[CA] Relative vs True

[Chuck - Raymarine - Moderator]

Tom,

This subject is addressed within Chapter 12.5 AIS vectors and Chapter 16.11 Vessel vectors (CPA graphics) overview of the LightHouse II Release 19 Operating Instructions. Please click here to view a FAQ addressing the limitations of information within product documentation and support. True motion is the motion (speed and direction) that an object is moving with respect to the ground. Relative motion is the motion (speed and direction) that another object is moving with respect to your vessel. While most operate the equipment with motion mode and vector in Relative reference frame (the default setting), the option to display information in the True reference frame is provided to support those having this preference. These subjects are covered in much greater detail within the reference materials and training specified within the FAQ referenced above.

Example: Your own vessel's speed over ground and course over ground are 10 kts and 000 respectively. A vessel located dead ahead of your has a true speed over ground and course over ground of 5 kts and 000 respectively. These are the magnitude and direction of each vessel's true vectors. The relative vector of the other vessel would have a magnitude of 5 kts and a direction of 180.