Missing Values

Recognition

GQ recognizes IEEE NaNs and declared NetCDF _FillValue and missing_value attributes. Use +n<value> on an input source when a file uses an undeclared sentinel such as -99999. For text input, GMT’s -di option may also identify the input nodata value.

Output missing values are written as NaN.

Merge precedence

Interpolation, when enabled, is attempted before optional replacement from a paired secondary. Any holes left after interpolation may be filled by the secondary only when the corresponding replacement option is selected. The default is to preserve primary missing values.

At an ordinary merge node where one member of an active primary/secondary pair is finite and the other is missing, the finite member is used according to the module’s selected missing-value behavior. If neither member is finite, the result is NaN.

Gap interpolation

merge1d, ssh1d, and vertical interpolation in merge3d and ssh3d remain within contiguous valid runs by default. Their -S+g gap modifiers bridge internal runs subject to an optional maximum coordinate span, but do not extrapolate beyond the first or last valid sample.

merge2d, merge3d, topobath, elygtl, ssh2d, and ssh3d use -H to interpolate strictly internal horizontal holes before resampling and subsequent operations. Edge-connected missing regions remain missing. In the three-dimensional modules, -H operates independently on each native x-y layer, while -S+g controls internal vertical gaps.

SSH application

After requested gap filling and resampling, unresolved model values remain missing when an SSH field is applied. Valid values are multiplied by the perturbation factor.