Finding Distances Between two Grids of Points

We are attempting to find the distance between 2 grids of points.  1 grid of points is based on a flat surface while the other grid of points is an intersection on a topography.  What we need to find is the vertical distance between the point on the 1st grid to its' corresponding point on the 2nd grid. We tried to measure the distance between the flattened grid lists, but it's not measuring the vertical distance.  We don't know how to measure the distances for each point without list item'ing each of the points.