Plot base maps and frames
gmt basemap -Jparameters -Rwest/east/south/north[/zmin/zmax][+r][+uunit] [ -A[file] ] [ -B[p|s]parameters ] [ -Fbox ] [ -Jz|Zparameters ] [ -Lscalebar ] [ -Trose ] [ -Tmag_rose ] [ -U[stamp] ] [ -V[level] ] [ -X[a|c|f|r][xshift[u]] ] [ -Y[a|c|f|r][yshift[u]] ] [ -fflags ] [ -pflags ] [ -ttransp ] [ --PAR=value ]
Note: No space is allowed between the option flag and the associated arguments.
Creates a basic or fancy basemap with axes, fill, and titles. Several map projections are available, and the user may specify separate tick-mark intervals for boundary annotation, ticking, and [optionally] gridlines. A simple map scale or directional rose may also be plotted. At least one of the options -B, -L, or -T must be specified.
- -Jparameters (more …)
- Select map projection.
- -Rxmin/xmax/ymin/ymax[+r][+uunit] (more …)
- Specify the region of interest.
For perspective view -p, optionally append /zmin/zmax. (more …)
- No plotting is performed. Instead, we determine the geographical coordinates of the polygon outline for the (possibly oblique) rectangular map domain. The plot domain must be given via -R and -J, with no other options allowed. The sampling interval is controlled via MAP_LINE_STEP parameter. The coordinates are written to file or to standard output if no file is specified.
- -B[p|s]parameters (more …)
- Set map boundary frame and axes attributes.
- Without further options, draws a rectangular border around any map inset (-D; classic mode only), map scale (-L) or map rose (-T) using MAP_FRAME_PEN; specify a different pen with +ppen. Add +gfill to fill the logo box [no fill]. Append +cclearance where clearance is either gap, xgap/ygap, or lgap/rgap/bgap/tgap where these items are uniform, separate in x- and y-direction, or individual side spacings between logo and border. Append +i to draw a secondary, inner border as well. We use a uniform gap between borders of 2p and the MAP_DEFAULT_PEN unless other values are specified. Append +r to draw rounded rectangular borders instead, with a 6p corner radius. You can override this radius by appending another value. Finally, append +s to draw an offset background shaded region. Here, dx/dy indicates the shift relative to the foreground frame [4p/-4p] and shade sets the fill style to use for shading [gray50]. Used in combination with -D, -L or -T. To specify separate parameters for the various map features, append d|l|t to -F to specify panel parameters for just that panel [Default uses the same panel parameters for all selected map features].
- Draws a simple map scale centered on the reference point specified using one of four coordinate systems: (1) Use -Lg for map (user) coordinates, (2) use -Lj or -LJ for setting refpoint via a 2-char justification code that refers to the (invisible) map domain rectangle, (3) use -Ln for normalized (0-1) coordinates, or (4) use -Lx for plot coordinates (inches, cm, etc.). Scale is calculated for latitude slat (optionally supply longitude slon for oblique projections [Default is central meridian]), length is in km, or append unit from e|f|k|M|n|u. Change the label alignment with +aalign (choose among l(eft), r(ight), t(op), and b(ottom)). Append +f to get a “fancy” scale [Default is plain]. By default, the anchor point on the map scale is assumed to be the center of the scale (MC), but this can be changed by appending +j followed by a 2-char justification code justify (see text for list and explanation of codes). Append +l to select the default label, which equals the distance unit (meter, foot, km, mile, nautical mile, US survey foot) and is justified on top of the scale [t]. Change this by giving your own label (append +llabel). Add +o to offset the map scale by dx/dy away from the refpoint in the direction implied by justify (or the direction implied by -Dj or -DJ). Select +u to append the unit to all distance annotations along the scale (for the plain scale, +u will instead select the unit to be appended to the distance length). Cartesian projections: Origin +c is not required, +f is not allowed, and no units should be specified in +w. You must set any Cartesian data units via +l. For a vertical rather than horizontal Cartesian scale, append +v. Note: Use FONT_LABEL to change the label font and FONT_ANNOT_PRIMARY to change the annotation font. The height of the map scale is controlled by MAP_SCALE_HEIGHT, and the pen thickness is set by MAP_TICK_PEN_PRIMARY. See -F on how to place a panel behind the scale.
