Larry Marek used to be frustrated trying to find crappie cover using distances displayed by his side-imaging (SI) sonar – that is until he figured out exactly why his SI unit seemed to be lying. (Contributed Photo)
Side-Imaging Sometimes Tells Lies
by Larry Marek
I love to fish brush piles. Tossing a jig out and slowly reeling it back in, or a minnow under a slip bobber suspended over a brush pile routinely rewards me with keeper crappie. My dog Gizzmo loves to bark at the birds and lick the crappie when I get one in the boat.
However, to fish brush piles or other crappie-holding structure you’ve got to find it first. For me, there is nothing better than to idle around the lake looking for structures on my Side Imaging (SI) screen. When I find something of interest and want to fish there, I like to drop a marker buoy on the spot.
I do this by switching to Down Imaging (DI), and driving directly over that spot, dropping my marker buoy over the back of the boat just as the structure starts to show on the screen. With the buoy in place, I can target the buoy as the boat shifts around on spot lock. Easy enough, right?
What many people do not realize is that, even though your range lines on your side-imaging display may show the structure is 20-feet away, that is actually not perfectly accurate.
That 20-feet is actually the straight-line distance from the transducer to the structure – NOT the horizontal distance from the boat. (See image below)
Imagine a triangle. The distance from the transducer to the structure (Sonar Distance) is 20 feet. The distance from the transducer to the bottom (Depth) is 10 feet in this example. That means the Lateral distance (Actual distance along the bottom to the structure) is only 17.3 feet.
Depending upon the size of the structure or brush pile, a distance variance of 2.7 feet may not matter much. However, the deeper the water and closer to the boat means a greater variance between your SI display and the actual distance from your boat. The chart below shows the distance variance in different depths.
You can see in some cases the difference between to SI display and the actual difference can be up to 17 feet… a large enough distance that you may easily miss it when trying to move directly over the top of the structure.
To know the exact lateral distance, or the distance from your boat, requires some calculation. We will skip all the detailed mathematical jargon and share the final formula anglers need to understand:
The square root of the (Sonar Distance squared) – (Depth squared)) = Lateral Distance, which is the is the actual distance from boat.
In our example of structure displayed 20 feet away in 10 feet of water, the equation looks like this:
400 (20 squared) – 100 (Depth squared) = 300 The Square Root of 300 equals 17.3 feet, the exact distance from your boat to the structure.
You probably have a calculator on your smartphone, but do you always need to use it?
Probably not, but you do need to be aware that the number of feet shown on your SI unit is not the exact lateral distance to the structure and you must adjust accordingly.
WHAT ABOUT WAYPOINTS?
Why not just set the waypoint on the SI screen then drive to that spot, or use the trolling motor to “Go to Waypoint”?
Waypoints suffer the exact same problem. If you mark a waypoint using your SI display, they too are set at the sonar distance, not the true lateral distance.
Until I figured this out, this drove me crazy! I’d carefully set the waypoint, then not be able to re-find the brush pile despite being right on the waypoint.
Just remember: the deeper the water and the closer to the boat that the brush pile shows on the SI screen, the less you need to jog the boat over to get on top of it.
Wishing my fellow anglers luck as they navigate to their own brush piles.
Larry Marek is a retired AT&T Programmer living Dayton, Tennessee with his loving wife and dog, Gizzmo. Larry enjoys crappie fishing on Watts Bar and Chickamauga lakes on the Tennessee River. You can e-mail him here.
