**-I**\ *x_inc*\ [**+e**\|\ **n**][/\ *y_inc*\ [**+e**\|\ **n**]]
    Set the grid spacing as *x_inc* [and optionally *y_inc*].
    
    **Geographical (degrees) coordinates**: Optionally, append an increment unit.
    Choose among:

        - **d** - Indicate arc degrees
        - **m** - Indicate arc minutes
        - **s** - Indicate arc seconds

        If one of **e** (meter), **f** (foot), **k** (km), **M** (mile), **n** (nautical
        mile) or **u** (US survey foot), the the increment 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 not given or given but set to 0 it will be reset equal to *x_inc*;
        otherwise it will be converted to degrees latitude.
    
    **All coordinates**: The following modifiers are supported:
    
        - **+e** - Slightly adjust the max *x* (*east*) or *y* (*north*) to fit
          exactly the given increment if needed [Default is to slightly adjust
          the increment to fit the given domain].
        - **+n** - Define the *number of nodes* rather than the increment, in
          which case the increment is recalculated from the number of nodes,
          the *registration* (see :ref:`GMT File Formats`), and the domain.
          **Note**: If **-R**\ *grdfile* is used then the grid spacing and
          the registration have already been initialized; use |-I| and |-r|
          to override these values.
