\begin{table}%t3
\caption{ Estimated mean errors in {\it UBVRI} gradients for various datasets. \label{tab3}}
%\centerline
{
\begin{tabular}{lll}
\hline   \hline     
 X & Source & $\sigma$  \\
 \hline
$\Delta_{BV}$ &IMP02+& 0.010 \\
$\Delta_{BV}$ &Gal94 & 0.017 \\
$\Delta_{VI}$ &IMP02+& 0.017 \\
$\Delta_{VI}$ &Gal94 & 0.025 \\
$\Delta_{BR}$ &IMP02+& 0.012 \\
$\Delta_{BR}$ &PF++  & 0.020 \\
$\Delta_{UB}$ &IMP02+& 0.022 \\
$\Delta_{UB}$ &PF++  & 0.035 \\
$\Delta_{UR}$ &IMP02+& 0.030 \\
$\Delta_{UR}$ &PF++  & 0.040 \\
\hline
\end{tabular}}
\par
\smallskip
(1) Gradient studied $X$; (2) source of data; (3) estimated mean error~$\sigma$.   
The source PF++ results from merging the Fal89 and Pal90 data; IMP02+ contains 
a few OHP images besides those treated in the refered paper.
\par
\end{table}