[pstricks] psdots and dataplot

[Herbert Voss]

Adam Fenn wrote:
<>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?

rounding problems in fact of the missing floating point operations
inside TeX. Dataplot and psdot calculates the PS-coordinates in a different
way. Compare it with

\psset{unit=.1pt}%
\pspicture(28800,24000)(32500,29000)
\savedata{\mydata}[{{31560, 25315}}]
\dataplot[plotstyle=dots,dotstyle=+]{\mydata}
\psdots[dotstyle=+](31560,25315)
\endpspicture

or with

\psset{unit=.01pt}%
\pspicture(288000,240000)(325000,290000)
\savedata{\mydata}[315600, 253150]
\dataplot[plotstyle=dots,dotstyle=+]{\mydata}
\psdots[dotstyle=+](315600,253150)
\endpspicture

<>
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Herbert