Re: geographic plotting

[Allin C. <cot...@wf...>]

On Tue, 14 Apr 2020, Ethan A Merritt wrote:

> 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.

In general the data one wants to represent via color on a map will 
not be present in the outlines file; they'll come from some other 
source (as in my population data).

>   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

Nice, thanks. Using a gnuplot array for the "extra" data is clearly 
a good solution. My only problem is that I'm trying to come up with 
something that works for gnuplot >= 5.0 (and arrays are new in 5.2).

I've found one alternative to my original approach: In preparing the 
data for gnuplot, I append a third column after the coordinate pairs 
for the vertices of the outlines. The values in this column repeat 
the population (or whatever) value for each entity. So now I can do 
something like:

set cbrange [0:1]
set palette defined (0 'white', 1 'steelblue')
...
plot for [i=0:*] 'states.dat' index i with filledcurves \
  fc palette, 'states.dat' using 1:2 with lines

where a little slice of one of the states in 'states.dat' looks 
like this:

# longitude, latitude, popratio
-85.799227 41.763535 0.25275361
-86.068302 41.764628 0.25275361
-86.234565 41.764864 0.25275361
-86.525181 41.765540 0.25275361
-86.834829 41.765504 0.25275361

Allin Cottrell