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				Spatial Variation
				------- ---------

It is often convenient to vary parameters between shots/rps because the
seismic data varies with the geology (e.g. dip).  Think of the data as
a seismic line with time going down (-y direction) and shot distance in
the horizontal (x) direction.  Each shot or rp defines a unique place

         <---------    distance  (shots)  ------------>
shots        1  2  3  4  5  6 ........   

The shots/rps on which the user defines the parameter are called control
points.  Shots/rps between control points receive parameters that are
interpolated values of the control points, based on the number of
shots/rps between the control points and the current shot.  Shots/rps
outside ALL controls points will receive parameters that are held
constant from the end control point.

e.g. In order to vary the velocity function along the seismic line, the
velocity function (parameter VTP) is given at defined rps.
              FNO 10 vtp 1450 4.5 2200 9.0 end
              FNO 20 vtp 1400 4.0 2200 9.0 end
In this case, rps 1-10 will receive the vtp associated with rp 10.
Rps 11-19 receive interpolated velocity functions based on the distance
the rp is from rp 10 and rp 20.  Rps 20 and larger get the velocity
defined at rp 20.

The parameter LNO (last shot/rp number) DEFAULTS to FNO in the SIOSEIS
processes that utilize spatial variation so that LNO does not have to
be given on every control point.  Default means that the parameter
value is reset on every parameter list (terminated by END).  (Refer to
the SYNTAX documention for a description of parameter list).