

from django.contrib.gis.geos import Polygon, MultiPolygon
from my_project.my_app.models import Photo

a = Polygon.from_bbox((30, -80, 180, 80))  # the part to the east of 180
b = Polygon.from_bbox((-180, -80, -160, 80))  # a part to the west of 180
c = Polygon.from_bbox((-180, -80, -100, 80))  # a larger part to the west of 180

ok   = MultiPolygon(a,b)
ok2  = MultiPolygon(c)
boom = MultiPolygon(a,c)

# This works
Photo.objects.filter(location__coveredby=ok)[:1]
# This also works so c is ok
Photo.objects.filter(location__coveredby=ok2)[:1]
# This gives "BOOM! Could not generate outside point!"
Photo.objects.filter(location__coveredby=boom)[:1]

# splitting c doesn't help
c1 = Polygon.from_bbox((-180, -80, -140, 80))
c2 = Polygon.from_bbox((-140, -80, -100, 80))
test = MultiPolygon(a,c1,c2)
Photo.objects.filter(location__coveredby=test)[:1]
# BOOM! Could not generate outside point!

By changing the numbers I can make this error come and go. (-180, -80, x, 80) works where x <= -140 for example. For every number there is a threshold like this but I can't find a pattern. For boxes with the same area, some will work and others not. For boxes with the same width some will work and others not.

I can look at the SQL being generated but the areas are represented in binary (EWKB) and I'm not sure how to read it.

Can anyone explain this?

Upvotes: 2

Views: 767

Answers (1)

Jake
Jake

Reputation: 13151

After asking this question I found out about gis.stackexchange.com, so I asked there too. With the help of the good folks there I found out what the problem is (I think) and a solution.

See: https://gis.stackexchange.com/questions/9217/postgis-certain-multipolygons-cause-boom-could-not-generate-outside-point/9257#9257

Upvotes: 1

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