Extract raster data into a hexagonal grid

Primary function for extracting raster values into hexagonal grid cells. CRS transformations are handled automatically: each raster is reprojected (or the grid is reprojected to the raster's CRS) so that the underlying call to [`exactextractr::exact_extract()`] always sees matching CRSs.

Usage

extract_raster_data(
  raster_files,
  study_area = NULL,
  cell_size = NULL,
  hex_grid = NULL,
  sample_fraction = 1,
  random_seed = 42,
  fun = "mean"
)

Arguments

raster_files

Named character vector of file paths OR named list of `terra::SpatRaster` objects.

study_area

Optional sf polygon used for cropping each raster and to define the grid CRS.

cell_size

Hex cell size, in the units of the grid CRS. Required when `hex_grid` is not supplied.

hex_grid

Optional sf hexagonal grid to use instead of creating one.

sample_fraction

Fraction of grid cells to keep (default 1).

random_seed

Seed for reproducible sampling.

fun

Aggregation function passed to `exactextractr::exact_extract()` (default `"mean"`).

Value

List with components

`data`

Data frame with `cell_id`, `x`, `y` and one column per raster.

`hex_grid`

The sf grid that was used (sampled, if applicable).

`cell_size`

The cell size used.

`extent`

Extent of the first raster (after cropping).

`variables`

Names of the rasters.

`n_cells`

Number of cells in `data`.

Input flexibility

`raster_files` may be either a named character vector of file paths or a named list of `terra::SpatRaster` objects. All inputs may be in different CRSs from one another and from the grid - the function handles cropping and transformation per raster.

CRS handling

- If `study_area` is supplied, the grid is created in the study area's CRS. - Otherwise the grid is created in the first raster's CRS. - For each raster, the grid is transformed to the raster's CRS before extraction (so cell-size units are honoured exactly once, in the grid CRS).