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Asked: 2026-05-16T19:49:28+00:00

I’m using PostGIS (1.5.1) with Rails (3.0.0rc) and a Google Map (v3), and I’m

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I’m using PostGIS (1.5.1) with Rails (3.0.0rc) and a Google Map (v3), and I’m looking for a way to query the database to retrieve all the points within a polygon (specifically, the bounds of the map viewport). At present I am sending the coordinates of the bounds to the server as such:

google.maps.event.addListener(map, 'bounds_changed', function() {
  var bounds = map.getBounds();
  var ne = bounds.getNorthEast();
  var sw = bounds.getSouthWest();
  var yMaxLat = ne.lat();
  var xMaxLng = ne.lng();
  var yMinLat = sw.lat();
  var xMinLng = sw.lng();
  //alert("bounds changed");
  updateMap(yMaxLat, xMaxLng, yMinLat, xMinLng);
});

function updateMap(yMaxLat, xMaxLng, yMinLat, xMinLng) {
  jQuery.getJSON("/places", { "yMaxLat": yMaxLat, "xMaxLng": xMaxLng, "yMinLat": yMinLat, "xMinLng": xMinLng }, function(json) {
    if (json.length > 0) {
      for (i=0; i<json.length; i++) {
        var place = json[i];
        var category = json[i].tag;
        addLocation(place,category);
      }
    }
  });
}

Then in my controller:

format.js do
  xMinLng = params[:xMinLng]
  yMinLat = params[:yMinLat]
  xMaxLng = params[:xMaxLng]
  yMaxLat = params[:yMaxLat]

  @map_places = Place.find(:all, :conditions => ["geom && ?", Polygon.from_coordinates([[[xMinLng, yMinLat], [xMinLng, yMaxLat], [xMaxLng, yMaxLat], [xMaxLng, yMinLat], [xMinLng, yMinLat]]], 4326)])
  render :json => @map_places
end

I’m basing my query on this example, but it isn’t really behaving as expected – most points are shown when the map is zoomed right out, but not those nearish to the international date line (ie. in Australia and New Zealand). The points aren’t shown at all when the map is zoomed in, for any region of the globe. I’ve read something about PostGIS’s geography type being more appropriate for this kind of query, but I don’t think that such a type is supported by GeoRuby (it is supported by GeoDjango). The output from my console for such a request is below:

Started GET "/places?yMaxLat=63.93767251746141&xMaxLng=43.033828125&yMinLat=36.88016688406946&xMinLng=-39.407578125" for 127.0.0.1 at Tue Aug 24 12:11:41 +0100 2010
Processing by PlacesController#index as JSON
Parameters: {"xMinLng"=>"-39.407578125", "yMaxLat"=>"63.93767251746141", "xMaxLng"=>"43.033828125", "yMinLat"=>"36.88016688406946"}
Place Load (0.4ms)  SELECT "places".* FROM "places"
Place Load (0.8ms)  SELECT "places".* FROM "places" WHERE (geom && '0103000020E6100000010000000500000052B81E852BB443C0DF0CF74EA970424052B81E852BB443C0686D2EA705F84F40AE47E17A54844540686D2EA705F84F40AE47E17A54844540DF0CF74EA970424052B81E852BB443C0DF0CF74EA9704240')
Completed 200 OK in 136ms (Views: 3.4ms | ActiveRecord: 1.2ms)

What is the best way to go about constructing such a query? I’m using the GeoRuby and spatial_adapter gems with my project, but I’m struggling to find much information on using them as I’ve described above. Would a distance query be the better approach to take perhaps? Thanks!

EDIT: my migration is as below:

class CreatePlaces < ActiveRecord::Migration
  def self.up
    create_table :places do |t|
      t.string :city_name
      t.integer :country_id, :null => false
      t.point :geom, :null => false, :with_z => true, :srid => 4326
      t.string :name, :null => false
      t.string :tag, :null => false

      t.timestamps
    end
add_index :places, :geom, :spatial => true
  end

  def self.down
    drop_table :places
  end
  remove_index :places, :geom
end
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1 Answer

  1. Editorial Team

    Thanks for all the input – I’m afraid this actually turned out to be a massive n00b error on my part, in getting the correlation between x/y and lng/lat mixed up, hence why the points on the map showed fine when zoomed out, but disappeared when zoomed in. I’d thoroughly recommend QGIS (thanks to scw on https://gis.stackexchange.com/ for suggesting it) for anyone starting out with GIS, I immediately saw the error when the points were shown upside down!! Thanks again all!

    BTW, slightly surprisingly, the points in NZ/Australia show up fine even when the bounding box crosses the international date line, although that might change if I add some more points in that area.

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