gnuplot 2D polar plot with heatmap from 3D dataset - possible?

Question

I'd like to take a plot that I have done as a surface in 3D using cartesian coordinates and view it as a heatmap in 2D in POLAR coordinates. The reason for this is I am obtaining data on a system in that coordinate system. I've only found some related examples, but am getting stuck trying to get it to work with my data. I am currently using the matrix format, and I can reformat the data set if this would help get the chart working.

The data is SPL measurements taken on a loudspeaker. The microphone is positioned at a fixed distance away (e.g. fixed radius) and measurements are made at every 10 degrees around the entire loudspeaker horizontally. The SPL measurement is obtained as a function of frequency, 20Hz to 20kHz.

I would like to use gnuplot to create a 2D polar plot. Frequency would be plotted as the radius, the angle around the loudspeaker would be the angle, and the "height" would be the SPL level. This should generate a surface, however, I would like to create a heat map pm3d and then view that from above (e.g. view 0,0) or as a 2D plot. I also need to add contour lines that show intervals of SPL and superimpose that on the heat map.

I found something similar for cartesian coordinates here: http://gnuplot-tricks.blogspot.com/2009/07/maps-contour-plots-with-labels.html When I tried this approach using polar coordinates for the final 2D plot, I got an error message that the "with image" option is not supported for polar plots. Can someone try this or confirm this?

I have been able to plot my polar data as a heatmap+contour lines in 3D using splot and view from directly above (set view 0.0). If I first convert my existing polar coordinate data into cartesian coordinates I will probably get something like what is shown in this web page: how to create a 3d polar graph with gnuplot I could view this from above, too, but I would like to add in the polar grid and have labels for the angle and radius. Would I have to do this manually or could I use multiplot to overlay a 2D grid and the 3D plot viewed from 0.0?

I am not sure how to approach this. Any advice about what direction to take would be appreciated.

-Charlie

Answer 1

The image plotting works only for equally distributed rectangular grids, just like any bitmap image is arranged. Otherwise you must use splot with pm3d.

The set grid polar works only for 2D, so you must use multiplot to overlay your heatmap with the polar grid. Here a, quite lengthy, example to show you how it might work:

reset
set terminal pngcairo size 800,800
set output '3d-polar.png'

set lmargin at screen 0.05
set rmargin at screen 0.85
set bmargin at screen 0.1
set tmargin at screen 0.9

set pm3d map
unset key

set multiplot

# plot the heatmap
set parametric
set isosamples 500

unset border
unset xtics
unset ytics

set angles degree
r = 6
set urange[0:r] # radius
set vrange[0:360] # angle
set xrange[-r:r]
set yrange[-r:r]
set colorbox user origin 0.9,0.1 size 0.03,0.8
splot u*cos(v), u*sin(v), (cos(v)*besj0(2*u))**2

# now plot the polar grid only
set style line 11 lc rgb 'white' lw 2
set grid polar ls 11
set polar
set rrange[0:r]
unset raxis
set rtics format '' scale 0
unset parametric
set for [i=0:330:30] label at first (r+0.35)*cos(i), first (r+0.35)*sin(i)
center sprintf('%d', i)
plot NaN w l
unset multiplot

The result is:

And now some details about some tricks:

• In order to get a square size, you can't use set size ratio 1, because the margins differ for the 2D and 3D plots, even if you would specify some absolute margins. Therefore, I set a square canvas size (terminal option size 800,800), and set appropriate absolute margins.

• You cannot unset rtics because then the grid would disappear.

• The grid labels must be set manually.

• The colorbox was also set manually because otherwise it would have overlapped with the 0 label.

• Plotting NaN does only plot the grid