Sonar Question

This is kind of a follow up to Dave McConkey's question about sonar. I use
it, but know I'm only scraping the surface. I plan on ordering the video
from Bass Clazz, but am wondering if someone can answer a question first. I
may have asked this before, not sure.Ok here it is.

So I'm idleing along and watching my sonar graph and see all the fishies
moving across the screen. . . . so i STOP my boat. . . but the screen is
moving. How come? I mean if the boat is still shouldn't the screen show the
same thing over and over again ? For instance if I cross over a brush pile
shouldn't the brush pile just stay there and not move off screen. All very
confusing to my old simple brain, but somthing I've not been able to figure
out and a question many first timers n my boat have asked.

as usual thanks guys.

--
God Bless America

Josh The Bad Bear