Able to plot on map but having trouble with st_contains

Question

I am new to working with GIS, and am building a rails app that uses the rgeo gem and PostGIS.

I am trying to use geospatial data to determine the school district for a house. The house is represented by a point, and the school districts are represented by multipolygons. I have successfully plotted both on a map, and all appears to be working correctly, with the point falling inside of one of the polygons.

However, when I try to run the following code to determine which district the house is in, I get back an empty array:

>>  point = @property.coords
>> ES_Zone.where{ st_contains(geometry, point) }
=> #<ActiveRecord::Relation []>

I think that the SRIDs are the same on the points and polygons, because the factories for each show SRID 3785.

>> point.class
=> RGeo::Geos::CAPIPointImpl
>> point.factory
=> #<RGeo::Geos::CAPIFactory:0x860c5838 srid=3785 bufres=1 flags=8>

>> ES_Zone.first.geometry.class
=> RGeo::Geos::CAPIMultiPolygonImpl
>> ES_Zone.first.geometry.factory
=> #<RGeo::Geos::CAPIFactory:0x84889648 srid=3785 bufres=1 flags=8>

Any ideas on why this is happening, and suggestions for how to fix it?

Answer 1

Try the && operator:

scope :containing, -> (point) { where("#{self.table_name}.geometry && ?", point) }

From http://postgis.net/docs/manual-1.3/ch03.html:

In general, you will want to use the "intersects operator" (&&) which tests whether the bounding boxes of features intersect. The reason the && operator is useful is because if a spatial index is available to speed up the test, the && operator will make use of this. This can make queries much much faster.

You will also make use of spatial functions, such as Distance(), ST_Intersects(), ST_Contains() and ST_Within(), among others, to narrow down the results of your search. Most spatial queries include both an indexed test and a spatial function test. The index test serves to limit the number of return tuples to only tuples that might meet the condition of interest. The spatial functions are then use to test the condition exactly.

SELECT id, the_geom  FROM thetable
WHERE
the_geom && 'POLYGON((0 0, 0 10, 10 10, 10 0, 0 0))'
AND
ST_Contains(the_geom,'POLYGON((0 0, 0 10, 10 10, 10 0, 0 0))');

Answer 2

ST_Contains() returns a boolean (true/false) whether one geometry falls within another. Suggest you use ST_Intersects() because it's faster and you don't have to be so careful with the order of the arguments. Note:

There are certain subtleties to ST_Contains and ST_Within that are not intuitively obvious. For details check out Subtleties of OGC Covers, Contains, Within