- Draw a map directional rose on the map at the location defined by the reference and anchor points: Give the reference point on the map for the rose using one of four coordinate systems: (1) Use g for map (user) coordinates, (2) use j for setting refpoint via a 2-char justification code that refers to the (invisible) map domain rectangle, (3) use n for normalized (0-1) coordinates, or (4) use x for plot coordinates (inches, cm, etc.) [Default]. You can offset the reference point by dx/dy in the direction implied by justify. By default, the anchor point on the directional rose is assumed to be the center of the rose (MC), but this can be changed by appending +j followed by a 2-char justification code justify (see text for list and explanation of codes). Note: If -Dj is used then justify defaults to the same as refpoint, if -DJ is used then justify defaults to the mirror opposite of refpoint. Add +o to offset the directional rose by dx/dy away from the refpoint in the direction implied by justify (or the direction implied by -Dj or -DJ). Append +wwidth to set the width of the rose in plot coordinates (in inches, cm, or points). Add +f to get a “fancy” rose, and specify in level what you want drawn. The default  draws the two principal E-W, N-S orientations, 2 adds the two intermediate NW-SE and NE-SW orientations, while 3 adds the eight minor orientations WNW-ESE, NNW-SSE, NNE-SSW, and ENE-WSW. Label the cardinal points W,E,S,N by adding +l and append your own four comma-separated strings to override the default. Skip a specific label by leaving it blank. See Placing directional map roses and -F on how to place a panel behind the directional rose.
- Draw a map magnetic rose on the map at the location defined by the reference and anchor points: Give the reference point on the map for the rose using one of four coordinate systems: (1) Use g for map (user) coordinates, (2) use j for setting refpoint via a 2-char justification code that refers to the (invisible) map domain rectangle, (3) use n for normalized (0-1) coordinates, or (4) use x for plot coordinates (inches, cm, etc.) [Default]. You can offset the reference point by dx/dy in the direction implied by justify. By default, the anchor point on the magnetic rose is assumed to be the center of the rose (MC), but this can be changed by appending +j followed by a 2-char justification code justify (see text for list and explanation of codes). Note: If -Dj is used then justify defaults to the same as refpoint, if -DJ is used then justify defaults to the mirror opposite of refpoint. Add +o to offset the color magnetic rose by dx/dy away from the refpoint in the direction implied by justify (or the direction implied by -Dj or -DJ). Append +wwidth to set the width of the rose in plot coordinates (in inches, cm, or points). Use +d to assign the magnetic declination and set dlabel, which is a label for the magnetic compass needle (Leave empty to format a label from dec, or give - to bypass labeling). With +d, both directions to geographic and magnetic north are plotted [Default is geographic only]. If the north label is * then a north star is plotted instead of the north label. Annotation and two levels of tick intervals for both geographic and magnetic directions default to 30/5/1 degrees; override these settings by appending +tints, and append six slash-separated intervals to set both the geographic (first three) and magnetic (last three) intervals. Label the cardinal points W,E,S,N by adding +l and append your own four comma-separated strings to override the default. Skip a specific label by leaving it blank. Number GMT default parameters control pens, fonts, and color. See Placing magnetic map roses and -F on how to place a panel behind the magnetic rose.
- -U[label][+c][+jjust][+odx/dy] (more …)
- Draw GMT time stamp logo on plot.
- -V[level] (more …)
- Select verbosity level [c].
- -Y[a|c|f|r][yshift[u]] (more …)
- Shift plot origin.
- -f[i|o]colinfo (more …)
- Specify data types of input and/or output columns. This applies only to the coordinates specified in the -R option.
- -p[x|y|z]azim[/elev[/zlevel]][+wlon0/lat0[/z0]][+vx0/y0] (more …)
- Select perspective view.
- -t[transp] (more …)
- Set transparency level in percent.
- -^ or just -
- Print a short message about the syntax of the command, then exits (NOTE: on Windows just use -).
- -+ or just +
- Print an extensive usage (help) message, including the explanation of any module-specific option (but not the GMT common options), then exits.
- -? or no arguments
- Print a complete usage (help) message, including the explanation of all options, then exits.
- Temporarily override a GMT default setting; repeatable. See gmt.conf for parameters.
Note: Since many GMT plot examples are very short (i.e., one module call between the gmt begin and gmt end commands), we will often present them using the quick modern mode GMT Modern Mode One-line Commands syntax, which simplifies such short scripts.
The following section illustrates the use of the options by giving some examples for the available map projections. Note how scales may be given in several different ways depending on the projection. Also note the use of upper case letters to specify map width instead of map scale.
