[R-sig-Geo] Combining multiple polylines into a single polyline

[Nikhil Kaza]

Not a R solution. But does v.generalize in grass help?

http://grass.osgeo.org/wiki/V.generalize_tutorial



Nikhil Kaza
Asst. Professor,
City and Regional Planning
University of North Carolina

nikhil.list at gmail.com

On Jun 12, 2010, at 8:05 PM, Don MacQueen wrote:

> I have an input shapefile representing roads in a city, read into R  
> where it becomes a SpatialLInesDataFrame.
>
> Looking at just one road at a time, it looks fine when plotted. But  
> in the underlying structure, the single real-world road has been  
> broken up into many shorter polylines, and they are not included in  
> the "correct" order, i.e., end to end.
>
> When I sample along the road using spsample() with type='regular',  
> the points aren't equally spaced. I assumed this was because it is  
> broken up into separate lines. So I thought to combine the many  
> short polylines into a single long polyline, i.e., matching  
> endpoints of one polyline with the beginning of the next, dropping  
> redundant nodes, and creating a new SLDF object.
>
> I'm wondering if there's already an algorithm for this somewhere (in  
> R or GRASS, preferably!).
> (That's my main question)
>
>
> However, in my toy example below, consisting of just three polylines  
> making up a single road, spsample() does appear to equally space the  
> points along the entire length. So perhaps my real roads have some  
> other structural problem. Perhaps some endpoints don't match.
>
> Thanks
> -Don
>
> Here's a toy example:
>
> l1 <- matrix(c(1,1, 6,6, 4,8), byrow=TRUE, ncol=2)
> l2 <- matrix(c(4,8, 7,9), byrow=TRUE, ncol=2)
> l3 <- matrix(c(7,9, 11,10, 13,13), byrow=TRUE, ncol=2)
>
> LL1 <- Lines( list( Line(l1)) , ID='32')
> LL2 <- Lines( list( Line(l2)) , ID='147')
> LL3 <- Lines( list( Line(l3)) , ID='21')
>
> ## deliberately constructed out of order
> SL <- SpatialLines( list( LL2, LL3, LL1) )
> df <- data.frame( name=rep('Road',3))
> row.names(df) <- c('147','21','32')
> SLDF <- SpatialLinesDataFrame( SL, df)
>
> ## I would like to reduce to a structure like this:
> lc <- rbind(l1 , l2[-1,] , l3[-1,])
> df1 <- data.frame( name='Road')
> row.names(df1) <- 'a'
> SLDFfix <-  
> SpatialLinesDataFrame( SpatialLines( list( Lines( list( Line(lc)) ,  
> ID='a'))) , df1)
>
> -- 
> --------------------------------------
> Don MacQueen
> Environmental Protection Department
> Lawrence Livermore National Laboratory
> Livermore, CA, USA
> 925-423-1062
>
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