Animating a line in a flyover video

Hello All!

I am hoping I can get some advice, I am relatively new to Motion. In my current project I have some video that flies over a local area. I would animate a line showing the progress of someone walking the route. My issue is that the background is constantly changing and I don't know how to "fix" the line to the moving background, especially since there isn't any constant for frame of reference. Ideally I would like to have the line grow along with the flight path of the video, what I mean by that is that the line isn't really getting much longer it is just staying towards the center of the frame.

Welcome, Jeremy, to fcp.co

Is that flyover of constant speed and in a straight direction?
Or, 'drone flight', lots of curves, and rolls and ups and downs?

Is that fly-over a real recording, or a map-animation in Motion?

diff. tasks, diff. options to solve that problem
could be a simple Write-On Behaviour or a complex, tracking, $$ 3rd party solution like DriveX …

This will be a drone flight of real footage. It is a relatively constant speed but not a straight line.