July 11th, 2010 | 5 Comments
In the Gnuplot demo files folder that comes with your Gnuplot installation exists the file world.dat which contains data in order to plot a map of the world. Therefore we remove the key from the figure and set a grid (the dashed line in Fig. 1). Also we remove the tics by setting the format to nothing and the scale to zero. We could also remove the tics with
unset tics, but the grid depends on the tics positions. After that we just plot the data:
unset key set grid set format '' set tics scale 0 plot 'world.dat' with lines linestyle 1
Here you can see a problem of the svg terminal of Gnuplot: it can’t produce dashed lines. In order to fix this, we can use Inkscape and open the svg file. Then pressing CRTL+F and type gray into the Style field and hit Enter. Now all the grid lines should be selected and you can set their stroke style to dashed by typing CRTL+Shift+F and choose one under Dashes. Doing so will lead to a figure shown in Fig. 2.
We can also easily draw a whole globe in 3D from the given data. Therefore we first add a gray line style, unset the border and arrange the figure margins.
set style line 2 lc rgb '#c0c0c0' lt 1 lw 2 unset border set lmargin screen 0 set bmargin screen 0 set rmargin screen 1 set tmargin screen 1
The 3D plot needs a little more settings. We have to tell Gnuplot to map the data on a sphere and using angle values in degree. Also we want to have a non transparent world, therefore we need hidden3d. We arrange the appeareance of the plot by setting the xy-plane to the lowest z value in order to avoid an
offset between the lowest z vlaue an the xy-plane. To have Europe in the center we set also the viewport.
set mapping spherical set angles degrees set hidden3d set xyplane at -1 set view 56,81
For the grid we have to remove the set grid command, because it doesn’t work with splot. So we draw the grid by our own using the parametric mode and finally plot the whole globe:
set parametric set isosamples 25 set urange[0:360] set vrange[-90:90] splot cos(v)*cos(u),cos(v)*sin(u),sin(v) with lines linestyle 2, \ 'world.dat' with lines linestyle 1
As you can see we have some problems with the data for Africa which lies behind the grid at some points. To avoid this and to make the grid dashed again we draw a grid with tinier radius and use Inkscape.
r = 0.99 splot r*cos(v)*cos(u),r*cos(v)*sin(u),r*sin(v) with lines \ linestyle 2, \
In order to select the grid in Inkscape we have to search after the Style blue for some strange reason (on another PC green was the right color to search). You may have a look at the xml data to figure this out. Therefore under Edit you will find XML Editor. We not only set the stroke style to dashed we also lowered the selected objects
to avoid that any line of the grid covered a black world line. Having done all that we will finally get the nice globe in Fig. 4.