Re: geometry: knn search of point to many linestrings?

From: Adam Wulkiewicz (adam.wulkiewicz_at_[hidden])
Date: 2021-05-23 15:33:26

W dniu 18.05.2021 o 09:34, Jens Thiele via Boost-users pisze:
> I have lots of linestrings and want to find the k nearest linestrings to
> some point.
>
> Looking at the example
> /libs/geometry/doc/index/src/examples/rtree/polygons_shared_ptr.cpp
> I first thought this should be close to the solution and I just could
> replace the polygons with linestrings. But now I think the nearest query
> in that example only finds the nearest polygon bounding boxes and not
> the nearest polygons. Am I correct?
>
> If yes, how would one extend that example to find the nearest polygons?

Hi Jens,

Yes, your understanding is correct. Bounding boxes of polygons together 
with pointers to polygons are stored in the rtree. This is also what is 
returned by the query. So you need to calculate the distances to actual 
linestrings by yourself.

I propose you to use query iterators instead of query function. Then you 
can iterate over nearest boxes (passing the number of values stored in 
the rtree into the nearest predicate). In the loop calculate distances 
to linestrings and break when you have enough of them. You should 
probably break when the number of linestrings you have is equal to your 
K and the distance to the furthest linestring is lesser than the 
distance to the current box returned by the rtree query (because then 
you know that you will not get any closer linestring). To track the 
furthest linestring you can use std::priority_queue.

Adam