Raymarine ITC-5 Advanced Speed Calibration

[Tommo]

The Raymarine ITC-5 advanced speed calibration function allows user to create a table of five calibration points, using a multiplication factor applied to uncalibrated speed to give calibrated speed across a range of speeds.

Effectively this creates an x-y graph of uncalibrated-calibrated speeds.

Does anybody have any idea how the ITC-5 interpolates between the five calibration points?

I can think of two likely answers:

Either it assumes straight lines between the five calibration points.

Or it uses least-squares regression to generate a best fit straight line between the five points.

Anybody know which is correct?