def _image_url(gsv_point, heading, **kwargs):
    """ construct a url for retrieving gsv metadata 
    at obs point location"""
    npics = kwargs.get('npics', 6)
    size = kwargs.get('size', 400)
    pad = kwargs.get('pad', 0)
    lat = gsv_point.geometry.y
    lng = gsv_point.geometry.x
    base = 'https://maps.googleapis.com/maps/api/streetview?pitch=-5'
    size = 'size={}x{}'.format(size, size)
    heading = 'heading={}'.format(heading)
    fov = 'fov={}'.format(360.0 / npics + 2 * pad)
    latlng = 'location={},{}'.format(lat, lng)
    source = 'source=outdoor'
    url = '{}&{}&{}&{}&{}&{}'.format(base, size, heading, fov, latlng, source)
    return url
