slight wording clarification

git-svn-id: http://svn.einsteintoolkit.org/cactus/EinsteinInitialData/IDAxiBrillBH/trunk@90 0a4070d5-58f5-498f-b6c0-2693e757fa0f

1 files changed, 7 insertions, 5 deletions

diff --git a/doc/documentation.tex b/doc/documentation.tex
index d06fcd2..5cb2fff 100644
--- a/doc/documentation.tex
+++ b/doc/documentation.tex
@@ -173,8 +173,10 @@ takes the absolute value of $\eta$ before interpolating.
 The 2-D grid covers the region $|\eta| \in [0,\texttt{etamax}]$,
 $\theta \in [0,\pi]$, where the parameter \verb|etamax| defaults to 5.
 If any 3-D grid point's $(|\eta|,\theta)$ is outside this 2-D grid,
-this thorn will abort with a fatal error message from the interpolator,
-typically looking like this:
+this thorn will abort with a fatal error message from the interpolator.
+In practice, the most common cause of such an out-of-range point is
+the 3-D grid having a grid point at, or very near to, the origin.
+For example:
 \begin{verbatim}
 WARNING level 1 in thorn AEILocalInterp processor 0 host ic0087 (line 1007
 of /nfs/nethome/pollney/runs/CactusDev/arrangements/AEIThorns/AEILocalInterp/src
@@ -193,11 +195,11 @@ WARNING level 0 in thorn IDAxiBrillBH processor 0 host ic0087
   -> error in interpolator: ierror=   -1002
 \end{verbatim}
 
-What happened here is that the 3-D grid had a point right at the origin
+Here the 3-D grid had a point right at the origin
 (which by virtue of~\eqref{IDAxiBrillBH/eta-coord} would have given
 $\eta = -\infty$), but some software moved the grid point by $10^{-16}m$
-or so to avoid divisions by zero, giving
-$\eta \equiv \ln (2 \,{\times}\, 10^{-16}) \approx -36$.
+or so (the standard Cactus work-around to try to avoid divisions by zero),
+giving $\eta \equiv \ln (2 \,{\times}\, 10^{-16}) \approx -36$.
 The absolute value of this is the ``$x$'' coordinate the
 interpolator is complaining about.