\begin{table}%t3
%\begin{center}
\par
\newcommand{\lw}[1]{\smash{\lower2.ex\hbox{#1}}}
\caption{Mean LSR-velocities$^a$ ($V_{\rm mean}$) for the identified maser features.}
\label{tbl:spfeature}
$$
\begin{tabular}{lcrrrr}
\hline\hline
\lw{Feature} & & \multicolumn{4}{c}{$V_{\rm mean}$ (km s$^{-1}$)} \\
\cline{3-6}
        & &       I &       II &      III & IV      \\ \hline
2   & &  $-$1.2 &  $-$1.1  &  $-$1.8  & \nodata \\
4   & & $-$0.39 &  $-$0.30 &  $-$0.23 &  $-$0.1 \\
5   & &     1.6 &     1.6  &     1.6  &     1.6 \\
9   & &     7.5 &     7.0  &     6.4  &     6.1 \\
13  & & $-$0.30 &  $-$1.0  &  $-$1.6  &  $-$1.1 \\
SW  & &     3.5 &     2.9  &     2.9  &     1.8 \\
EHV & & $-$91.2 & $-$91.3  & $-$91.2  & $-$91.3 \\  
\hline
\end{tabular}
$$
%\end{center}
\par
$^a$ Errors are typically less than 0.03~\kms. 
\end{table}