1. Using a vex wrangle: save v@rest position of the topography geometry, the set @P.y = 0, create a high res grid, match size and match position it with the flattened topography geo, and run a nearpoint() to the topo geo. Set the @P.y of the grid points to the v@rest.y of the nearpoint found.
   
2. Use booelan node: extrude vertically the topo lines, and use them as cutting planes of a grid with 2x2 columns and rows. Repeat the process described on point 1)
   

cncverkstad
if you mean a Closed Surface you have Files on Odforce with Otl's. No Boolean.