Temperature gauge calibration...

Is there any way to calibrate the temperature gauges on a Humminbird graph. I've got 2 Helix 10's, one on the trolling motor and one on the transom and they read about 3-4 degrees different. I know there's the Temperature Offset function but how do I determine which one's correct???... Dan

You measure the water with a known accurate thermometer at the depth that the transducers are.

The trolling motor transducer has to be submerged long enough to actually measure the water temp accurately.