Units

GMT uses two unit groups: dimension units (plot units) and distance units (Earth distances).

Dimension Units (Plot Units)

Use these for map widths, offsets, symbol sizes, pen widths, etc.

Code	Unit	Notes
c	Centimeter	Default for most dimensions
i	Inch	1 inch = 2.54 cm
p	Point	1 point = 1/72 inch

Defaults: If no unit is given, GMT uses PROJ_LENGTH_UNIT (default is c). Fonts and pen thicknesses default to p.

Distance Units

GMT supports various distance units for geographic calculations.

Code	Unit	Notes
d	Degree	Arc degree
m	Minute of arc	1/60 degree
s	Second of arc	1/3600 degree
e	Meter	Default distance unit
k	Kilometer	1000 meters
f	Foot	0.3048 meters
u	US Survey Foot	1200/3937 meters
M	Statute Mile	1.60934 km
n	Nautical Mile	1.852 km