4.2.1 Overview and the .gmtdefaults4 file

Figure 4.1: Some GMT parameters that affect plot appearance.

There are about 100 parameters which can be adjusted individually to modify the appearance of plots or affect the manipulation of data. When a program is run, it initializes all parameters to the GMT defaults^4.2, then tries to open the file .gmtdefaults4 in the current directory^4.3. If not found, it will look for that file in your home directory. If successful, the program will read the contents and set the default values to those provided in the file. By editing this file you can affect features such as pen thicknesses used for maps, fonts and font sizes used for annotations and labels, color of the pens, dots-per-inch resolution of the hardcopy device, what type of spline interpolant to use, and many other choices (A complete list of all the parameters and their default values can be found in the gmtdefaults manual pages). Figures 4.1, 4.2, and 4.3 show the parameters that affect plots). You may create your own .gmtdefaults4 files by running gmtdefaults and then modify those parameters you want to change. If you want to use the parameter settings in another file you can do so by specifying +<defaultfile> on the command line. This makes it easy to maintain several distinct parameter settings, corresponding perhaps to the unique styles required by different journals or simply reflecting font changes necessary to make readable overheads and slides. Note that any arguments given on the command line (see below) will take precedent over the default values. E.g., if your .gmtdefaults4 file has x offset = 1i as default, the -X1.5i option will override the default and set the offset to 1.5 inches.

There are at least two good reasons why the GMT default options are placed in a separate parameter file:

1. It would not be practical to allow for command-line syntax covering so many options, many of which are rarely or never changed (such as the ellipsoid used for map projections).

2. It is convenient to keep separate .gmtdefaults4 files for specific projects, so that one may achieve a special effect simply by running GMT commands in a sub-directory whose .gmtdefaults4 file has the desired settings. For example, when making final illustrations for a journal article one must often standardize on font sizes and font types, etc. Keeping all those settings in a separate .gmtdefaults4 file simplifies this process. Likewise, GMT scripts that make figures for PowerPoint presentations often use a different color scheme and font size than output intended for laser printers. Organizing these various scenarios into separate .gmtdefaults4 files will minimize headaches associated with micro-editing of illustrations.

   Figure 4.2: More GMT parameters that affect plot appearance.