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<H1><A NAME="SECTION0012190000000000000000"></A>
<A NAME="18601"></A>
<A NAME="18602"></A>
<BR>
4.19 The NaN data value
</H1>

<P>
For a variety of data processing and plotting tasks there is a need to acknowledge that
a data point is missing or unassigned.  In the ``old days'' such information was passed
by letting a value like -9999.99 take on the special meaning of ``this is not really a
value, it is missing''.  The problem with this scheme is that -9999.99 (or any other
floating point value) may be a perfectly reasonable data value and in such a scenario
would be skipped.  The solution adopted in <A NAME="tex2html1228"
  HREF="http://gmt.soest.hawaii.edu"><B>GMT</B></A> is to use the IEEE concept Not-a-Number
(NaN) for this purpose.  Mathematically, a NaN is what you get if you do an undefined
mathematical operation like <IMG
 WIDTH="28" HEIGHT="31" ALIGN="MIDDLE" BORDER="0"
 SRC="img84.png"
 ALT="$0/0$">; in ASCII data files they appear as the textstring NaN.
This value is internally stored with a particular bit pattern
defined by IEEE so that special action can be taken when it is encountered by programs.
In particular, a library function called <TT>isnan</TT> is used to test if a floating point
is a NaN.  <A NAME="tex2html1229"
  HREF="http://gmt.soest.hawaii.edu"><B>GMT</B></A> uses these tests extensively to determine if a value is suitable for plotting
or processing (if a NaN is used in a calculation the result would become NaN as well).  Data points
whose values equal NaN are not normally plotted (or plotted with the special NaN color given in
<U>.gmtdefaults4</U>).  Several tools such as <A NAME="tex2html1230"
  HREF="../man/xyz2grd.html"><I><B>xyz2grd</B></I></A><A NAME="19632"></A>, <A NAME="tex2html1231"
  HREF="../man/gmtmath.html"><I><B>gmtmath</B></I></A><A NAME="19637"></A>, and
<A NAME="tex2html1232"
  HREF="../man/grdmath.html"><I><B>grdmath</B></I></A><A NAME="19642"></A> can convert user data to NaN and vice versa, thus facilitating arbitrary
masking and clipping of data sets.  Note that a few computers do not have native IEEE hardware
support.  At this point, this applies to some of the older Cray super-computers.  Users on such
machines may have to adopt the old `-9999.99'' scheme to achieve the desired results.

<P>
Data records that contain NaN values for the <I>x</I> or <I>y</I> columns (or the <I>z</I> column
for cases when 3-D Cartesian data are expected) are usually skipped during reading.  However,
the presence of these bad records can be interpreted in two different ways, and this behavior
is controlled by the <B>NAN_RECORDS</B> defaults parameter.  The default setting (<I>gap</I>)
considers such records to indicate a gap in an otherwise continuous series of points (e.g., a line),
and programs can act upon this information, e.g., not to draw a line across the gap or to break the line
into separate segments.  The alternative setting (<I>bad</I>) makes no such interpretation and
simply reports back how many bad records were skipped during reading.

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<ADDRESS>
Paul Wessel
2010-07-14
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