The ground overlay image requires four coordinates in order to draw a rectangle on the map by using your custom image. The rectangle is composed of two points, south-west and north-east. In order to define the best position for your ground overlay image, you will need to provide a coordinate for: the south point (first point latitude), the west point (first point longitude), the north point (second point latitude) and the east point (second point longitude). You can click the helper button to use the current map bounds as the bounds for your overlay image.