Creating a marker file NOT wrt the ground frame

[Sietse Achterop]

I understand that the coordinates/positions in a marker file are with respect to the ground frame, correct?

In my use case, a simulation of a rower in a boat, I am interested in the speed of the rowing wrt to the ground frame.
To control the rower I want to describe its motion using a marker file that can be used with the IK tool and then the CMC tool.
But because the position wrt to the ground is the unknown here, it cannot be used in the marker data.

I would like to describe the motion of the rower and oars using a marker file wrt to the boat instead of to the ground.
If that is possible I can perform the simulation and determine the speed of the boat.

The boat is of course not relevant, the same would apply to simulating a runner where you would like to determine the speed of the runner.
Think about trying to optimize running (how muscles a used, small steps vs large steps) in a simulation environment.

How do I setup such a marker file, is it possible at all?
Thanks again, Sietse