RGEOS Polygon Hole Comment Functions | R Documentation |
Utility functions for assigning ownership of holes to parent polygons
createSPComment(sppoly,which=NULL,overwrite=TRUE)
createPolygonsComment(poly)
set_do_poly_check(value)
get_do_poly_check()
sppoly |
Object that inherits from the |
which |
Vector, which subset of geometries in sppoly should comment attributes be generated |
overwrite |
Logical, if a comment attribute already exists should it be overwritten |
poly |
Object of class |
value |
logical value to set “do_poly_check” option, default TRUE |
In order for rgeos to function properly it is necessary that all holes within a given POLYGON or MULTIPOLYGON geometry must belong to a specific polygon. The SpatialPolygons
class implementation does not currently include this information. To work around this limitation rgeos uses an additional comment attribute on the Polygons
class that indicates which hole belongs to which polygon.
Under the current implementation this comment is a text string of numbers separated by spaces where the order of the numbers corresponds to the order of the Polygon
objects in the Polygons
slot of the Polygons
object. A 0
implies the Polygon
object is a polygon, a non-zero number implies that the Polygon
object is a hole with the value indicating the index of the Polygon
that “owns” the hole.
createPolygonsComment
attempts to create a valid comment for a Polygons
object by assessing which polygons contain a given hole (using gContains
). Ownership is assigned to the smallest polygon (by area) that contains the given hole. This is not guaranteed to generate valid polygons, always check the resulting objects for validity.
createSPComment
attempts to create valid comments for all or a subset of polygons within a SpatialPolygons
object. Refer to Bivand et al. (2013), pages 47-48 and 132-133 for a further discussion and SpatialPolygons
The helper functions get and set the imposition of checking of objects inheriting from SpatialPolygons
for proper assignment of hole interior rings to exterior rings in Polygons
objects. The internal GEOS representation defines a POLYGON object as a collection of only one exterior ring and from zero to many interior rings, so an sp Polygons
object corresponds to a GEOS MULTIPOLYGON object, but without proper hole assignment. By default do_poly_check
is TRUE; if it is set to FALSE using set_do_poly_check(FALSE)
, and the Polygons
object is not valid in GEOS terms, GEOS may crash R, which is why checks are imposed by default.
The details below show how hole assignment is handled in the package; here we assume that the hole status slots of all Polygon objects in a given Polygons object are set correctly. In the examples below, we use a data set from maptools which has the holes correctly assigned, and we see that the SpatialPolygons object is geometrically valid both initially and after removing the comment attribute on the only Polygons object in usa
- regenerating internally within rgeos from the hole status slots since do_poly_check
is TRUE.
If we modify usa
by setting all hole status slots to FALSE, the SpatialPolygons object is geometrically invalid even though a comment attribute can be created - the function createSPComment
is deceived by the incorrect hole status slots. To rectify this, we use checkPolygonsHoles
from maptools on each Polygons
object. This function calls gContains
, gContainsProperly
, gEquals
and createPolygonsComment
from rgeos to check whether the hole status slots are set correctly. Experience shows that many imported datasets from for example publically provided shapefiles have incorrect hole status values. Running checkPolygonsHoles
is time-consuming when the number of member Polygon
objects is large - attempts will be made to make this more efficient.
The geometries handled by GEOS are assumed to be planar, so that any rgeos functions making measurements will give incorrect results when used on geographical coordinates measured in decimal degrees. Topological functions may work satisfactorily, but will not understand spherical wrap-around.
