Question

How do I query MongoDB for nearby geographic points using the C# driver and the GeoNear method?

The following returns points with an incorrect Distance value:

var results = myCollection.GeoNear(
    Query.GT("ExpiresOn", now), // only recent values
    latitude,
    longitude,
    20
);

I suspect I should be telling Mongo to query on the double[] Location field, but I don’t know the query syntax.

Answer 1

Found the answer via this and this:

var earthRadius = 6378.0; // km
var rangeInKm = 3000.0; // km

myCollection.EnsureIndex(IndexKeys.GeoSpatial("Location"));

var near =
    Query.GT("ExpiresOn", now);

var options = GeoNearOptions
    .SetMaxDistance(rangeInKm / earthRadius /* to radians */)
    .SetSpherical(true);

var results = myCollection.GeoNear(
    near,
    request.Longitude, // note the order
    request.Latitude,  // [lng, lat]
    200,
    options
);