R: Create polygon lists and bounding boxes from imported shapefiles

Map2poly {maptools}

R Documentation

Create polygon lists and bounding boxes from imported shapefiles

Description

Map2poly() is a simple function to make imported GIS vector polygons into a "polylist" object from a "Map" object, and Map2bbs() retrieves polygon bounding boxes; analogous functions for the "shapefiles" package are shape2poly() and shape2bbs() (thanks to Stéphane Dray for his contribution); convert.pl() serves to convert the deprecated "multipart" "polylist" format to an NA-separated format now used by plot() for polylist objects and poly2nb(). *2lines() and *2points() do the same as *2poly() for shapefiles of types 3=lines and 1=points; polygons are type=5.

Usage

Map2poly(Map, region.id = NULL)
Map2lines(Map)
Map2points(Map)
Map2bbs(Map)
shape2poly(shape, region.id = NULL)
shape2lines(shape)
shape2points(shape)
shape2bbs(shape)
convert.pl(pl)

Arguments

`Map`	object of class `"Map"`, imported from a GIS polygon vector file using unpublished package "maptools"
`shape`	list returned by `read.shapefile()`, imported from a GIS polygon vector file using the package "shapefiles"
`region.id`	character vector of region ids to be added to the neighbours list as attribute `region.id`
`pl`	list of old-style multipart polygons of class `polylist`

Details

http://sal.agecon.uiuc.edu/datasets/columbus.zip

Value

Map2poly, shape2poly, convert.pl return polylist objects, lists of polygon boundary coordinates (divided by NA where the polygon object is represented by more than one polygon); Map2lines, shape2lines return lineslist objects; Map2points, shape2points return (n x 2) matrices; and Map2bbs, shape2bbs return bounding box matrixes, c(x1, y1, x2, y2) with coordinates for the lower left corner and upper right corner.

Author(s)

Roger Bivand and Stéphane Dray, Roger.Bivand@nhh.no

Examples

shp <- try(library(shapefiles))
if (class(shp) != "try-error") {
  ShapeDir <- system.file("shapes", package="maptools")[1]
  try1 <- read.shapefile(paste(ShapeDir, "columbus", sep="/"))
  shppolys <- shape2poly(try1, as.character(try1$dbf$dbf$NEIGNO))
  plot(shppolys)
  title(main="Polygons for Columbus OH from shapefiles package")
  try3a <- read.shapefile(paste(ShapeDir, "baltim", sep="/"))
  baltptsa <- shape2points(try3a)
  xylims <- attr(baltptsa, "maplim")
  plot(xylims$x, xylims$y, asp=1, type='n', xlab="", ylab="")
  points(baltptsa)
  title(main="Baltimore points from shapefiles package")
  try4a <- read.shapefile(paste(ShapeDir, "fylk-val", sep="/"))
  fylk.vala <- shape2lines(try4a)
  xylims <- attr(fylk.vala, "maplim")
  plot(xylims$x, xylims$y, asp=1, type='n', xlab="", ylab="")
  for (i in 1:length(fylk.vala)) lines(fylk.vala[[i]])
  title(main="Norwegian river centrelines from shapefiles package")
}
try2 <- read.shape(system.file("shapes/columbus.shp", 
  package="maptools")[1])
mappolys <- Map2poly(try2, as.character(try2$att.data$NEIGNO))
plot(mappolys)
title(main="Polygons for Columbus OH from maptools package")
if (class(shp) != "try-error") {
  plot(shppolys)
  plot(mappolys, add=TRUE, border="red", lty="dotted")
  title(main="Polygons for Columbus OH overplotted from both packages")
}
try3 <- read.shape(system.file("shapes/baltim.shp", package="maptools")[1])
baltpts <- Map2points(try3)
xylims <- attr(baltpts, "maplim")
plot(xylims$x, xylims$y, asp=1, type='n', xlab="", ylab="")
points(baltpts)
title(main="Baltimore points from maptools package")
try4 <- read.shape(system.file("shapes/fylk-val.shp", package="maptools")[1])
fylk.val <- Map2lines(try4)
xylims <- attr(fylk.val, "maplim")
plot(xylims$x, xylims$y, asp=1, type='n', xlab="", ylab="")
for (i in 1:length(fylk.val)) lines(fylk.val[[i]])
title(main="Norwegian river centrelines from maptools package")

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