st_extract function

Extract cell values at point locations

st_extract(x, ...)

## S3 method for class 'stars'
st_extract(
  x,
  at,
  ...,
  bilinear = FALSE,
  time_column = attr(at, "time_column") %||% attr(at, "time_col"),
  interpolate_time = bilinear,
  FUN = mean,
  resampling = c("nearest", "bilinear", "cubic", "cubicspline")
)

Arguments

• x: object of class stars or stars_proxy
• ...: passed on to aggregate.stars when geometries are not exclusively POINT geometries
• at: object of class sf or sfc with geometries, or two-column matrix with coordinate points in rows, indicating where to extract values of x
• bilinear: logical; use bilinear interpolation rather than nearest neighbour?
• time_column: character or integer; name or index of a column with time or date values that will be matched to values of the first temporal dimension (matching classes POSIXct, POSIXt, Date, or PCICt), in x, after which this dimension is reduced. This is useful to extract data cube values along a trajectory; see https://github.com/r-spatial/stars/issues/352 .
• interpolate_time: logical; should time be interpolated? if FALSE, time instances are matched using the coinciding or the last preceding time in the data cube.
• FUN: function used to aggregate pixel values when geometries of at intersect with more than one pixel
• resampling: character; resampling method; for method cubic or cubicspline, stars_proxy objects should be used and GDAL should have version >= 3.10.0

Returns

if at is of class matrix, a matrix with extracted values is returned; otherwise: if x has more dimensions than only x and y (raster), an object of class stars with POINT geometries replacing x and y raster dimensions, if this is not the case, an object of sf with extracted values.

Details

points outside the raster are returned as NA values. For large sets of points for which extraction is needed, passing a matrix as to at may be much faster than passing an sf or sfc object.

Examples

tif = system.file("tif/L7_ETMs.tif", package = "stars")
r = read_stars(tif)
pnt = st_sample(st_as_sfc(st_bbox(r)), 10)
st_extract(r, pnt)
st_extract(r, pnt) %>% st_as_sf()
st_extract(r[,,,1], pnt)
st_extract(r, st_coordinates(pnt)) # "at" is a matrix: return a matrix