Select the 
button from the Surface
toolbar or the Spline (Surface
menu) command. The dialog box for the command is displayed.
Mark the Laced check box if you want to digitise points in a laced pattern, as shown in this example:
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Splines are used when large fit point arrays are available as source data; for example, data from CMM (co-ordinate measuring machine) systems. In this case, the data given is real (that is, the points lie on the surface).
Spline surfaces can be altered using the Entity Data (Edit menu) command.
To create a spline surface
Select the button from the Surface
toolbar or the Spline (Surface
menu) command. The dialog box for the command is displayed.
Mark the Laced check box if you want to digitise points in a laced pattern, as shown in this example:
Clear the Points check box if you want to define the surface by using entities rather than points. Entities can be curves, lines, arcs or continuous entities.
Type the U and V point data. If the Points check box is marked, type the number of fit points you will digitise in the U and V directions into the U Points and V Points boxes, respectively. If the Points check box is empty, enter the number of fit points to be created automatically into the U Points box. Enter the number of entities to be used into the V Points box.
Type a name for the surface, if required.
Select Display to set the surface display parameters. The Display parameters now appear.
Enter the required number of patches in the U and V directions into the U Patch and V Patch boxes respectively.
Enter the required number of display points in the U and V directions into the U Display and V Display boxes respectively.
Select Slopes to enter edge slope values. Enter the slope data, type the start and end slope conditions in the U direction into the U Start Angle and U End Angle boxes, respectively. Type the start and end slope conditions in the V direction into the V Start Angle and V End Angle boxes, respectively.
Click OK to accept the parameters for the command.
Make digitises as directed by the prompts. If the Points check box is marked, digitise fit points in response to the ‘Digitise fit points for grid (or return)’ prompt. The first row of points defines the U axis direction. If the Points check box is empty, make entity digitises in response to the ‘Digitise entities defining spline surface’ prompt. The first entity establishes the direction of the U axis. The UV Marker appears at the end nearest to the entity digitise.
Perform a finish.
A Spline passes through all fit points. Compare this example with those given for the B-Spline and Bezier surfaces: all these examples were created using the same set of fit points.