[R] Removing polygons from shapefile of Scotland and Islands

[Jan van der Laan]

I believe mapshaper has functionality for removing small 'islands'. 
There is a webinterface for mapshaper, but I see there is also an 
R-package (see 
https://search.r-project.org/CRAN/refmans/rmapshaper/html/ms_filter_islands.html 
for island removal).

If you want to manually select which islands to keep and which to 
remove, you can split multipolygons into single polygons. I believe that 
is possible using st_cast.

But if it is just getting the relevant portion of the map on screen. 
With the plot-command and using st_viewport it is possible to set the 
part of the map that is drawn.

HTH,
Jsn


On 14-05-2024 15:16, Nick Wray wrote:
> Hello  I have a shapefile of Scotland, including the islands.  The river
> flow data I am using is only for the mainland and for a clearer and larger
> map I would like to not plot Orkney and Shetland to the north of the
> mainland, as I don't need them.
> 
> The map I have I got from
> https://borders.ukdataservice.ac.uk/easy_download_data.html?data=infuse_ctry_2011
> 
> then I put the uk shapefile onto my laptop with no problems (I have sf
> running)
> 
> the_uk<-st_read(dsn="C:/Users/nickm/Desktop/Shapefiles/infuse_ctry_2011.shp")
> 
> scotland<-the_uk[2,]
> 
> plot(scotland$geometry)
> 
> This gives me a nice map of Scotland  plus islands but obviously there are
> lots of separate polygons and if I go into the points with something like
> 
> scot_pts<-unlist(as.data.frame(scotland$geometry))
> 
> it's not at all clear how I can get rid of the points I don't want as they
> don't seem to be listed in any easy way to find where one polygon stops and
> another starts
> 
> I am wondering whether this approach is right anyway or whether there is
> some sf function which would allow me to identify the polygons I want -
> essentially the big one which is the mainland without lots of elaborate
> conversions and manipulations
> 
> Any pointers, thoughts etc much appreciated
> 
> Thanks Nick Wray
> 
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