These functions may be used to convert map objects returned by the map function in the maps package to suitable objects defined in the sp package. In the examples below, arguments are shown for retrieving first polygons by name, then lines by window.

map2SpatialPolygons(map, IDs, proj4string = CRS(as.character(NA)), checkHoles=FALSE)
map2SpatialLines(map, IDs=NULL, proj4string = CRS(as.character(NA)))
pruneMap(map, xlim=NULL, ylim=NULL)



a map object defined in the maps package and returned by the map function


Unique character ID values for each output Polygons object; the input IDs can be an integer or character vector with duplicates, where the duplicates will be combined as a single output Polygons object


Object of class "CRS"; holding a valid proj4 string


default=FALSE, if TRUE call checkPolygonsHolesinternally to check hole assignment, (by default no polygon objects are holes)


limits for pruning a map object - should only be used for lines, because polygons will not be closed


Any zero area output geometries are dropped, and warnings are issued.


map2SpatialPolygons returns a SpatialPolygons object and map2SpatialLines returns a SpatialLines object (objects defined in the sp package); pruneMap returns a modified map object defined in the maps package


Roger Bivand


As the examples show, retrieval by name should be checked to see whether a window is not also needed: the "norway" polygons include "Norway:Bouvet Island", which is in the South Atlantic. Here, the IDs argument is set uniformly to "Norway" for all the component polygons, so that the output object contains a single Polygons object with multiple component Polygon objects. When retrieving by window, pruning may be needed on lines which are included because they begin within the window; interior=FALSE is used to remove country boundaries in this case.

See also


run <- FALSE if(require(maps)) run <- TRUE if (run) { nor_coast_poly <- map("world", "norway", fill=TRUE, col="transparent", plot=FALSE) range(nor_coast_poly$x, na.rm=TRUE) }
#> [1] -9.098877 33.629295
if (run) { range(nor_coast_poly$y, na.rm=TRUE) }
#> [1] 58.02095 80.47783
if (run) { nor_coast_poly <- map("world", "norway", fill=TRUE, col="transparent", plot=FALSE, ylim=c(58,72)) nor_coast_poly$names }
#> [1] "Norway:1" "Norway:2" "Norway:Smola" #> [4] "Norway:Froya" "Norway:5" "Norway:6" #> [7] "Norway:7" "Norway:8" "Norway:Moskenesoya" #> [10] "Norway:Vestvagoy" "Norway:Langoya" "Norway:Hinnoya" #> [13] "Norway:Senja" "Norway:14" "Norway:Arnoy" #> [16] "Norway:16" "Norway:Vannoy" "Norway:Seiland" #> [19] "Norway:Kvaloya" "Norway:Soroya" "Norway" #> [22] "Norway:Mageroya" "Norway:Jan Mayen"
if (run) { IDs <- sapply(strsplit(nor_coast_poly$names, ":"), function(x) x[1]) } if (run) { nor_coast_poly_sp <- map2SpatialPolygons(nor_coast_poly, IDs=IDs, proj4string=CRS("+proj=longlat +datum=WGS84")) sapply(slot(nor_coast_poly_sp, "polygons"), function(x) length(slot(x, "Polygons"))) }
#> [1] 23
if (run) { plot(nor_coast_poly_sp, col="grey", axes=TRUE) }
if (run) { nor_coast_lines <- map("world", interior=FALSE, plot=FALSE, xlim=c(4,32), ylim=c(58,72)) plot(nor_coast_lines, type="l") }
if (run) { nor_coast_lines <- pruneMap(nor_coast_lines, xlim=c(4,32), ylim=c(58,72)) lines(nor_coast_lines, col="red") nor_coast_lines_sp <- map2SpatialLines(nor_coast_lines, proj4string=CRS("+proj=longlat +datum=WGS84")) plot(nor_coast_poly_sp, col="grey", axes=TRUE) }
if (run) { plot(nor_coast_lines_sp, col="blue", add=TRUE) }
if (run) { worldmap <- map("world", fill=TRUE, plot=FALSE) worldmapPolys <- map2SpatialPolygons(worldmap, IDs=sapply(strsplit(worldmap$names, ":"), "[", 1L), proj4string=CRS("+proj=longlat +datum=WGS84")) if (rgeosStatus()) { require(rgeos) vals <- rgeos::gIsValid(worldmapPolys, byid=TRUE, reason=TRUE) print(table(sapply(strsplit(vals, "\\["), "[", 1))) } }
#> Loading required package: rgeos
#> rgeos version: 0.5-7, (SVN revision 666) #> GEOS runtime version: 3.10.0dev-CAPI-1.15.0 #> Linking to sp version: 1.4-6 #> Polygon checking: TRUE
#> #> Duplicate Rings Nested shells Self-intersection Valid Geometry #> 2 8 4 239