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5.1.1.3 Calendar time coordinates
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<DIV ALIGN="CENTER"><A NAME="fig:GMT_linear_cal"></A><A NAME="20591"></A>
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<CAPTION ALIGN="BOTTOM"><STRONG>Figure 5.3:</STRONG>
Linear transformation of calendar coordinates.</CAPTION>
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Several particular issues arise when we seek to make linear plots using calendar date/time as
the input coordinates. As far as setting up the coordinate transformation we must indicate whether
our input data have absolute time coordinates or relative time coordinates. For the
former we append <B>T</B> after the axis scale (or width), while for the latter we append <B>t</B> at the
end of the <B>-Jx</B> (or <B>-JX</B>) option.
However, other command line arguments (like the <B>-R</B> option) may already specify whether the time
coordinate is absolute or relative. An
absolute time entry must be given as [<I>date</I>]<B>T</B>[<I>clock</I>]
(with <I>date</I> given as <I>yyyy</I>[-<I>mm</I>[-<I>dd</I>]], <I>yyyy</I>[-<I>jjj</I>], or <I>yyyy</I>[-<B>W</B><I>ww</I>[-<I>d</I>]], and <I>clock</I> using
the <I>hh</I>[:<I>mm</I>[:<I>ss</I>[<I>.xxx</I>]]] 24-hour clock format) whereas the relative time is simply
given as the units of time since the epoch followed by <B>t</B> (see <B>TIME_UNIT</B> and <B>TIME_EPOCH</B> for information
on specifying the time unit and the epoch). As a simple example, we will make a plot of a school week
calendar (Figure <A HREF="#fig:GMT_linear_cal">5.3</A>).
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<PRE>gmtset PLOT_DATE_FORMAT o TIME_WEEK_START Sunday PLOT_CLOCK_FORMAT -hham TIME_FORMAT_PRIMARY full
psbasemap -R2001-9-24T/2001-9-29T/T07:0/T15:0 -JX4i/-2i -Ba1Kf1kg1d/a1Hg1hWsNe -P > GMT_linear_cal.ps
</PRE>
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When the coordinate ranges provided by the <B>-R</B> option and the projection type given by <B>-JX</B>
(including the optional <B>d</B>, <B>g</B>, <B>t</B> or <B>T</B>) conflict, <A NAME="tex2html1224"
HREF="http://gmt.soest.hawaii.edu"><B>GMT</B></A> will warn the
users about it. In general, the options provided with <B>-JX</B> will prevail.
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<ADDRESS>
Paul Wessel
2010-01-14
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