Is it possible to tie the start and finish of a write on stroke to positions on vector background that is being moved and rescaled at various keyframes. For example, if you draw a line between two points on a map where the map position and scale is different at the two keyframes the write on tool does not anchor the start position to the original position on the map. Is there a way to tie the coordinates of the start position to the coordinates of the underlie map rather than the screen position at timeline position that you keyframe the start of the write on. thanks.
You are making things way too complicated. Simply create your line animation in a sufficiently sized pre-comp and use the pre-comp as a layer in teh main comp.
Providing the layers are precisely lined up when you start you can use parenting. Make the map the parent of the Write On layer, then move and scale the map to your hearts content and everything should line up.
Thank you for this guidance. I added a solid layer and parented it to the map. In the write on effect on chose on transparency. It worked as you said - of course:)