title: Decent (r-tree, quad-tree or similar) library in ruby for searching spatial data
body: <p>I have a database of 20k+ cities with latitude and longitude and I need to make lot of nearest point queries (<em>which city is the nearest to certain lat,long point</em>) against this dataset. </p>
  
  <p>I guess an R-Tree or QuadTree would be a perfect data structure for this, but I haven't been able to find a working ruby implementation. Do you know any?</p>