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

Custom Labels or Intervals
--------------------------

The **-B** option sets up a regular annotation interval and the
annotations derive from the corresponding *x*, *y*, or *z* coordinates.
However, some applications requires special control on which annotations
to plot and even replace the annotation with other labels. This is
achieved by using **c**\ *intfile* in the **-B** option, where *intfile*
contains all the information about annotations, ticks, and even
gridlines. Each record is of the form *coord* *type* [*label*], where
*coord* is the coordinate for this annotation (or tick or gridline),
*type* is one or more letters from **a** (annotation), **i** interval
annotation, **f** tickmark, and **g** gridline. Note that **a** and
**i** are mutually exclusive and cannot both appear in the same
*intfile*. Both **a** and **i** requires you to supply a *label* which
is used as the plot annotation. If not given then a regular formatted
annotation based on the coordinate will occur.

Restrictions
------------

For some projections, a spherical earth is implicitly assumed. A warning
will notify the user if **-V** is set.

Bugs
----

The **-B** option is somewhat complicated to explain and comprehend.
However, it is fairly simple for most applications (see examples).