Creating a line that contours a grid with three.js

I have a simple square grid that has elevation values ​​derived from a dataset of ground elevation values. Is there a way that gives two (or more) X, Y coordinates to draw a line that will follow the grid surface (i.e. represent a road on the ground)?

I followed these blog posts to create an elevation mesh and I would like to add a series of lines (points of which have no height values) to the mesh without drawing them onto the texture.

Is there a way to do this without indexing the height values ​​for each point that are on a different X / Y scale?