Marking structure when using SI

Excuse my ignorance, but I've just started learning about imaging sonars. I ran across an old post by Humminbird Greg where he explains a structure marked at the edge of an SI screen could be a hundred feet off location since the gps receiver is located in the boat. I guess I just assumed that if you marked the structure with your cursor, the fishfinder would have enough intelligence to realize the cursor is pointing to a location 100' away and would adjust the waypoint location accordingly. Is this not the way it works? (see post #9) http://www.crappie.com/crappie/fishi...7-crappie-rig/

Arkie
I am also new at this, but this is how I understand the GPS error on marking. The variance is the horizontal distance from the Transducer which sends out the SI signal to GPS module that marks the GPS location. So for example if your transducer is on the transom and your GPS is at the console then there is a horizontal distance of 6 to 8 feet separating those two points. Multiple that by 2 and you could have an error of 12 -16 feet on the water when you try to find that point.
If you mount an external GPS directly above the Transducer on the boat, then you reduce the horizontal space between the transducer and the GPS module. So again transducer on the transom and the external GPS within 1 horizontal foot of the transducer multiple by 2 equals an error of 2 feet on the water.
This is a good video that explains the concept.
https://www.youtube.com/watch?v=4btE-Vm4fmw
Hope this helps
Dennis

Plus the GPS position of your boat can be off 10' or so (No GPS gets perfect accuracy). Then when you go to the marked waypoint that position can be anothe 10' off. Lots of error involved but it will get you in the general area where you can find it with traditional sonar and downvu

Dennis, I agree with the variance between transducer and gps receiver location. What caught my attention in the post I listed was the comment "With a Side imaging unit what you see on display could be hundreds of feet off to one side or the other. The system assumes that the transducer and GPS Receiver are in the same location so that when you Mark a waypoint the unit calculates where it is based on the GPS Receiver location." I understood that to mean your marked location (waypoint) could be logged as hundreds of feet away from it's actual location. I think what was meant is there could be a 20-40 foot error due to transducer/gps receiver locations as well as the normal satellite gps inaccuracies as MonteSS points out.

I have never used a fishfinder with internal gps and SI so not exactly sure of the process to mark something, but will assume you use the cursor to mark what you want on the SI screen (brush pile), then the fishfinder system logs the "mark" lat/lon based on the gps receiver's current position plus an offset based on where your cursor was on the SI screen. At least that seems to make sense to me and yes there will be some variance due to the factors mentioned but hopefully it will be in 10's of feet, not hundreds.

I think you misread Greg's post ..... He is talking about the error introduced as covered by cause in his post.

Greg is talking about a waypoint that may be hundreds of feet off to the side who's accuracy could be affected .... he didn't say the waypoint could be hundreds of feet inaccurate...........What he is indicating is that the further off to the side, the more your inaccuracy will be affected by GPS / transducer not being close to one another.

Yes I'm sure I did now that I've watched the video and gotten feedback. Thanks to those who commented.