GMT tutorial for beginners (codes included)

Utpal Kumar   5 minute read      

The Generic Mapping Tools is widely used across Earth and Planetary and other fields of studies to process data and generate high quality illustrations. This post is first introduction to beginners for getting started in GMT.

Please note that recently GMT version 6 has been released but this post is focussed on GMT version 5. For readers interested in GMT 6, I recommend beginners to get started with pyGMT.

Key idea — a GMT map is built by layering commands. Every GMT figure starts by fixing the region (-R) and projection (-J), then stacks layers on top: a frame (-B), coastlines, data points (psxy), contours (grdcontour), a colorbar (psscale). The examples below use GMT 5 classic mode, where each ps* command appends to a single PostScript file and you thread them together with -K (“more layers coming”) and -O (“overlay”). GMT 6’s modern mode wraps the same idea in gmt begin … gmt end and drops that bookkeeping — but the mental model (region → projection → layers) is identical.

GMT 5 classic mode versus GMT 6 modern mode GMT 5 classic mode chains separate ps commands that each append to a PostScript file, using -K to signal more layers are coming and -O to overlay, then converts the PostScript to an image. GMT 6 modern mode wraps the same map in gmt begin and gmt end, drops the -K and -O bookkeeping, and writes the image directly. GMT 5 · classic mode (what this tutorial uses) gmt psbasemap -R -J -B -K > map.ps gmt pscoast -O -K ... >> map.ps gmt psxy data -O ... >> map.ps gmt psconvert map.ps -Tg -K = "more layers coming" -O = "overlay on existing PostScript" you manage the PostScript by hand GMT 6 · modern mode (recommended today — GMT 6.6) gmt begin map png gmt basemap -R -J -B gmt coast ... gmt plot data ... gmt end show no -K / -O, no .ps file to convert the session handles the PostScript same map, less bookkeeping — classic still runs in GMT 6, but modern mode is the way forward
GMT 5 classic mode chains PostScript layers with -K/-O; GMT 6 modern mode wraps the same map in gmt begin … gmt end.

GMT 5 is old — the current release is GMT 6.6. This tutorial targets GMT 5 classic mode, which still runs, but GMT has been on the 6.x series for years (latest 6.6.0) with modern mode as the recommended interface: the ps* commands became basemap/coast/plot, -K/-O are gone, and figures are wrapped in gmt begin … gmt end. See the companion post on the modern interface to GMT, or drive GMT 6 from Python with PyGMT. The concepts and projections below carry straight over.

How to install GMT

In Ubuntu: sudo apt-get install gmt gmt-dcw gmt-gshhg

In Mac: brew install gmt

For other operating systems, check the GMT download page

Requirements:

Below is a list of examples, originally inspired from the GMT documentation. For successfully running the codes, the user need to fulfill following requirements:

  • Pre-installed GMT-5, check by typing gmt in terminal.
  • Pre-installed netcdf-5, check by typing ncdump.
  • Pre-installed ghostview package, check by typing gv.
  • Downloaded ETOPO1_Bed_g_gmt4.grd in the Data directory from the NOAA website.

Examples:

Readers can download the package containing all the following examples from here. The package consists of three directories: Data, Scripts and Figures.

  • The Data directory contains the data files required to run the scripts in the Scripts directory.
  • The Scripts directory consists of all the bash scripts for all the later examples.
  • The Figures directory consists of all the example plots from 1-24.

Linear Plots

This script contains commands for making basemap for linear projections including the log-log plot. It also explains how to add title, xlabel, ylabel, tick-marks, background-color to the plot.

GMT example figure 1: linear basemap example GMT example figure 2: linear basemap example

GMT example figure 3: linear/log basemap example GMT example figure 4: linear/log basemap example GMT example figure 5: linear/log basemap example

Plotting maps with different projections

This example explains how to plot the Mercator projection, Alber’s projection, Orthographic projection, Eckert projection.

GMT example figure 6: map projection example GMT example figure 7: map projection example

GMT example figure 8: map projection example GMT example figure 9: map projection example GMT example figure 10: map projection example

Plotting lines and symbols

This example explains the use of psxy command to plot the lines and symbols. It also contains the commands to plot the earthquake epicenter with colors representing depths and symbol size representing magnitude.

GMT example figure 11: psxy lines and symbols example GMT example figure 12: earthquake epicenters coloured by depth GMT example figure 13: earthquake epicenters sized by magnitude

Customizing plots with texts

This example explains how to type texts onto the plots. The user can even type mathematical equations.

GMT example figure 14: plot annotated with text and equations

Contour lines in GMT

This bash example explains how to plot the contour lines using the command grdcontour. It also explains how to cut the large data set using the grdcut command and obtain the information about it using the grdinfo. It also explains how to do interpolation of data (nearest neighbour and spline).

GMT example figure 15: contour lines with grdcontour GMT example figure 16: contour lines with grdcontour

GMT example figure 17: interpolated grid (nearest neighbour) GMT example figure 18: interpolated grid (spline)

Colorbars and colorscales

This example contains the description of how to make the cpt files, and plot the colorbars using psscale command. It also explains plotting the relief data.

GMT example figure 19: colorbar and cpt example GMT example figure 20: colorbar and cpt example GMT example figure 21: shaded relief with colorbar

Plotting NetCDF data in GMT

This example shows how to plot the multidimensional netcdf data in GMT.

GMT example figure 22: NetCDF data plotted in GMT

Three dimensional plots: Mesh and Surface plots

This examples script includes how to plot the data as 3D plots using two methods: mesh plot, color-coded surface.

GMT example figure 23: 3-D mesh plot GMT example figure 24: 3-D color-coded surface plot

Quick check: In GMT 5 classic mode, what do the -K and -O flags do?

  • They set the map’s colour and opacity
  • They chain PostScript layers — -K says “more layers follow” and -O says “overlay onto the existing file”
  • They control the output resolution
  • They are required only for 3-D plots

Recap

  • Every GMT figure is built the same way: set the region (-R) and projection (-J), then layer on a frame (-B), coastlines, data (psxy), contours (grdcontour), and a colorbar (psscale).
  • These examples use GMT 5 classic mode, where ps* commands append to a PostScript file and -K/-O thread the layers together, followed by psconvert to a raster image.
  • The tutorial covers the essentials: linear/log basemaps, projections (Mercator, Albers, Orthographic, Eckert), lines and symbols, text/equations, contours and interpolation, colorbars and CPTs, NetCDF grids, and 3-D mesh/surface plots.
  • GMT 5 is legacy — the maintained release is GMT 6.6 with modern mode (gmt begin/gmt end, no -K/-O); the concepts transfer directly.

Where to go next

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