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

[Patrick Giraudoux]

Le 28/07/2019 à 17:08, Roger Bivand a écrit :
> 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 


Ups... How  can I have missed this chapter of my bible ;-) ? (must admit 
I had been on google first). Will re-read it carefully and come back to 
the list with a solution or a reproducible example, indeed.

Best,

Patrick