Saturday the 22nd provided a rare but welcome mild weather day with air temps in the mid-40’s and fairly low winds making it a perfect opportunity for a trip back to the bridge at La Cygne for some more crappie action. Here are some sonar pictures for you using Garmin’s new DownVu imaging features:
Although the above images make the bridge appear to be arched – it is not. The bridge is flat all the way across. The reason it appears this way is explained here. The sonar from the transducer emits sound waves in a cone shaped pattern and listens for return signals. The electronics calculate the distance (or depth) based on the time of flight from when the echo pulse is transmitted and received back from an object underwater:
The sonar is picking up return signals from the bridge at multiple different points within the cone. The echo returns from the bridge at the outer edge of the cone have to travel farther, taking more time, than the echo returns from directly under the boat. The electronics measures the time of flight from the boat to points A and C to be longer and therefore farther away (deeper) than the signal path to point B and back. Therefore, the resulting image on the screen shows points A & C to be deeper than point B which represents the actual true depth; making the bridge appear arched instead of flat. This is basically the same reason that fish echoes appear as arches when the boat is moving. The fish arch on the screen is made of up multiple signals all measured at different distances as the boat passes by the fish.