saving longitude and latitude etc…

i’ve been following this post which tells you how to get add more meta data from your images’ exif data.

I’m trying to add longitude and lat – the below nearly works and adds the metadata fields into the table _postmeta but the values are empty.

i’m not sure how to express the $file variable in add_filter(‘wp_read_image_metadata’, ‘add_geo_exif’ );
the post says to use
add_filter(‘wp_read_image_metadata’, ‘add_geo_exif’, , 3); but that doesn’t work?

(placed in the templates’ functions.php)

add_geo_exif($meta,$file,$sourceImageType) {
	list(,,$sourceImageType) = getimagesize( $file );
		$exif = exif_read_data( $file );
			$meta['latitude'] = $exif['GPSLatitude'] ;
			$meta['latitude_ref'] = trim( $exif['GPSLatitudeRef'] );
			$meta['longitude'] = $exif['GPSLongitude'] ;
			$meta['longitude_ref'] = trim( $exif['GPSLongitudeRef'] );

	return $meta;
}
add_filter('wp_read_image_metadata', 'add_geo_exif');

any help much appreciated!