Here is the deal with marking a waypoint using si and di on your Helix. It will be close but not spot on. If you mark a waypoint using si you are marking the distance it is away from boat. It is slant range. HB could make it exact range but they don't...more chips and code to write. If you mark the waypoint using di it is not exact either.....the core series and all of the Helix models use the two piezo's in the transducer to form a composite di image....it is not spot on either.....once again slant range. For what you are wanting to do it would be better to use the 2d sonar to mark your waypoint....more accurate but still not spot on....and as Travis said...you are after a moving target.
Regards
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