The parameter DOTS_PR_INCH can be set by the user through the .gmtdefaults4 file or gmtset. By default it is equal to the value in the gmt_defaults.h file, which is supplied with 300 when you get GMT from us. This seems a good size for most applications, but should ideally reflect the resolution of your hardcopy device (most laserwriters have at least 300 dpi, hence our default value). GMT computes what the plot should look like in double precision floating point coordinates, and then converts these to integer coordinates at DOTS_PR_INCH resolution. This helps us find out that certain points in a path lie on top of other points, and we can remove these, making smaller paths. Small paths are important for the laserwriter bugs above, and also to make fill operations compute faster. Some users have set their DOTS_PR_INCH to very large numbers. This only makes the PostScript output bigger without affecting the appearence of the plot. However, if you want to make a plot which fits on a page at first, and then later magnify this same PostScript file to a huge size, the higher DPI is important. Your data may not have the higher resolution but on certain devices the edges of fonts will not look crisp if they are not drawn with an effective resolution of 300 dpi or so. Beware of making an excessively large path. Note that if you change dpi the linewidths produced by your -W options will change, unless you have appended p for linewidth in points.