gmt-fork/doc/rst/source/explain_-I.rst_

**-I**\ *xinc*\ [**+e**\|\ **n**][/\ *yinc*\ [**+e**\|\ **n**]]
    *x_inc* [and optionally *y_inc*] is the grid spacing. Optionally,
    append a suffix modifier. **Geographical (degrees) coordinates**: Append
    **m** to indicate arc minutes or **s** to indicate arc seconds. If one
    of the units **e**, **f**, **k**, **M**, **n** or **u** is appended
    instead, the increment is assumed to be given in meter, foot, km, Mile,
    nautical mile or US survey foot, respectively, and will be converted to
    the equivalent degrees longitude at the middle latitude of the region
    (the conversion depends on :term:`PROJ_ELLIPSOID`). If *y_inc* is given
    but set to 0 it will be reset equal to *x_inc*; otherwise it will be
    converted to degrees latitude. **All coordinates**: If **+e** is appended
    then the corresponding max *x* (*east*) or *y* (*north*) may be slightly
    adjusted to fit exactly the given increment [by default the increment
    may be adjusted slightly to fit the given domain]. Finally, instead of
    giving an increment you may specify the *number of nodes* desired by
    appending **+n** to the supplied integer argument; the increment is then
    recalculated from the number of nodes and the domain. The resulting
    increment value depends on whether you have selected a
    gridline-registered or pixel-registered grid; see :ref:`GMT File Formats` for
    details. **Note**: If **-R**\ *grdfile* is used then the grid spacing (and registration) have
    already been initialized; use **-I** (and **-r**) to override the values.