1.6.1 The -B option

This is by far the most complicated option in GMT, but most examples of its usage are actually quite simple. Given as -–Bxinfo[/yinfo][:."title string":][W$\vert$w][E$\vert$e][S$\vert$s][N$\vert$n], this switch specifies a map boundary to be plotted by using the selected tick-mark intervals. xinfo and yinfo are of the form

[a]tick[m$\vert$c][ftick[m$\vert$c]][gtick[m$\vert$c]][l$\vert$p][:"axis label":][:,"unit label":]

where a, f, and g stand for annotation, frame, and grid interval. The m$\vert$c indicates minutes (m) or seconds (c). By default, all 4 boundaries are plotted (denoted W, E, S, N). To change this selection, append the codes for those you want (e.g., WSn). Upper case (e.g., W) will annotate in addition to draw axis/tick-marks. The title, if given, will appear centered above the plot^1.1.

Figure: Geographic map border using separate selections for anotation, frame, and grid intervals. Formatting of the anotation is controlled by the parameter DEGREE_FORMAT in your .gmtdefaults file.

Figure 1.6: Linear Cartesian projection axis. Long tickmarks accompany anotations, shorter ticks indicate frame interval. The axis label is optional. We used -R0/12/0/1 -JX3/0.4 -Ba4f2g1:Frequency::,%:.