Cut polygons using the outline line below the polygon layers.

Question

I would like to cut the polygonal layer, according to the height, into two parts (upper and lower part). The polygon may be convex or concave, and the position of the cut may differ from each other. The contour line has an interval of 5 m, which means that I may need to create a contour with significant condensed contour lines, for example, with an interval of 1 m. Any idea how to do this is better in ArcGIS or in R? The following is an example of performing Q:

library(sp) library(raster) r<-raster(ncol=100,nrow=100) values(r)<-rep(1:100,100) plot(r) ### I have no idea why half of the value is negative... p1<-cbind(c(-100,-90,-50,-100),c(60,70,30,30,60)) p2<-cbind(c(0,50,100,0),c(0,-25,10,0)) p1p<-Polygons(list(Polygon(p1,hole=T)),"p1") p2p<-Polygons(list(Polygon(p2,hole=T)),"p2") p<-SpatialPolygons(list(p1p,p2p),1:2) plot(p,add=T) segments(-90,80,-90,20) ##where the polygon could be devided segments(50,20,50,-30) ## 

Thanks in advance ~

Marco

Answer 1

If I understand correctly, you can use the rgeos package and its associated Spatial tools in R.

I took the trick to buffer the intersected line, and then generated a “difference” polygon from this site:

http://www.chopshopgeo.com/blog/?p=89

Generate a raster example and an overlying polygon.

 vdata <- list(x = 1:nrow(volcano), y = 1:ncol(volcano), z = volcano) ## raw polygon data created using image(vdata); xy <- locator() xy <- structure(list(x = c(43.4965355534823, 41.7658494766076, 36.2591210501883, 25.560334393145, 13.7602020508178, 18.7949251835441, 29.179041644792, 40.6645037913237, 44.2832110429707, 47.272577903027, 47.5872480988224 ), y = c(30.0641086410103, 34.1278207016757, 37.6989616034726, 40.900674136118, 32.7732500147872, 27.4781100569505, 22.5523984682652, 22.7986840476995, 24.5226831037393, 29.3252519027075, 33.8815351222414 )), .Names = c("x", "y")) ## close the polygon coords <- cbind(xy$x, xy$y) coords <- rbind(coords, coords[1,]) library(sp) ## create a Spatial polygons object poly <- SpatialPolygons(list(Polygons(list(Polygon(coords, hole = FALSE)), "1"))) ## create a contour line that cuts the polygon at height 171 cl <- contourLines(vdata, levels = 171) ## for ContourLines2SLDF library(maptools) clines <- ContourLines2SLDF(cl) 

Now, cross the polygon with the line, then buffer the line a bit and the difference, which is again with the polygon, to give the polynomial.

 library(rgeos) lpi <- gIntersection(poly, clines) blpi <- gBuffer(lpi, width = 0.000001) dpi <- gDifference(poly, blpi) 

Select the raw data and halves of the polygon manually extracted from the Spatial object.

 par(mfrow = c(2,1)) image(vdata) plot(poly, add = TRUE) plot(SpatialPolygons(list(Polygons(list(dpi@polygons[[1]]@Polygons[[1]]), "1"))), add = TRUE, col = "lightblue") image(vdata) plot(poly, add = TRUE) cl <- contourLines(vdata, levels = 171) plot(SpatialPolygons(list(Polygons(list(dpi@polygons[[1]]@Polygons[[2]]), "2"))), add = TRUE, col = "lightgreen") 

This works for this rather simple case; it may be useful for your scenario.