sentences:  Line arrows are placed to help the divers to establish the direction they come from, when they later need to find the way out, but should not alter the general navigation information of the system, so line arrows are added only where they cannot cause other groups to become confused about the direction to the nearest air source.An arrow would be added at the point of attachment of a reel line when tying off to explore new areas, and may be removed after return and removal of the temporary line.If the place is to be marked for future reference, the arrow may be left in place.