[pstricks] psdots and dataplot

[Adam Fenn]

I have been using pstricks with plain tex to produce geographical maps based 
upon six-figure grid references.

However, if I input the same point using \psdots and \dataplot using the 
following code the resulting points are slightly offset.

\psset{unit=.01pt}%
\pspicture(288000,240000)(325000,290000)

\savedata{\mydata}[{%
{315600, 253150}}]
\dataplot[plotstyle=dots,dotstyle=+]{\mydata}

\psdots[dotstyle=+](315600,253150)

I also use the \fileplot command to input boundaries and these too seem to 
be slightly offset. Why is this and how can I correct it?

Thanks in advance,
Adam Fenn.