Build polygons from rings

Returns one polygon per row of shell, with shell as its exterior ring and the lines on the same row of holes as its holes. This matches PostGIS ST_MakePolygon().

Usage

ga_make_polygon(shell, holes = NULL, close = TRUE)

Arguments

shell

a GeoArrow linestring array of exterior rings

holes

a GeoArrow multilinestring array the same length as shell, holding the holes of each polygon, or NULL for none

close

whether to close each open ring by repeating its first coordinate

Value

a GeoArrow polygon array the same length as shell

Details

With close = TRUE, ga_make_polygon() repeats the first coordinate at the end of any ring whose last coordinate differs from it, as ga_polygon() does. It returns a null polygon for a null shell and for a row with a ring of fewer than four coordinates once closed, so the result always has the same length as shell. It returns an empty polygon for an empty shell, and skips null and empty holes.

ga_make_polygon() keeps the z and m values and the CRS of shell. Use ga_make_line_agg() to join points into the rings first.

See also

Other construct: ga_linestring(), ga_make_envelope(), ga_make_grid(), ga_make_line(), ga_make_line_agg(), ga_multilinestring(), ga_multipoint(), ga_multipolygon(), ga_point(), ga_polygon()

Examples

# join points into rings, then rings into polygons
pts <- ga_point(c(0, 10, 10, 0, 20, 30, 20), c(0, 0, 10, 10, 0, 0, 10))
rings <- ga_make_line_agg(pts, by = c(1, 1, 1, 1, 2, 2, 2))
sf::st_as_sfc(geoarrow::as_geoarrow_vctr(ga_make_polygon(rings)))
#> Geometry set for 2 features 
#> Geometry type: POLYGON
#> Dimension:     XY
#> Bounding box:  xmin: 0 ymin: 0 xmax: 30 ymax: 10
#> CRS:           NA
#> POLYGON ((0 0, 10 0, 10 10, 0 10, 0 0))
#> POLYGON ((20 0, 30 0, 20 10, 20 0))

# a square with a square hole
shell <- ga_make_line_agg(ga_point(c(0, 10, 10, 0), c(0, 0, 10, 10)))
hole <- ga_make_line_agg(ga_point(c(2, 4, 4, 2), c(2, 2, 4, 4)))
sf::st_as_sfc(geoarrow::as_geoarrow_vctr(
  ga_make_polygon(shell, ga_as_multilinestring(hole))
))
#> Geometry set for 1 feature 
#> Geometry type: POLYGON
#> Dimension:     XY
#> Bounding box:  xmin: 0 ymin: 0 xmax: 10 ymax: 10
#> CRS:           NA
#> POLYGON ((0 0, 10 0, 10 10, 0 10, 0 0), (2 2, 4...