Combine several feature geometries into one, without unioning or resolving internal boundaries

## Arguments

- x
object of class

`sf`

,`sfc`

or`sfg`

- y
object of class

`sf`

,`sfc`

or`sfg`

(optional)- ...
ignored

- by_feature
logical; if TRUE, union each feature, if FALSE return a single feature that is the geometric union of the set of features

- is_coverage
logical; if TRUE, use an optimized algorithm for features that form a polygonal coverage (have no overlaps)

## Value

`st_combine`

returns a single, combined geometry, with no resolved boundaries; returned geometries may well be invalid.

If `y`

is missing, `st_union(x)`

returns a single geometry with resolved boundaries, else the geometries for all unioned pairs of x[i] and y[j].

## Details

`st_combine`

combines geometries without resolving borders, using c.sfg (analogous to c for ordinary vectors).

If `st_union`

is called with a single argument, `x`

, (with `y`

missing) and `by_feature`

is `FALSE`

all geometries are unioned together and an `sfg`

or single-geometry `sfc`

object is returned. If `by_feature`

is `TRUE`

each feature geometry is unioned. This can for instance be used to resolve internal boundaries after polygons were combined using `st_combine`

. If `y`

is provided, all elements of `x`

and `y`

are unioned, pairwise (and `by_feature`

is ignored). The former corresponds to `rgeos::gUnaryUnion`

, the latter to `rgeos::gUnion`

.

Unioning a set of overlapping polygons has the effect of merging the areas (i.e. the same effect as iteratively unioning all individual polygons together). Unioning a set of LineStrings has the effect of fully noding and dissolving the input linework. In this context "fully noded" means that there will be a node or endpoint in the output for every endpoint or line segment crossing in the input. "Dissolved" means that any duplicate (e.g. coincident) line segments or portions of line segments will be reduced to a single line segment in the output. Unioning a set of Points has the effect of merging all identical points (producing a set with no duplicates).

## Examples

```
nc = st_read(system.file("shape/nc.shp", package="sf"))
#> Reading layer `nc' from data source
#> `/home/runner/work/_temp/Library/sf/shape/nc.shp' using driver `ESRI Shapefile'
#> Simple feature collection with 100 features and 14 fields
#> Geometry type: MULTIPOLYGON
#> Dimension: XY
#> Bounding box: xmin: -84.32385 ymin: 33.88199 xmax: -75.45698 ymax: 36.58965
#> Geodetic CRS: NAD27
st_combine(nc)
#> Geometry set for 1 feature
#> Geometry type: MULTIPOLYGON
#> Dimension: XY
#> Bounding box: xmin: -84.32385 ymin: 33.88199 xmax: -75.45698 ymax: 36.58965
#> Geodetic CRS: NAD27
#> MULTIPOLYGON (((-81.47276 36.23436, -81.54084 3...
plot(st_union(nc))
```