The empty value

GeoCoordinate.Empty, the default value, means "no location":

var unknown = GeoCoordinate.Empty;
unknown.IsEmpty;                      // true
unknown.Latitude;                     // double.NaN
unknown.DistanceTo(oslo);             // double.PositiveInfinity
unknown.IsWithin(oslo, 100_000);      // false: never matches

Empty coordinates are left out of the spatial index. A venue without a location never shows up in a "within 5 km" search.