Opinions on how to mark Waypoints on structure

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.

I see the same results that you do and I have asked lots of "Tech" people about it. The general answer is that Humminbird's Gps does not recognize altitude, only longitude and latitude. Therefore, there is no difference in marking the top of the water, the top of the brush, or the lake bottom as long as those points are vertically plumb. I hate it, but that's the answer I've gotten. Maybe someone else will chime in with some more favorable info.

I will mark them different depending on what it is .... sometimes on an underwater laydown, I will mark both ends.

In general, if i mark things by moving the cursor on Si and want a more accurate waypoint I will later try to get directly over it with 200Khz 2D sonar...or 455 if your unit has that 2D frequency available ... and mark the waypoint again and delete the original (There is a whole other world of theory if you want to do some research on slant range technology)

Shadboy is correct ... The altitude position does not affect the lat/lon position solution in the 2d view ...

http://www.crappie.com/crappie/side-...onar-quiz-8-a/

And Waypoints marked in the SI view are calculated as slant range distance from the xducer (with an extra calculated algorithm to compensate for the SI image compression of the near field bottom detail echoes in the SI view) ...

Marking Waypoints in the SI view should be placed where the structure meets the bottom ... Then it's a very good idea to readjust that waypoint when the boat is actually over that waypoint on the water ...

Due to inherent inaccuracies in our consumer grade marine GPS ... Accuracy can only be expected to 8-15ft anyway ...

One way to help find structure related to its marked waypoint is by "double Marking" the structure when the boat is over the structure ...

*Mark the structure well with buoys ...and then pass over the structure in a straight line ... dropping a waypoint at the beginning of the structure and dropping a waypoint at the end of the structure ...

This "double waypoint" gives a better visual reference of where the structure actually should be on the Chart view ... The structure should be between the 2 Waypoints ... And not just "somewhere around 1 waypoint" ...

Rickie