The chart might tell that these structures are 300 m apart. The objects are 3 kilometres away. Other physical relationships are useful for quick reference. The image on the right shows us that it is possible to estimate the height of any object that crosses the horizon as seen from our own point of view. This picture of the 'Pigeon Rocks' near Beirut harbour was taken from a crow's nest at a height of 34 meters.
All tops crossing the horizon and with bases at sea level are on eye level.
Due to leeway, currents or other factors the two course lines need not be parallel to or of same length as each other. The locus of points along which a ship's position must lie. A minimum of two LOP's are necessary to establish a fix. It is standard practice to use at least three LOP's when obtaining a fix, to guard against the possibility of and, in some cases, remove ambiguity. A position determined by crossing lines of position obtained at different times and advanced or retired to a common time.