gmt-fork/doc/rst/source/explain_grd_output.rst_

Grid File Formats
-----------------

By default GMT writes out grid as single precision floats in a
COARDS-complaint netCDF file format. However, GMT is able to produce
grid files in many other commonly used grid file formats and also
facilitates so called "packing" of grids, writing out floating point
data as 1- or 2-byte integers. To specify the precision, scale and
offset, the user should add the suffix
=\ *ID*\ [**+s**\ *scale*][**+o**\ *offset*][**+n**\ *invalid*], where
*ID* is a two-letter identifier of the grid type and precision, and
*scale* and *offset* are optional scale factor and offset to be applied
to all grid values, and *invalid* is the value used to indicate missing
data. See :doc:`grdconvert` and Section
:ref:`grid-file-format`
of the GMT Technical Reference and Cookbook for more information.

When writing a netCDF file, the grid is stored by default with the
variable name "z". To specify another variable name *varname*, append
**?**\ *varname* to the file name. Note that you may need to escape the
special meaning of **?** in your shell program by putting a backslash in
front of it, or by placing the filename and suffix between quotes or
double quotes.