Build a polygonal clip target from points and/or a boundary

Resolves a single polygon to tessellate within. With a boundary it is that boundary (optionally buffered by expand); without one it is the axis-aligned bounding box of points_sf (the rectangle in the working CRS), again optionally buffered, or a small buffer around the points when they all share one x or one y, or nearly so (the short side of their bounding box below a millionth of the long side). Reach for it to build the boundary that method = "hex" and "square" require, to check that a study-area polygon actually contains the observations before tessellating, or to pass the same envelope to create_voronoi_polygons() and create_grid_polygons() so that two tessellations of one dataset cover identical ground.

Usage

clip_target_for(points_sf, boundary = NULL, expand = 0, quiet = FALSE)

Arguments

points_sf: An sf object with POINT/MULTIPOINT geometry.
boundary: Optional polygonal sf object. One with no CRS, given with points that have one, is interpreted in the points' own CRS, with a warning. With points in a projected CRS it is read as harmonize_crs() does: coordinates that look like lon/lat are taken as EPSG:4326 and reprojected, anything else is stamped with the points' CRS. With lon/lat points it is read as lon/lat when its coordinates fit the lon/lat envelope, and refused with an error otherwise.
expand: Numeric expansion distance or fraction (0–1 = fraction of extent). Absolute values are expressed in the units of the CRS the clip target is built in. Because sf::st_buffer() interprets dist as metres for lon/lat data while the fraction-of-extent form is derived from a bounding box measured in degrees, lon/lat input is projected to a local projected CRS first (see @return), so both forms agree.
quiet: Logical; suppress this function's progress message()s. It does not silence R warnings, nor the package's console log echo (see spatialkit_quiet for that). Default FALSE.

Value

An sf polygon layer representing the clip target. For lon/lat input the layer is returned in the automatically selected local projected CRS, not the input CRS; a message reports this unless quiet = TRUE.

Details

It is not the target build_tessellation() derives on its own: method = "voronoi" without a boundary clips to the convex hull of the points buffered by 2 percent of its diagonal, and there expand is always a distance. A bounding box over a non-rectangular point cloud includes corners with no data, so a hex or square grid laid over it has cells that hold no points; pass the study-area polygon when there is one.

Examples

library(sf)
set.seed(1)
pts <- st_as_sf(
  data.frame(x = 5e5 + runif(30, 0, 100), y = 5e6 + runif(30, 0, 100)),
  coords = c("x", "y"), crs = 32632
)
# No boundary: the bounding box, expanded by 10% of the extent
box <- clip_target_for(pts, expand = 0.1, quiet = TRUE)
st_area(box)
#> 12054.18 [m^2]