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<H3><A NAME="SECTION001311100000000000000">
5.1.1.1 Regular floating point coordinates</A>
</H3>
<P>
Selection of the Cartesian linear transformation with regular floating point coordinates
will result in a simple linear scaling <IMG
WIDTH="76" HEIGHT="32" ALIGN="MIDDLE" BORDER="0"
SRC="img93.png"
ALT="$u' = au + b$"> of the input coordinates. The projection
is defined by stating
<P>
<UL>
<LI>scale in inches/unit (<B>-Jx</B>) or axis length in inches (<B>-JX</B>)
</LI>
</UL>
<P>
If the <I>y</I>-scale or <I>y</I>-axis length is different from that of
the <I>x</I>-axis (which is most often the case), separate the two
scales (or lengths) by a slash, e.g., <B>-Jx</B>0.1i/0.5i or <B>-JX</B>8i/5i.
Thus, our <IMG
WIDTH="51" HEIGHT="33" ALIGN="MIDDLE" BORDER="0"
SRC="img94.png"
ALT="$y = \sqrt{x}$"> data sets will plot as shown in Figure <A HREF="#fig:GMT_linear">5.1</A>.
<P>
<DIV ALIGN="CENTER"><A NAME="fig:GMT_linear"></A><A NAME="20573"></A>
<TABLE>
<CAPTION ALIGN="BOTTOM"><STRONG>Figure 5.1:</STRONG>
Linear transformation of Cartesian coordinates.</CAPTION>
<TR><TD>
<DIV ALIGN="CENTER"><IMG
WIDTH="391" HEIGHT="209" ALIGN="BOTTOM" BORDER="0"
SRC="img95.png"
ALT="\includegraphics{scripts/GMT_linear}"></DIV></TD></TR>
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<P>
The complete commands given to produce this plot were
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<PRE>psxy -R0/100/0/10 -JX3i/1.5i -Ba20f10g10/a2f1g2WSne -Wthick,- -P -K sqrt.d > GMT_linear.ps
psxy -R -J -St0.075i -Glightgray -W -O sqrt.d10 >> GMT_linear.ps
</PRE>
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<HR><A NAME="20367"></A>
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Normally, the user's <I>x</I>-values will increase to the right
and the <I>y</I>-values will increase upwards. It should be noted
that in many situations it is desirable to have the direction of
positive coordinates be reversed. For example, when plotting
depth on the <I>y</I>-axis it makes more sense to have the positive
direction downwards. All that is required to reverse the sense of
positive direction is to supply a negative scale (or axis length).
Finally, sometimes it is convenient to specify the width (or height)
of a map and let the other dimension be computed based on the implied
scale and the range of the other axis. To do this, simply specify
the length to be recomputed as 0.
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<ADDRESS>
Paul Wessel
2010-01-14
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