Code:
// receives the terrain height(y) at point x,z
function TTerrain.GetHeight(x,z:single) : single;
var
  rx, rz : integer;
  fx, fz,
  A, B, C, D : single;

begin
   if &#40;x <0> &#40;TerrainMaxX-1&#41;&#41; or &#40;z <0> &#40;TerrainMaxZ-1&#41;&#41; then
   begin
     result &#58;= 0.0;
     exit;
   end;
   rx &#58;= trunc&#40;x&#41;;
   rz &#58;= trunc&#40;z&#41;;
   fx &#58;= x - rx;
   fz &#58;= z - rz;

   A &#58;= HeightMap&#91;rx   ,rz&#93;;
   B &#58;= HeightMap&#91;rx+1 ,rz&#93;;
   C &#58;= HeightMap&#91;rx   ,rz+1&#93;;
   D &#58;= HeightMap&#91;rx+1 ,rz+1&#93;;

   result &#58;= &#40;A + &#40;B -A&#41; * fx&#41; + &#40;&#40;C + &#40;D - C&#41; * fx&#41; - &#40;A + &#40;B - A&#41; * fx&#41;&#41; * fz;
end;
Heightmap is a 2dimensional array containing the heights at your "vertices". calling this function gives the height at any point within the terraindimensions.

Greetings,
Dirk