# Getting the elevation of a specific point of a elevation gridline

In a highway Project, I have to design a road connecting to roads together. The map I have on AutoCAD is a elevation gridlines map and I have x, y and z coordinates but I don't know how to get an elevation for a point that is not exactly on a clear grid point, as the points where the grid lines intersect, the elevation is given.

If you use ACad civil engineering, C3D, it automatically creates contour lines and surfaces from the grid data.

One way would be to draw circles from the point, P to the 4 grid points surrounding it and measure their radius, say r1,r2...

then a good estimate is $$Z_P= \frac{Z1r1+Z2r2+Z3r3+Z4r4}{r1+r2+r3+r4}$$