gistfile1.js

// degrees to radians
var deg2rad = function(degrees) {
    return Math.PI * degrees / 180.0
  }
  // radians to degrees
var rad2deg = function(radians) {
  return 180.0 * radians / Math.PI
}

// Semi-axes of WGS-84 geoidal reference
var WGS84_a = 6378137.0 // Major semiaxis [m]
var WGS84_b = 6356752.3 // Minor semiaxis [m]

// Earth radius at a given latitude, according to the WGS-84 ellipsoid [m]
var WGS84EarthRadius = function(lat) {
  // http://en.wikipedia.org/wiki/Earth_radius
  var An = WGS84_a * WGS84_a * Math.cos(lat)
  var Bn = WGS84_b * WGS84_b * Math.sin(lat)
  var Ad = WGS84_a * Math.cos(lat)
  var Bd = WGS84_b * Math.sin(lat)
  return Math.sqrt((An * An + Bn * Bn) / (Ad * Ad + Bd * Bd))
}

// Bounding box surrounding the point at given coordinates,
// assuming local approximation of Earth surface as a sphere
// of radius given by WGS84
var boundingBox = function(latitudeInDegrees, longitudeInDegrees, halfSideInKm) {
  var lat = deg2rad(latitudeInDegrees)
  var lon = deg2rad(longitudeInDegrees)
  var halfSide = 1000 * halfSideInKm

  // Radius of Earth at given latitude
  var radius = WGS84EarthRadius(lat)
    // Radius of the parallel at given latitude
  var pradius = radius * Math.cos(lat)

  var latMin = lat - halfSide / radius
  var latMax = lat + halfSide / radius
  var lonMin = lon - halfSide / pradius
  var lonMax = lon + halfSide / pradius

  var bounds = [
    [rad2deg(latMin), rad2deg(lonMin)],
    [rad2deg(latMax), rad2deg(lonMax)]
  ];
  return bounds;
}