From the help document, spline interpolation is supposed to produce a surface runnin through the exact input point. i have about 100 point and run spline interpolation. i also extract the interpolated value from the spline surface at the 100 input points location and compared with the original value. i found that there are some siginificant difference between the interpolated value and original value. Did anybody came across this before? Why the spline surface does not run through the input points. P.S. i used tension spline with 15 as weight and 12 as searching points, cellsize 20m of 3D analyst of ArcGIS10.