Dimension and Distance Units

As a geospatial analysis and mapping package, GMT needs to distinguish between two types of units: those used for plotting and those used for geographic distances. The former are called dimension units (or plot units) and the latter are called distance units.

Dimension Units

Dimension units are used for map dimensions, 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