Many people wonder: why does a fishfinder need a compass, especially when working with side imaging?
The side imaging transducer is rigidly mounted to the boat’s hull, most often on the transom. Its task is to emit a signal strictly perpendicular to the vessel’s axis. For example, the popular GT56 transducer forms an image exactly this way.
However, a standard fishfinder does not have a built-in magnetic sensor. This means it cannot determine the actual orientation of the boat relative to the cardinal directions.
Instead, it only has:
Based on this data, the unit calculates the course, but not the boat’s true orientation.
The course calculated by the fishfinder is based on the boat’s movement over time. In other words, the device analyzes changes in coordinates and “understands” where the vessel is heading.
But here’s the problem:
the fishfinder “thinks” the boat is moving straight ahead, while in reality it may be sideways or even stern-first.
As a result, the side imaging data becomes distorted because the unit does not know where “left” and “right” actually are relative to the boat’s position.
To understand why a compass is essential for correct side imaging operation, let’s look at a simple example.
Example: Boat and River Current
Suppose you are crossing a river at a perfect right angle.
However, the boat’s speed relative to the ground (earth) will be the vector sum of these two speeds. To visualize this, imagine them as a right triangle:
Calculation Using the Pythagorean Theorem
V = √(Vboat² + Vcurrent²)
V = √(4² + 3²) = √(16 + 9) = √25 = 5 km/h
Thus, in order to cross the river “strictly perpendicular” at a boat speed of 4 km/h, the actual movement relative to the ground will occur diagonally, with a resulting speed of 5 km/h.
The side imaging transducer always works at a right angle to the boat’s axis. But under such conditions, the course calculated via GPS will deviate from the boat’s actual orientation.
As a result:
This means the coordinates you mark on the side imaging screen will be highly inaccurate.