algorithm for locating services providers in closest proximity to the Homeless Helper user

##In resource.py

• def list(self, kind, lat, lng, radius):

  db.resource.find({'location': {'$within':{'$center':[[float(lat),float(lng)],radius/100]}}, 'resource_type':kind}) returns all resources of type variable kind within radius

• calls distance_on_unit_sphere(self, lat1, long1, lat2, long2) to calculate distance from user

• sort resource (of type kind) based on distance

distance calculation: def distance_on_unit_sphere(self, lat1, long1, lat2, long2):

    # Convert latitude and longitude to
    # spherical coordinates in radians.
    degrees_to_radians = math.pi/180.0

    # phi = 90 - latitude
    phi1 = (90.0 - lat1)*degrees_to_radians
    phi2 = (90.0 - lat2)*degrees_to_radians

    # theta = longitude
    theta1 = long1*degrees_to_radians
    theta2 = long2*degrees_to_radians

    # Compute spherical distance from spherical coordinates.

    # For two locations in spherical coordinates
    # (1, theta, phi) and (1, theta, phi)
    # cosine( arc length ) =
    #    sin phi sin phi' cos(theta-theta') + cos phi cos phi'

Compute spherical distance from spherical coordinates.

    # For two locations in spherical coordinates
    # (1, theta, phi) and (1, theta, phi)
    # cosine( arc length ) =
    #    sin phi sin phi' cos(theta-theta') + cos phi cos phi'
    # distance = rho * arc length

    cos = (math.sin(phi1)*math.sin(phi2)*math.cos(theta1 - theta2) +
           math.cos(phi1)*math.cos(phi2))
    arc = math.acos( cos )

    # Remember to multiply arc by the radius of the earth
    # in your favorite set of units to get length.
    return arc