Tricky $centerSphere problem

[Christopher Noyes]

I want to do something with

db.<collection>.find( { <location field> :
                         { $geoWithin :
                            { $centerSphere : [ [ <x>, <y> ] , <radius> ] }
                      } } )


The collection will have a location field (long/lat) and a radius field in addition to some other fields

I want to do something like

db.<collection>.find( { [<x>,<y>] :
                         { $geoWithin :
                            { $centerSphere : [ this.location , this.radius ] }
                      } } )


to find those locations that my x,y is inside the circle


which is really the inverse of how this type of query is specified. 

[Andrew Ryder]

Hi Christopher!

You will need to do 2 queries to achieve what you want. This makes sense given that you want to search over the whole collection once to locate a specific document at the epicenter to retrieve it's radius property (what happens if there is none or multiple matches?) and then using that value, do a second query (geoWithin) over the whole collection for items within that range.

I hope this helps.

Kind regards,
Andrew

[Christopher Noyes]

I wound up storing keys and radius separately, using the geoNear command using the known long/lat and some max radius and using the distance that comes back and comparing with the radius data as a separate step. Works pretty well, not the cleanest answer.

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[Nicolai Kilian]

Hi,

I had the same problem and solved it using http://docs.mongodb.org/manual/reference/operator/query/geoIntersects/

Instead of defining your location data as point and radius, you can simulate GeoJSON-circles using this JS

function generateGeoJSONCircle(center, radius, numSides){

  var points = [], degreeStep = 360 / numSides;

  for(var i = 0; i < numSides; i++){

    var gpos = google.maps.geometry.spherical.computeOffset(center, radius, degreeStep * i);

    points.push([gpos.lng(), gpos.lat()]);

  };

  // Duplicate the last point to close the geojson ring

  points.push(points[0]);

  return {

    type: 'Polygon',

    coordinates: [ points ]

  };

};

console.log(JSON.stringify(generateGeoJSONCircle(

    new google.maps.LatLng(47.567444, 6.9999), 5000, 30)));

Then, use this function to convert your starting point into a little triangle and use mongo geoIntersect to find which circles intersect / include your triangle.

Cheers!