Linear x-y plot¶
To make a linear x/y frame with all axes, but with only left and bottom axes annotated, using xscale = yscale = 1.0, ticking every 1 unit and annotating every 2, and using xlabel = “Distance” and ylabel = “No of samples”, use
gmt begin linear gmt basemap -R0/9/0/5 -Jx1 -Bf1a2 -Bx+lDistance -By+l"No of samples" -BWeSn gmt end show
As mentioned above, such simple modern mode script can take advantage of the one-liner format. We repeat the same example using the one-liner format and then only show this format for the remaining examples:
gmt basemap -R0/9/0/5 -Jx1 -Bf1a2 -Bx+lDistance -By+l"No of samples" -BWeSn -pdf linear
To make a log-log frame with only the left and bottom axes, where the x-axis is 25 cm and annotated every 1-2-5 and the y-axis is 15 cm and annotated every power of 10 but has tick-marks every 0.1, run
gmt basemap -R1/10000/1e20/1e25 -JX25cl/15cl -Bx2+lWavelength -Bya1pf3+lPower -BWS -pdf loglog
To design an axis system to be used for a depth-sqrt(age) plot with depth positive down, ticked and annotated every 500m, and ages annotated at 1 my, 4 my, 9 my etc, use
gmt basemap -R0/100/0/5000 -Jx1p0.5/-0.001 -Bx1p+l"Crustal age" -By500+lDepth -pdf power
Polar (theta,r) plot¶
For a base map for use with polar coordinates, where the radius from 0 to 1000 should correspond to 3 inch and with gridlines and ticks intervals automatically determined, use
gmt basemap -R0/360/0/1000 -JP6i -Bafg -pdf polar
Cylindrical Map Projections¶
A 10-cm-wide basemap using the Cassini projection may be obtained by
gmt basemap -R20/50/20/35 -JC35/28/10c -Bafg -B+tCassini -pdf cassini
A Mercator map with scale 0.025 inch/degree along equator, and showing the length of 5000 km along the equator (centered on 1/1 inch), may be plotted as
gmt basemap -R90/180/-50/50 -Jm0.025i -Bafg -B+tMercator -Lx1i/1i+c0+w5000k -pdf mercator
A global Miller cylindrical map with scale 1:200,000,000 may be plotted as
gmt basemap -Rg -Jj180/1:200000000 -Bafg -B+tMiller -pdf miller
Oblique Mercator [conformal]¶
To create a page-size global oblique Mercator basemap for a pole at (90,30) with gridlines every 30 degrees, run
gmt basemap -R0/360/-70/70 -Joc0/0/90/30/0.064cd -B30g30 -B+t"Oblique Mercator" -pdf oblmerc
Transverse Mercator [conformal]¶
A regular Transverse Mercator basemap for some region may look like
gmt basemap -R69:30/71:45/-17/-15:15 -Jt70/1:1000000 -Bafg -B+t"Survey area" -pdf transmerc
Equidistant Cylindrical Projection¶
This projection only needs the central meridian and scale. A 25 cm wide global basemap centered on the 130E meridian is made by
gmt basemap -R-50/310/-90/90 -JQ130/25c -Bafg -B+t"Equidistant Cylindrical" -pdf cyl_eqdist
Universal Transverse Mercator [conformal]¶
To use this projection you must know the UTM zone number, which defines the central meridian. A UTM basemap for Indo-China can be plotted as
gmt basemap -R95/5/108/20r -Ju46/1:10000000 -Bafg -B+tUTM -pdf utm
First select which of the cylindrical equal-area projections you want by deciding on the standard parallel. Here we will use 45 degrees which gives the Gall projection. A 9 inch wide global basemap centered on the Pacific is made by
gmt basemap -Rg -JY180/45/9i -Bafg -B+tGall -pdf gall
Conic Map Projections¶
A basemap for middle Europe may be created by
gmt basemap -R0/90/25/55 -Jb45/20/32/45/0.25c -Bafg -B+t"Albers Equal-area" -pdf albers
Another basemap for middle Europe may be created by
gmt basemap -R0/90/25/55 -Jl45/20/32/45/0.1i -Bafg -B+t"Lambert Conformal Conic" -pdf lambertc
Yet another basemap of width 6 inch for middle Europe may be created by
gmt basemap -R0/90/25/55 -JD45/20/32/45/6i -Bafg -B+t"Equidistant conic" -pdf econic
A basemap for north America may be created by
gmt basemap -R-180/-20/0/90 -JPoly/4i -Bafg -B+tPolyconic -pdf polyconic
Azimuthal Map Projections¶
A 15-cm-wide global view of the world from the vantage point -80/-30 will give the following basemap:
gmt basemap -Rg -JA-80/-30/15c -Bafg -B+t"Lambert Azimuthal" -pdf lamberta
Follow the instructions for stereographic projection if you want to impose rectangular boundaries on the azimuthal equal-area map but substitute -Ja for -Js.
