def make_url(idx, df, GOOGLE_API_KEY):
    """Helper function to generate the urls for the Google Static Maps API.

    Args:
        index (str): Identifies an image.
        df (pandas.DataFrame): Stores image info.
        GOOGLE_API_KEY (str)

    Returns:
        url (str): The URL to the image.
    """
    params = {
        'center': ('{:.6f},{:.6f}'
                   .format(df.loc[idx, 'lat'], df.loc[idx, 'lon'])),
        'zoom': '19',
        'size': '640x640',
        'scale': '2',
        'maptype': 'satellite',
        'key': GOOGLE_API_KEY}
    params_str = '&'.join(['{}={}'.format(k, v) for k, v in params.items()])
    return '?'.join(['https://maps.googleapis.com/maps/api/staticmap',
                     params_str])