[R-sig-Geo] how to specify the size of device so that it fits a SpatialPolygonsDataFrame plot exactly

[Roger Bivand]

On Sat, 27 Jul 2019, Patrick Giraudoux wrote:

> Dear listers,
>
> I would like to write jpeg (or png, etc.) maps using the function 
> graphics:jpeg (png, etc.), but with the device window exactly fitting the map 
> (= no white strip above, below or on the sides of the map). The map is 
> derived from a SpatialPolygonsDataFrame (WGS84)
>
> However, even if I set
>
> par(mar=c(0,0,0,0) and pass xaxs='i', yaxs='i' in the plot, there are still 
> white strips
>
> I tried several tricks but none are fully satisfying.
>
> Trial #1: I computed the ratio of the bounding box and using this ratio in 
> the function jpeg with the height and width arguments. I did it both in WGS84 
> and in UTM47:
>
> ratio<-(max(bbox(ChinaUTM)[1,])-min(bbox(ChinaUTM)[1,]))/(max(bbox(ChinaUTM)[2,])-min(bbox(ChinaUTM)[2,])) 
> # ratio width/height
>
> then
>
> jpeg("./MapChina.jpeg",height=8,width=8*ratio,res=300,units="in")
>
> par(mar=c(0,0,0,0)) # no margin
> plot(China,col="grey",border="white",lwd=2,xaxs='i', yaxs='i')
>
> dev.off()
>
> The best is obtained with UTM47 (planar projection), however I get (almost) 
> rid of any white strip only if I add an additionnal 1.04 coefficient 
> (obtained by trial-error)
>
> Hence:
>
> jpeg("./MapChina.jpeg",height=8,width=8*ratio*1.04,res=300,units="in")
>
> par(mar=c(0,0,0,0)) # no margin
> plot(China,col="grey",border="white",lwd=2,xaxs='i', yaxs='i')
>
> dev.off()
>
> Trial #2: The other way has been to pick up the parameter values like that:
>
> par(mar=c(0,0,0,0)) # no margin
> plot(China,col="grey",border="white",lwd=2,xaxs='i', yaxs='i')
> pt<-par()$pin
>
> ratio<-pt[2]/pt[1]
>
> jpeg("./MapChina.jpeg",height=8,width=8*ratio,res=300,units="in")
> par(mar=c(0,0,0,0)) # no margin
> plot(China,col="grey",border="white",lwd=2,xaxs='i', yaxs='i') #
>
> dev.off()
>
> Does not work either
>
> Any idea about how to deal with this ? In short how to get the exact size of 
> a SpatialPolygonDataFrame plot to make the device window exactly this  size?

This feels like a section in ASDAR, ch. 3, and code chunks 24-27 in 
https://asdar-book.org/book2ed/vis_mod.R. Could you please try that first 
by way of something reproducible?

Best wishes,

Roger

>
> Best,
>
> Patrick
>
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-- 
Roger Bivand
Department of Economics, Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
voice: +47 55 95 93 55; e-mail: Roger.Bivand using nhh.no
https://orcid.org/0000-0003-2392-6140
https://scholar.google.no/citations?user=AWeghB0AAAAJ&hl=en