Thread: picture of crappie

No debate here crappiepappy, I’m just hoping it is a chance for us to learn – and I don’t rule myself out from the learning part here. I don’t think that the comparison of your car and thumb to sonar is accurate. Your brain uses the visual reference of what your eyes see and interprets size from that mainly based on past experiences. Sonar measures only distance and can show signal strength. Your thumb would show up on sonar closer and possibly as a stronger sonar signal which is expressed in colors or shading but not size or distance. The screen snapshot that Kosmo posted was with the Upper Range menu set to 7 feet deep and the Lower Range menu set to 30 feet deep. This is not a zoom function so the distances shown by the sonar should be accurate.

My calculations using a more accurate ruler and zooming in on the image this time:
- Overall height of the image = 8 inches.
- Vertical distance displayed in the image: 23 feet [30 feet – 7 = 23] or 276 inches.
- Each physical inch of the image represents 2.875 feet [23 (feet) /8 (inches)] of water distance or 34.5 inches [2.875 x 12].
- 1/10th of an inch represents 3.45 inches of water distance [34.5/10].
- The fish measured 2/10 to 3/10 of an inch in height.
- So the fish was somewhere between 6.9 to 10.35 inches in height [2 x 3.45, 3 x 3.45].

Now, since I was not working from the original image and I was using a ruler to measure things with I could be off a bit due to blurring the image by zooming in on it, but I believe that my calculations are in order. If not, please tell me where I messed up.

Something else to ponder…

You know what amazes me most about these screen pics? The orientation of the fish to the transducer and therefore surface of the water. Stay with here: when we look at these pictures we are looking at them as if we were looking into an aquarium – from the side. That is not how conventional 2D or down imaging sonar looks at them though; they are looking at them from above. So while we see the side of the fish (if he is swimming normally), the sonar sees only the top of the fish. Try this: take your wallet and hold it up in front so that you can see the side of it. That is what you see looking into an aquarium. Now hold it so that you are looking down at it but hold it so that you can only see an edge. This is what the sonar should be seeing. Look at the picture that we have been discussing or even the School of LM Bass picture. Looks like we are looking into an aquarium doesn’t it? So doesn’t that mean that these fish were at least partially turned on their sides?

Now the last picture that Kosmo posted (should have grabbed a screen snapshot Kosmo, it would have been clearer!). This is a Di image from the Side Imaging sonar beams. These may be hitting the fish from the sides, due to being pointed at an angle, and so seeing the profile of the fish should happen more often.