as.polygons results in clockwise polygons

[srfall]

I'm not an expert on the wkt standard but as far as I am aware of, the exterior boundary of a polygon should be defined counter clockwise and many other applications require this format (e.g see the issue in rgbif here: ropensci/rgbif/issues/662).

Observed problem:

terra::geom(terra::as.polygons(terra::ext(2, 18, 45, 55), crs = "+proj=longlat"), wkt = TRUE)

returns
"POLYGON ((2 45, 2 55, 18 55, 18 45, 2 45))"
which is clockwise, instead of the desired counter clockwise
"POLYGON ((2 45, 18 45,18 55, 2 55, 2 45))"

It looks like as.polygons is the cause:

wkt_polygon_input <- "POLYGON ((2 45, 18 45,18 55, 2 55,  2 45))"

terra::geom(terra::as.polygons(terra::vect(wkt_polygon_input, crs = "+proj=longlat")), wkt = TRUE)
# "POLYGON ((2 45, 2 55, 18 55, 18 45, 2 45))"
# undesired result

terra::geom(terra::vect(wkt_polygon_input, crs = "+proj=longlat"), wkt = TRUE)
# "POLYGON ((2 45, 18 45, 18 55, 2 55, 2 45))"
# desired result

[srfall]

If this is not a bug, but rather intended, it might be nice to have a parameter that can specify the desired winding order.

[edzer]

Some background on polygon orientation: r-spatial/sf#2096

[rhijmans]

I have added a new method forceCCW that returns counter-clockwise polygons.

library(terra)
#terra 1.7.42
p <- terra::as.polygons(terra::ext(2, 18, 45, 55), crs = "+proj=longlat")
x <- forceCCW(p)

geom(p, wkt = TRUE)
#[1] "POLYGON ((2 45, 2 55, 18 55, 18 45, 2 45))"
geom(x, wkt = TRUE)
#[1] "POLYGON ((2 45, 18 45, 18 55, 2 55, 2 45))"

#Also see
plot(x, axes=F, border="azure", lwd=4)
text(as.points(x), 1:4)

It would be trivial to change the order returned by as.polygons<SpatExtent> to CCW if that is helpful.