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4.19 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 ; 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 `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 values equal NaN are not normally plotted (or plotted with the special NaN color given in .gmtdefaults4). Several tools such as xyz2grd, gmtmath, and grdmath 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.

Data records that contain NaN values for the x or y columns (or the z 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 NAN_RECORDS defaults parameter. The default setting (gap) 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 (bad) makes no such interpretation and simply reports back how many bad records were skipped during reading.

Next: 4.20 GMT environment parameters Up: 4. General features Previous: 4.18 Options for COARDS-compliant Contents Index