Script for creating grid

I need to drag and drop some items onto a terrain for which I would need to create soem rectangular grids.

What should be the best way to create invisible multiple rectangle grids over a terrain grid?

 

If the rectangles are all in the same plane, you can just align one corner of the terrain with the <0,0,0> world position and then divide spatial positions by the width of a grid cell to get the grid position.