Cray pretty much answered the question. The fan shape of a DI sonar ping IS spread out left to right (side to side), and it IS shooting straight down, but it's also ~6 degrees thick .... so the leading edge of the "fan" is actually at an angle (front to back) and therefore will pick up the lack of any return signal "under" the crossmember of the bridge before it picks up the return signal bounced back from the crossmember.
Or, in other words, it actually STARTS drawing the image from the bottom up ... but, is processed so fast in the unit that it shows on the screen almost instantaneously.
At least, that's the way I understand how it can show the "empty space" under any object it passes over/by. If it was sending out a straight down razor thin line of sonar pings, everything it picked up a return signal from would look like solid bottom.
... cp
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Originally Posted by CrappiePappy
Cray pretty much answered the question. The fan shape of a DI sonar ping IS spread out left to right (side to side), and it IS shooting straight down, but it's also ~6 degrees thick .... so the leading edge of the "fan" is actually at an angle (front to back) and therefore will pick up the lack of any return signal "under" the crossmember of the bridge before it picks up the return signal bounced back from the crossmember.
Randy Andres
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