In this chapter I introduced the concept  of approximating the 
covariance matrix through a non-stationary steering filter.
When we add in the  tau-versus-depth
tomography of Chapter~\ref{chap:tau} we have four possible tomography
schemes to compare.  
The  four combinations I will refer to using the following
shorthand:
\begin{description}
\item [depth-laplacian]- Tomography in depth, smoothing
the slowness field (compared to the change in slowness smoothing of
Chapter~\ref{chap:tau}) with an isotropic smoother.

\item [tau-laplacian] - Tomography in  the tau domain, smoothing
slowness with an isotropic smoother.

\item [depth-steering] - Tomography in the depth domain, using steering
filter smoothing.

\item [tau-steering] - Tomography in the tau domain, using steering filter
smoothing.
\end{description}

For the comparison I will use the same data and initial model as used
Chapter~\ref{chap:tau}. I will show the resulting velocity model
and migration after one and after two
non-linear iterations of each tomography  method.

\subsection{First iteration}
\input{../steer/first_act}
%\subsection{Regularization versus Parameterization}
%\input{../steer/param_act}
\subsection{Two iterations}
\input{../steer/final_act}
