qgis - How to find the polygon inside which a point lies?

I have a layer with polygonal features. Each feature has attributes and values. I also have a list of co-ordinates and I would like to know which feature (or polygon) the co-ordinates lie in.

Could someone please guide me on how to go about this? Is there a function in the API that can help me achieve my goal or should I use some computational geometry algorithm to do it myself? I know how to do the latter but it would save me some time if there was a built in function already.

Thanks.

Answer

Few features

What you probably want to do is:

1. Create a list of QgsPoint from your coordinates


2. Iterate over all your layer features (polygons)


3. Loop over the list of points (within the iteration)


4. Call feature.geometry().contains( point ) to check if you've got a match

Of course you can now improve performance if you e.g. know, that a point can only be contained by one polygon, you can remove a point from the list, once the appropriate polygon has been found.

Lots of features (Using SpatialIndex)

As pointed out in the comments, a spatial index can be used to speed up the process significantly.

The steps here would be

1. Create a list of QgsPoint from your coordinates


2. Create a QgsSpatialIndex


3. Iterate over all your layer features (polygons)


4. Add the features to your index with insertFeature



5. Iterate over all your points


6. Call index.intersects( QgsRectangle( point, point ) ) to check if you've got a match

There is also a code example by NathanW