In this chapter I showed three diferent ways to characterize
non-stationary covariance function:  PEFs, variograms, and steering filters.
I showed that the geophysicist's space-varying
PEFs,  in general, do a better job and our better suited
for tomography, than the 
geostatistician's varigorams, but require significantly more information
then is often available.  Steering fiters provide  a  way to
acurately characterize non-stationary covariance with limited
information.  I applying steering filters as a regularizer
to tomography and showed that our slowness estimate is 
significantly improved over symetric representations for the covariance
function.
