How do you guys mark structure you have found (using either the cursor or viewing software - i.e. Hummviewer). Do you mark the tip of the structure, do you mark "center of mass" or do you mark where the structure meets the bottom (shadow)? Which is the most accurate marking method?

I have set waypoints when I made BP drops, then run over the drop points and checked the BP with cursor and found coordinates to be different. Then viewed the video with HummView and gotten completely different coordinates. I run 2 different brand units in my boat, with one unit tied to a GPS trolling motor, and I have found that accuracy of waypoint coordinates is necessary for proper Goto and anchor functions. I understand that the drop coordinates are off due to mounting location of the units in the boat (internal GPS antenna on both) and the mounting location factors combined with the marking differences can throw my setup off a good deal. Once I figure out how to properly place marks using the curser and/or Hummviewer, I can make the proper offset compensations for my surface drop coordinates.