Extracts the values of cells in a RasterLayer that are covered by a
simple feature collection containing polygonal geometries, as well as the
fraction of each cell that is covered by the polygon. Returns either
the result of a summary operation or function applied to the values
and coverage fractions (if fun is specified), or a data frame
containing the values and coverage fractions themselves (if fun
is NULL.)
# S4 method for Raster,sf
exact_extract(x, y, fun = NULL, ...,
include_xy = FALSE, progress = TRUE, max_cells_in_memory = 3e+07)# S4 method for Raster,sfc_MULTIPOLYGON
exact_extract(x, y, fun = NULL, ...,
include_xy = FALSE, progress = TRUE, max_cells_in_memory = 3e+07)
# S4 method for Raster,sfc_POLYGON
exact_extract(x, y, fun = NULL, ...,
include_xy = FALSE, progress = TRUE, max_cells_in_memory = 3e+07)
a RasterLayer
a sf object with polygonal geometries
an optional function or character vector, as described below
additional arguments to pass to fun
if TRUE, augment the returned data frame with
columns for cell center coordinates (x and
y) or pass them to fun
if TRUE, display a progress bar during processing
the maximum number of raster cells to load at
a given time when using a named summary operation
for fun (as opposed to a function defined using
R code). If a polygon covers more than max_cells_in_memory
raster cells, it will be processed in multiple chunks.
a vector or list of data frames, depending on the value of fun
(see Details)
The value of fun may be set to a string (or vector of strings)
representing summary operations supported by the exactextract library.
If a single summary operation is specified, exact_extract will
return a vector with the result of the summary operation for each
feature in the input. If multiple summary operations are specified,
exact_extract will return a data frame with the result of each
summary operation for each feature.
The following summary operations are supported:
min - the minimum defined value in any raster cell wholly or
partially covered by the polygon
max - the maximum defined value in any raster cell wholly or
partially covered by the polygon
count - the sum of fractions of raster cells with defined values
covered by the polygon
sum - the sum of defined raster cell values, multiplied by
the fraction of the cell that is covered by the polygon
mean - the mean cell value, weighted by the fraction of each cell
that is covered by the polygon
mode - the most common cell value, weighted by the fraction of
each cell that is covered by the polygon. Where multiple
values occupy the same maximum number of weighted cells,
the largest value will be returned.
majority - synonym for mode
minority - the least common cell value, weighted by the fraction
of each cell that is covered by the polygon. Where
multiple values occupy the same minimum number of
weighted cells, the smallest value will be returned.
variety - the number of distinct values in cells that are wholly
or partially covered by the polygon.
Alternatively, an R function may be provided as fun. The function will be
called for each feature with with vectors of cell values and weights as arguments.
exact_extract will then return a vector of the return values of fun.
If fun is not specified, exact_extract will return a list with
one data frame for each feature in the input feature collection. The data
frame will contain a column with values from each layer in the input `Raster*`,
and a final column indicating the fraction of the cell that is covered by the
polygon.
# NOT RUN {
rast <- raster::raster(matrix(1:100, ncol=10), xmn=0, ymn=0, xmx=10, ymx=10)
poly <- sf::st_as_sfc('POLYGON ((2 2, 7 6, 4 9, 2 2))')
exact_extract(rast, poly, 'mean')
# two summary operations, returns data frame
exact_extract(rast, poly, c('min', 'max'))
# custom summary function, returns vector
exact_extract(rast, poly, function(value, cov_frac) length(value[cov_frac > 0.9]))
# }
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