[R-sig-Geo] [R] polygon shapefile from line edge coordinate list

[Murray Richardson]

Hi Roger

Thanks so much for your help.  I can definitely work with this.  On your 
advice I'm responding via r-sig-geo.

Unfortunately, there is no guarantee that the line segments will come in 
any particular order from the alpha shapes routine.  In fact, there will 
also likely be line segments from  different polys mixed together, so I 
need to write the code so that the polygons are assembled only for 
connected line segments.  I was naive when I started thinking about the 
problem.  As you suggest, this could get messy. 

As an alternative I may load the segments into a postGIS linestring 
table and polygonize that, all via RODBC/postGIS.  An added benefit is 
that it would clean up dangling segments, which may also exist.

Thanks again

Murray



Roger Bivand wrote:
> Murray Richardson <murray.richardson <at> utoronto.ca> writes:
>
>   
>> Hello,
>>
>> I am looking for advice on a task I am trying to complete.
>>
>> I have a 4 column dataframe defining the start and end coordinates of 
>> line edges (from a CGAL alpha shapes function to define concave hulls 
>> from point clusters).  I would like to create polygon shapefiles from 
>> these line edges, presumably creating lines first and then polygons.
>>     
>
> How many polygons in each data.frame? If more than one, how are they 
> separated? You note below that there are multiples, are they flagged by 
> for example an NA row, or is there a jump from (endX endY) to the next 
> (startX startY)?
>
>   
>> e.g. columns are:
>>
>> startX  startY endX endY
>>
>> where each row represents start and end coordinates of a line segment.
>>     
>
> Are the line segments ordered in sequence. If they are, something like:
>
> xy0 <- data.frame(sX=c(1,2,2,1,3,4,4,3), sY=c(1,1,2,2,3,3,4,4), 
>   eX=c(2,2,1,1,4,4,3,3), eY=c(1,2,2,1,3,4,4,3))
>
> brks <- logical(nrow(xy0))
> for (i in 2:nrow(xy0)) brks[i] <- (xy0$sX[i] != xy0$eX[i-1]) & 
>   (xy0$sY[i] != xy0$eY[i-1])
> cbrks <- cumsum(brks)+1
> # find the separate polygons
>
> xy1 <- split(xy0, cbrks)
>
> library(sp)
> Plist <- vector(mode="list", length=length(xy1))
> for (i in seq(along=Plist)) {
>   crds <- rbind(cbind(xy1[[i]]$sX, xy1[[i]]$sY), 
>     c(xy1[[i]]$eX[nrow(xy1[[i]])], xy1[[i]]$eY[nrow(xy1[[i]])]))
> #   close the polygon
>   Plist[[i]] <- Polygons(list(Polygon(crds)), ID=names(xy1[i]))
> }
> # make a list of Polygons objects with IDs
>
> Plist1 <- SpatialPolygons(Plist)
> plot(Plist1, axes=TRUE)
> SPDF <- SpatialPolygonsDataFrame(Plist1, data=data.frame(i=names(xy1), 
>   row.names=names(xy1)))
> # convert this to a SpatialPolygonsDataFrame object for export
>
> library(maptools)
> writePolyShape(SPDF, "chulls")
>
> If the segments are not in "join up the dots" sequence, it will be 
> (even) more messy. Please also consider whether the R-sig-geo list 
> might not be more relevant.
>
> Roger
>
>   
>> I am new to R spatial packages and I am not sure which one is best 
>> suited to this task. There seems to be a lot of options, which is great 
>> but hard to know which one to start with.  Any suggestions on the best 
>> way to proceed with this?  One further challenge is that the list 
>> includes line segments defining multiple polygons.
>>
>> Thanks for any advice.
>>
>> Murray
>>
>> ______________________________________________
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>>
>>
>>     
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
>