\begin{table}%t4
%\begin{center}
\par
\newcommand{\lw}[1]{\smash{\lower2.ex\hbox{#1}}}
\caption{Position shifts and proper motions for the 7 maser features.}
\label{tbl:pm}
$$
\begin{tabular}{lr@{$~\pm~$}l r@{$~\pm~$}l r@{$~\pm~$}l r@{$~\pm~$}l r@{$~\pm~$}l r@{$~\pm~$}l r@{$~\pm~$}l}
\hline\hline
\lw{Feature} & \multicolumn{2}{c}{$\Delta\alpha^a$} & \multicolumn{2}{c}{$\Delta\delta^b$} & \multicolumn{2}{c}{$|\mu|^c$} & \multicolumn{2}{c}{PA$^d$} & 
\multicolumn{2}{c}{$V_{\rm trans}^e$} & \multicolumn{2}{c}{$V_{\rm 3D}^f$} & \multicolumn{2}{c}{$i^g$} \\
   & \multicolumn{2}{c}{(mas)}    & \multicolumn{2}{c}{(mas)}   & \multicolumn{2}{c}{(mas)} & 
   \multicolumn{2}{c}{(deg)}   & \multicolumn{2}{c}{(km s$^{-1}$)}
    & \multicolumn{2}{c}{(km s$^{-1}$)}
     & \multicolumn{2}{c}{(deg)} \\ \hline
2$^h$  & $-$0.018 & 0.31  & $-$0.15 & 0.16 & (0.15&0.35) & \multicolumn{2}{c}{$\cdot\cdot\cdot$}
 & \multicolumn{2}{c}{$\cdot\cdot\cdot$} & \multicolumn{2}{c}{$\cdot\cdot\cdot$} & \multicolumn{2}{c}{$\cdot\cdot\cdot$} \\
4  &    0.013&0.13  & $-$0.017&0.011 & (0.021&0.14) & \multicolumn{2}{c}{$\cdot\cdot\cdot$}
 & \multicolumn{2}{c}{$\cdot\cdot\cdot$} & \multicolumn{2}{c}{$\cdot\cdot\cdot$}
  & \multicolumn{2}{c}{$\cdot\cdot\cdot$} \\
5  & $-$0.17& 0.034  & 0.042&0.03 & (0.18&0.076) & \multicolumn{2}{c}{$\cdot\cdot\cdot$}
 & \multicolumn{2}{c}{$\cdot\cdot\cdot$} & \multicolumn{2}{c}{$\cdot\cdot\cdot$} & \multicolumn{2}{c}{$\cdot\cdot\cdot$} \\
9  & 0.28&0.058  & 0.098&0.043 & (0.30&0.094) & \multicolumn{2}{c}{$\cdot\cdot\cdot$} 
& \multicolumn{2}{c}{$\cdot\cdot\cdot$} & \multicolumn{2}{c}{$\cdot\cdot\cdot$}
 & \multicolumn{2}{c}{$\cdot\cdot\cdot$} \\
13 & $-$0.13& 0.017  & 0.19&0.062 & (0.23&0.06) & \multicolumn{2}{c}{$\cdot\cdot\cdot$}
 & \multicolumn{2}{c}{$\cdot\cdot\cdot$} & \multicolumn{2}{c}{$\cdot\cdot\cdot$}
  & \multicolumn{2}{c}{$\cdot\cdot\cdot$} \\
SW & $-$0.75&0.078  & $-$0.85&0.085 &    1.13&0.11 & \multicolumn{2}{c}{$-$131.4}
 & 14.4&1.4  &   14.4&1.4 &  4.7&3.5 \\
EHV & $-$0.50&0.12  & +1.26 & 0.085  & 1.36 & 0.16 &  \multicolumn{2}{c}{$-$68.2} & 
17.9 & 5.2  & $-$91.3 & 29.5 & 79.4 & 0.2  \\  
\hline
\end{tabular}
$$
%\end{center}
\par
$ ^a $ and  $^b$  total position shifts of the maser features over the observing
period, namely per 95 days, along RA and Dec  directions, respectively, 
derived from the least-square fitting in Fig.~\ref{fig:rdist}.
%
$ ^c $~$|\mu|=\sqrt{(\Delta\alpha)^2+(\Delta\delta)^2}$.
For the maser features where we could not detect well-defined proper motions,
apparent position shifts are given in the parenthesis.
%
$ ^d $ Position Angle of the proper motion vector in the plane of the sky.
$ ^e $ Transverse velocity on the plane of the sky converted from $|\mu|$.
%
$ ^f $ 3-Dimensional velocity obtained from 
$V_{\rm 3D}=\sqrt{V_{\rm trans}^2+(V_{\rm mean}-V_{\rm ref})^2}$ where 
$V_{\rm mean}$ is in Table~\ref{tbl:spfeature} and
$V_{\rm ref}$ is the LSR-velocity of the reference spot (1.6~km~s$^{-1}$).
Negative and positive represent approaching and receding motions, respectively.
%
$ ^g $ Inclination angle of the $V_{\rm 3D}$ vector with respect to the plane of 
the sky, obtained from $i=\tan^{-1}(|V_{\rm mean}-V_{\rm ref}|/V_{\rm trans})$.
%
$ ^h $ All the parameters for Feature 2 are derived from the data taken in the first 3 epochs.
\end{table}