[R-sig-Geo] gDifference

[Roger Bivand]

On Fri, 7 Jun 2013, Ross Ahmed wrote:

> Hi all
>
> I have this polygon and these lines:
>
> library(sp); library(rgeos)
> # polygon
> coords <- matrix(c(-1.798123, -1.793072, -1.805767, -1.804129, -1.798123,
> 55.68066, 55.67369, 55.67508, 55.68139, 55.68066), ncol=2)
> myPolygon <- Polygons(list(Polygon(coords)),  "myPolygon")
> myPolygon <- SpatialPolygons(list(myPolygon))
> proj4string(myPolygon) <- CRS("+proj=longlat +datum=WGS84 +ellps=WGS84
> +towgs84=0,0,0")
> plot(myPolygon)
>
> # lines
> line1 <- matrix(c(-1.79880, -1.79517, 55.67737, 55.67920), ncol=2)
> line2 <- matrix(c(-1.80231, -1.80679, 55.67764, 55.68004), ncol=2)
> line1L <- Line(line1)
> line2L <- Line(line2)
> my.lines <- Lines(list(line1L, line2L), ID="my.lines")
> myLines <- SpatialLines(list(my.lines))
> proj4string(myLines) <- CRS("+proj=longlat +datum=WGS84 +ellps=WGS84
> +towgs84=0,0,0")
> plot(myLines, add=T)
>
>
>
> I expected that the code below would remove parts of myPolygon that contain
> myLines:
>
> d <- gDifference(myPolygon, myLines)
> plot(d)
>
>
> However myPolygon seems unaffected. Why are the parts of myPolygon that
> contain myLines not being removed?

Well, lines have by definition zero width, so:

gIntersects(myPolygon, myLines)
gArea(gIntersection(myPolygon, myLines))

so there is no difference between myPolygon and d because nothing is being 
removed. If you buffer the lines, you see a difference:

gArea(myPolygon)
d <- gDifference(myPolygon, gBuffer(myLines, width=0.000001))
gArea(d)
plot(d)

http://www.vividsolutions.com/jts/bin/JTS%20Technical%20Specs.pdf
http://www.vividsolutions.com/jts/bin/JTS%20Developer%20Guide.pdf

Note that you get warnings for using operations on unprojected objects, 
because your assumption that apparently straight lines are straight is not 
justified.

Hope this clarifies,

Roger

>
> Thanks
> Ross
>
>
>
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-- 
Roger Bivand
Department of Economics, NHH Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no