Available with 3D Analyst license.
Summary
Interpolates a raster surface from points using a twodimensional minimum curvature spline technique.
The resulting smooth surface passes exactly through the input points.
Usage
The REGULARIZED option of Spline type usually produces smoother surfaces than those created with the TENSION option.
With the REGULARIZED option, higher values used for the weight parameter produce smoother surfaces. The values entered for this parameter must be equal to or greater than zero. Typical values used are 0, 0.001, 0.01, 0.1, and 0.5. The Weight is the square of the parameter referred to in the literature as tau (t).
With the TENSION option, higher values entered for the weight parameter result in somewhat coarser surfaces, but surfaces that closely conform to the control points. The values entered must be equal to or greater than zero. Typical values are 0, 1, 5, and 10. The Weight is the square of the parameter referred to in the literature as phi (Φ).
The greater the value of Number of Points, the smoother the surface of the output raster.

Some input datasets may have several points with the same x,y coordinates. If the values of the points at the common location are the same, they are considered duplicates and have no effect on the output. If the values are different, they are considered coincident points.
The various interpolation tools may handle this data condition differently. For example, in some cases, the first coincident point encountered is used for the calculation; in other cases, the last point encountered is used. This may cause some locations in the output raster to have different values than what you might expect. The solution is to prepare your data by removing these coincident points. The Collect Events tool in the Spatial Statistics toolbox is useful for identifying any coincident points in your data.
Syntax
Spline_3d (in_point_features, z_field, out_raster, {cell_size}, {spline_type}, {weight}, {number_points})
Parameter  Explanation  Data Type 
in_point_features 
The input point features containing the zvalues to be interpolated into a surface raster.  Feature Layer 
z_field  The field that holds a height or magnitude value for each point. This can be a numeric field or the Shape field if the input point features contain zvalues.  Field 
out_raster  The output interpolated surface raster. It is always a floatingpoint raster.  Raster Layer 
cell_size (Optional)  The cell size at which the output raster will be created. This will be the value in the environment if it is explicitly set; otherwise, it is the shorter of the width or the height of the extent of the input point features, in the input spatial reference, divided by 250.  Analysis Cell Size 
spline_type (Optional)  The type of spline to be used.
 String 
weight (Optional)  Parameter influencing the character of the surface interpolation. When the REGULARIZED option is used, it defines the weight of the third derivatives of the surface in the curvature minimization expression. If the TENSION option is used, it defines the weight of tension. The default weight is 0.1.  Double 
number_points (Optional)  The number of points per region used for local approximation. The default is 12.  Long 
Code Sample
Spline example 1 (Python window)
This example inputs a point shapefile and interpolates the output surface as a TIFF raster.
import arcpy
from arcpy import env
env.workspace = "C:/data"
arcpy.Spline_3d("ozone_pts.shp", "ozone", "C:/output/splineout.tif",
2000, "REGULARIZED", 0.1)
Spline example 2 (standalone script)
This example inputs a point shapefile and interpolates the output surface as a Grid raster.
# Name: Spline_3d_Ex_02.py
# Description: Interpolate a series of points onto a rectangular
# raster using a minimum curvature spline technique.
# Requirements: 3D Analyst Extension
# Import system modules
import arcpy
from arcpy import env
# Set environment settings
env.workspace = "C:/data"
# Set local variables
inPointFeatures = "ca_ozone_pts.shp"
zField = "ozone"
outRaster = "C:/output/splineout"
cellSize = 2000.0
splineType = "REGULARIZED"
weight = 0.1
# Check out the ArcGIS 3D Analyst extension license
arcpy.CheckOutExtension("3D")
# Execute Spline
arcpy.Spline_3d(inPointFeatures, zField, outRaster, cellSize,
splineType, weight)
Environments
Licensing Information
 ArcGIS for Desktop Basic: Requires 3D Analyst or Spatial Analyst
 ArcGIS for Desktop Standard: Requires 3D Analyst or Spatial Analyst
 ArcGIS for Desktop Advanced: Requires 3D Analyst or Spatial Analyst