I have a basic Lowrance sonar unit on my bow, with the transducer located on my trolling motor.  The depth readings are helpful, but I would like to use it more productively to locate and "stay on" brush piles.  When I encounter a brush pile on my screen (when it enters the area of the cone), the image appears on my graph and slowly moves across the graph and disappears.  Why does the image on my screen do this, even when my boat is stationary and I am directly above the brush pile?

If you stay above the brush pile it will continually be recorded at the depth of the bottom and the brush pile parts.

 

When you see it displayed and then not displayed, you are not where the brush pile is anymore.

 

You will not see the "shape" of it unless you are passing across it.

 

The display records history, only the very first vertical row of screen pixels on the right side is current data.

Thanks for the response Wayne.  I didn't think I was drifting off of the structure, but it is the only explanation.  I will practice with it over known bottom structure.

drop a marker buoy when your there. you never really realize how far you drift until you have a visible waypoiny in the water