4.18 The NaN data value

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 **GMT** 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 . This value is 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 `isnan` is used to test if a floating point
is a NaN. **GMT** 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 value is NaN are not normally plotted (or plotted with the special NaN color given in
__.gmtdefaults4__). Several tools such as * xyz2grd*,