Roger Bivand & Colin Rundel
Roger Bivand, Edzer Pebesma and Virgilio Gomez-Rubio, 2013. Applied spatial data analysis with R, Second edition. Springer, NY. https://asdar-book.org/
library(sp)
p1 <- Polygon(cbind(x=c(0, 0, 10, 10, 0), y=c(0, 10, 10, 0, 0)), hole=FALSE) # I
p2 <- Polygon(cbind(x=c(3, 3, 7, 7, 3), y=c(3, 7, 7, 3, 3)), hole=TRUE) # H
p8 <- Polygon(cbind(x=c(1, 1, 2, 2, 1), y=c(1, 2, 2, 1, 1)), hole=TRUE) # H
p9 <- Polygon(cbind(x=c(1, 1, 2, 2, 1), y=c(5, 6, 6, 5, 5)), hole=TRUE) # H
p3 <- Polygon(cbind(x=c(20, 20, 30, 30, 20), y=c(20, 30, 30, 20, 20)),
hole=FALSE) # I
p4 <- Polygon(cbind(x=c(21, 21, 29, 29, 21), y=c(21, 29, 29, 21, 21)),
hole=TRUE) # H
p5 <- Polygon(cbind(x=c(22, 22, 28, 28, 22), y=c(22, 28, 28, 22, 22)),
hole=FALSE) # I
p6 <- Polygon(cbind(x=c(23, 23, 27, 27, 23), y=c(23, 27, 27, 23, 23)),
hole=TRUE) # H
p7 <- Polygon(cbind(x=c(13, 13, 17, 17, 13), y=c(13, 17, 17, 13, 13)),
hole=FALSE) # I
p10 <- Polygon(cbind(x=c(24, 24, 26, 26, 24), y=c(24, 26, 26, 24, 24)),
hole=FALSE) # I
p11 <- Polygon(cbind(x=c(24.25, 24.25, 25.75, 25.75, 24.25),
y=c(24.25, 25.75, 25.75, 24.25, 24.25)), hole=TRUE) # H
p12 <- Polygon(cbind(x=c(24.5, 24.5, 25.5, 25.5, 24.5),
y=c(24.5, 25.5, 25.5, 24.5, 24.5)), hole=FALSE) # I
p13 <- Polygon(cbind(x=c(24.75, 24.75, 25.25, 25.25, 24.75),
y=c(24.75, 25.25, 25.25, 24.75, 24.75)), hole=TRUE) # H
lp <- list(p1, p2, p13, p7, p6, p5, p4, p3, p8, p11, p12, p9, p10)
# 1 2 3 4 5 6 7 8 9 10 11 12 13
# 0 1 11 0 6 0 8 0 1 13 0 1 0
# I H H I H I H I H H I H I
pls <- Polygons(lp, ID="1")
comment(pls)
comment(pls) = createPolygonsComment(pls)
comment(pls)
plot(SpatialPolygons(list(pls)), col="magenta", pbg="cyan")
title(xlab="Hole slot values before checking")
## Not run:
# running this illustration may be time-consuming
if (require(maptools)) {
data(wrld_simpl)
usa <- wrld_simpl[wrld_simpl$ISO3=="USA",]
lapply(slot(usa, "polygons"), comment)
gIsValid(usa, reason=TRUE)
comment(slot(usa, "polygons")[[1]]) <- NULL
lapply(slot(usa, "polygons"), comment)
gIsValid(usa)
any(c(sapply(slot(usa, "polygons"),
function(x) sapply(slot(x, "Polygons"), slot, "hole"))))
lapply(slot(createSPComment(usa), "polygons"), comment)
usa1 <- usa
Pls <- slot(usa1, "polygons")
pls <- slot(Pls[[1]], "Polygons")
pls1 <- lapply(pls, function(p) {slot(p, "hole") <- FALSE; return(p)})
slot(Pls[[1]], "Polygons") <- pls1
slot(usa1, "polygons") <- Pls
any(c(sapply(slot(usa1, "polygons"),
function(x) sapply(slot(x, "Polygons"), slot, "hole"))))
lapply(slot(createSPComment(usa1), "polygons"), comment)
gIsValid(usa1, reason=TRUE)
Pls <- slot(usa1, "polygons")
Pls1 <- lapply(Pls, checkPolygonsHoles)
slot(usa1, "polygons") <- Pls1
lapply(slot(usa1, "polygons"), comment)
gIsValid(usa1, reason=TRUE)
}
## End(Not run)
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