A 15-cm-wide global map in which distances from the center (here 125/10) to any point is true can be obtained by:
gmt basemap -Rg -JE125/10/15c -Bafg -B+tEquidistant -pdf equi
A view of the world from the vantage point -100/40 out to a horizon of 60 degrees from the center can be made using the Gnomonic projection:
gmt basemap -Rg -JF-100/40/60/6i -Bafg -B+tGnomonic -pdf gnomonic
A global perspective (from infinite distance) view of the world from the vantage point 125/10 will give the following 6-inch-wide basemap:
gmt basemap -Rg -JG125/10/6i -Bafg -B+tOrthographic -pdf ortho
The -JG option can be used in a more generalized form, specifying altitude above the surface, width and height of the view point, and twist and tilt. A view from 160 km above -74/41.5 with a tilt of 55 and azimuth of 210 degrees, and limiting the viewpoint to 30 degrees width and height will product a 6-inch-wide basemap:
gmt basemap -Rg -JG-74/41.5/160/210/55/30/30/6i -Bafg -B+t"General Perspective" -pdf genper
To make a polar stereographic projection basemap with radius = 12 cm to -60 degree latitude, with plot title “Salinity measurements”, using 5 degrees annotation/tick interval and 1 degree gridlines, run
gmt basemap -R-45/45/-90/-60 -Js0/-90/12c/-60 -B5g1 -B+t"Salinity measurements" -pdf stereo1
To make a 12-cm-wide stereographic basemap for Australia from an arbitrary view point (not the poles), and use a rectangular boundary, we must give the pole for the new projection and use the -R option to indicate the lower left and upper right corners (in lon/lat) that will define our rectangle. We choose a pole at 130/-30 and use 100/-45 and 160/-5 as our corners. The command becomes
gmt basemap -R100/-45/160/-5r -JS130/-30/12c -Bafg -B+t"General Stereographic View" -pdf stereo2
Miscellaneous Map Projections¶
The Hammer projection is mostly used for global maps and thus the spherical form is used. To get a world map centered on Greenwich at a scale of 1:200000000, use
gmt basemap -Rd -Jh0/1:200000000 -Bafg -B+tHammer -pdf hammer
To make a sinusoidal world map centered on Greenwich, with a scale along the equator of 0.02 inch/degree, use
gmt basemap -Rd -Ji0/0.02i -Bafg -B+tSinusoidal -pdf sinus1
To make an interrupted sinusoidal world map with breaks at 160W, 20W, and 60E, with a scale along the equator of 0.02 inch/degree, run the following sequence of commands:
gmt begin gmt basemap -R-160/-20/-90/90 -Ji-90/0.02i -Bx30g30 -By15g15 -BWesn gmt basemap -Bx30g30 -By15g15 -Bwesn -X2.8i gmt basemap -Bx30g30 -By15g15 -BwEsn -X1.6i gmt end show
Eckert IV [equal-area]¶
Pseudo-cylindrical projection typically used for global maps only. Set the central longitude and scale, e.g.,
gmt basemap -Rg -Jkf180/0.064c -Bafg -B+t"Eckert IV" -pdf eckert4
Eckert VI [equal-area]¶
Another pseudo-cylindrical projection typically used for global maps only. Set the central longitude and scale, e.g.,
gmt basemap -Rg -Jks180/0.064c -Bafg -B+t"Eckert VI" -pdf eckert6
Projection designed to make global maps “look right”. Set the central longitude and width, e.g.,
gmt basemap -Rd -JN0/8i -Bafg -B+tRobinson -pdf robinson
Yet another projection typically used for global maps only. You can set the central longitude, e.g.,
gmt basemap -R90/450/-90/90 -JR270/25c -Bafg -B+t"Winkel Tripel" -pdf winkel
The Mollweide projection is also mostly used for global maps and thus the spherical form is used. To get a 25-cm-wide world map centered on the Dateline:
basemap -Rg -JW180/25c -Bafg -B+tMollweide -pdf mollweide
Van der Grinten¶
The Van der Grinten projection is also mostly used for global maps and thus the spherical form is used. To get a 18-cm-wide world map centered on the Dateline:
gmt basemap -Rg -JV180/18c -Bafg -B+t"Van der Grinten" -pdf grinten
If you need to plot a map but have it rotated about a vertical axis then use the -p option. For instance, to rotate the basemap below 90 degrees about an axis centered on the map, try
gmt basemap -R10/40/10/40 -JM10c -Bafg -B+t"I am rotated" -p90+w25/25 -Xc -pdf rotated
Custom Labels or Intervals¶
The -B option sets up a regular annotation interval and the annotations derive from the corresponding x, y, or z coordinates. However, some applications requires special control on which annotations to plot and even replace the annotation with other labels. This is achieved by using cintfile in the -B option, where intfile contains all the information about annotations, ticks, and even gridlines. Each record is of the form coord type [label], where coord is the coordinate for this annotation (or tick or gridline), type is one or more letters from a (annotation), i interval annotation, f tickmark, and g gridline. Note that a and i are mutually exclusive and cannot both appear in the same intfile. Both a and i requires you to supply a label which is used as the plot annotation. If not given then a regular formatted annotation based on the coordinate will occur.
For some projections, a spherical earth is implicitly assumed. A warning will notify the user if -V is set.
The -B option is somewhat complicated to explain and comprehend. However, it is fairly simple for most applications (see examples).