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Asked: 2026-06-09T08:53:23+00:00

I am trying to superimpose an image to a world map. This is may

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I am trying to superimpose an image to a world map. This is may code

   require(reshape)
   require(mapdata)
   df <- read.table('year.dat',head=F)
   names(df) <- c("value", "x", "y", "t")
   dfc <- cast(df[ ,-4], x ~ y)
   filled.contour(as.matrix(dfc),color.palette = colorRampPalette(c("lightblue",       "blue","violet", "black")),
   xlab = "Longitude (°)", ylab = "Latitude (°)",
   xlim = c(0, 360), ylim = c(-90, 90), 
   nlevels = 25,
   plot.axes = {axis(1); axis(2);           
   map('world2Hires', 
   xlim = c(0, 360), 
   ylim = c(-90, 90), 
   add = T, col = "darkgrey")}
   )

the data contained in the file “year.dat” are in the form

 0.35217720               1         201           1
 0.26413283               1         209           1
 1.1665874                1         210           1
 ...
 0.30815500               2         194           1
 0.15407741               2         196           1
 0.15407741               2         197           1
 0.33016610               2         205           1
 ...

where the first column is scalar value, the second column is a longitude index, third column is the latitude index and the fourth column is time index. Namely the row

 0.35217720               1         1           1

means that at time 1, in the coordinate -90° N, 0° E we have the value 0.35217720. The whole lattice contains 480×241 points.
The problem is the following, if i try to execute the previous code in R, the world map and the bar legend are drown but no colours are superimposed. If i comment the line

#xlim = c(0, 360), ylim = c(-90, 90),

then the image well coloured is drawn but no world map is present. So i tried different limits in xlim and ylim and indeed the problem is the scale of the input matrix. Namely R take the input range from 0 to 1 for both axes while actually they goes from 0 to 360 and 0 to 180. In which way can i tell R that the input scale goes from 0 to 360 for x axes and from 0 to 180 for y axes?

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1 Answer

  1. Editorial Team

    The solution is:

    require(reshape) 
require(mapdata)
require(mapproj)
df <- read.table('year.dat',head=F)
names(df) <- c("value", "x", "y", "t")
dfc <- cast(df[ ,-4], x ~ y)
mm<-as.matrix(dfc,ncol=480,nrow=241)
#pdf('mappamondo.pdf')
 filled.contour(x=seq(0,360,length.out=480),y=seq(-90,90,length.out=241),mm,color.palette = colorRampPalette(c("lightblue", "blue","violet", "black")),
xlab = "Longitude (°)", ylab = "Latitude (°)",
plot.axes = {axis(1); axis(2);          
map('world2Hires',
xlim = c(0, 360), 
ylim = c(-90, 90), 
add = T, col = "black")}
)

#dev.off()
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