\begin{table}%t2
%\begin{center}
\par
\newcommand{\lw}[1]{\smash{\lower2.ex\hbox{#1}}}
\caption{Relative positions of the identified H$_2$O maser features.}
\label{tbl:posfeature}
\begin{tabular}{lrrrrrrrrrrrrrrrr}
\hline\hline
       & & \multicolumn{3}{c}{Epoch I} & & \multicolumn{3}{c}{Epoch II} & & \multicolumn{3}{c}{Epoch III} & & \multicolumn{3}{c}{Epoch IV} \\
\cline{3-5}\cline{7-9}\cline{11-13}\cline{15-17}   
Features & & $\Delta\alpha^a$ & $\Delta\delta^b$ & \multicolumn{1}{c}{$\sigma^c$} & & $\Delta\alpha$ & $\Delta\delta$ & \multicolumn{1}{c}{$\sigma$} & & $\Delta\alpha$ & $\Delta\delta$ & \multicolumn{1}{c}{$\sigma$} & & $\Delta\alpha$ & $\Delta\delta$ & \multicolumn{1}{c}{$\sigma$} \\
       & & (mas) & (mas) & (mas) & & (mas) & (mas) & (mas) & & (mas) & (mas) & (mas) & & (mas) & (mas) & (mas) \\
\hline
2     & & $-$0.046 &    3.928 & 0.011 & &    0.049 &    4.094 & 0.014 & & $-$0.133 &    3.860 & 0.012 & & \nodata & \nodata & \nodata \\
4     & & $-$0.160 &    1.500 & 0.031 & & $-$0.124 &    1.434 & 0.010 & & $-$0.154 &    1.308 & 0.012 & & $-$0.172 & 1.510    & 0.008 \\
5     & & $-$0.133 &    0.135 & 0.008 & & $-$0.034 &    0.113 & 0.009 & & $-$0.053 & $-$0.021 & 0.018 & & $-$0.071 & $-$0.037 & 0.019 \\
9     & & $-$1.794 & $-$2.481 & 0.014 & & $-$1.679 & $-$2.436 & 0.012 & & $-$1.659 & $-$2.398 & 0.019 & & $-$1.684 & $-$2.204 & 0.010 \\
13    & & $-$5.439 & $-$1.280 & 0.010 & & $-$5.392 & $-$1.464 & 0.016 & & $-$5.349 & $-$1.600 & 0.023 & & $-$5.235 & $-$1.395 & 0.019 \\
SW    & & $-$23.11 & $-$12.89 & 0.052 & & $-$23.43 & $-$13.10 &  0.04 & & $-$23.62 & $-$13.50 & 0.03 & & $-$23.97 & $-$13.60 & 0.05 \\
EHV   & &   4184.8 & $-$709.9 & 0.29  & &   4184.3 & $-$709.9 &  0.35 & &   4183.9 & $-$709.7 & 0.18  & &  4183.6  & $-$709.4 & 0.17 \\
\hline
\end{tabular}
%\end{center}
\par
$^a$~Right Ascension offset with respect to the 1.6~\kms \ spot.
$^b$~Declination offset.
$^c$~Position error.
\end{table}