Re: geographic plotting

[Ethan A M. <me...@uw...>]

On Monday, 13 April 2020 13:14:54 PDT Allin Cottrell wrote:
> On Fri, 6 Mar 2020, Ethan A Merritt wrote:
> 
> [a propos Manfred Schwarb's suggestion of using GeoJSON files
> for geographic plotting in gnuplot]
> 
> > Good call.  Geojson looks quite promising.
> > A real parser should be easy, but even a quick pass through sed
> > or a text editor makes it usable.
> > I have posted a proof-of-principle example using US state boundary
> > data from a sample file on the geojson development site:
> >
> >  http://skuld.bmsc.washington.edu/people/merritt/gnuplot/
> 
> Thanks for the example, Ethan.
> 
> I've recently returned to this issue, exploring the shapefiles 
> option. I've written some relevant utilities in C, designed to link 
> against Frank Warmerdam's lipshape. (I'll be happy to share these 
> once they're better tested.) With their help I've managed to create 
> quite a nice map of the US states, colorized by state population 
> levels. See 
> http://ricardo.ecn.wfu.edu/pub/gretl/datamaps/statepop.png .
> 
> But the method I'm using for the colorization is clunky and I'm sure 
> it can be improved upon. Here's my input file:
> 
> <gnuplot>
> set term pngcairo size 660,400
> set output 'statepop.png'
> set linetype 1 lc rgb "white" lw 1
> 
> set palette model HSV file 'statepop.hsv'
> set cbrange [0:50]
> unset colorbox
> 
> set xrange [95:210]
> set yrange [-175:-135]
> set noxtics
> set noytics
> set border 0
> unset key
> 
> plot for [i=0:*] 'merc.dat' index i with filledcurves \
>   fc palette cb i, 'merc.dat' with lines lt 1
> </gnuplot>
> 
> In the above, 'merc.dat' contains the state outlines from the .shp
> file. The population data get into the picture via the pre-processed
> 'statepop.hsv'. To produce this (with its shades of blue) I ran a
> loop across the states using H = 229/360, V = 93/100, and S[i] =
> population of state i divided by the max population value.
> 
> Any suggestions on the best way to do the colorization within 
> gnuplot? And also, perhaps, how to discretize it (by decile for 
> instance)? Is there a way to replace
> 
>    fc palette cb i
> 
> with something like
> 
>    fc palette cb <function of i and the data to be colorized>

Assuming you can get the state's population from somewhere
(maybe the header of the data block in the outlines file?)
the palette need not be involved.

   blue = 0x000044
   white = 0xffffff
   color(i) =  pop(i)/maxpop * blue + (maxpop-pop(i))/maxpop * white

If you use your array S[], that becomes
   color(i) = S[i] * blue + (1.0-S[i]) * white

Then you can plot using the color directly

   plot for [i=0:*] 'merc.dat' index i using 1:2:(color(i)) with filledcurves \
           fc rgb variable

		Ethan

> Allin Cottrell