This is a fascinating map showing a coastal view of the north side of Queen Charlotte's Island and the South Side of a Volcano six leagues to the north of that island.  It shows a detailed map of Swallow's Bay with sound depths, a scale of miles and where to obtain fresh water.  The map of Byron's Harbor is even more interesting.  There are rocks above water and mud very far from shore.  To find fresh water, you have to decide which river to go up.  The one on the left has salt water and the one on the right has fresh water.  You have 30 feet of water to work with on Swallows and 10-15' of mud to deal with on Byron's so I doubt it was used much.  Interestingly, this piece was printed in 1772 by Strahan and Cadell before Hawkesworth's book came out.  Enjoy your piece of history.