Create a spatial grid — get_grid • spatialgridr

Creates a spatial grid, in terra::rast() of sf format, for areas within the boundaries provided

Usage

get_grid(boundary, resolution = 5000, crs, output = "raster", touches = FALSE)

Arguments

boundary: sf object with boundary of the area(s) you want a grid for, e.g an EEZ or country. Boundaries can be obtained using get_boundary()
resolution: numeric; the desired grid cell resolution in same units (usually metres or degrees) as crs: sf::st_crs(crs, parameters = TRUE)$units_gdal
crs: a coordinate reference system: numeric (e.g. EPSG code 4326), character (e.g. "+proj=longlat +datum=WGS84 +no_defs"), or object of class sf or sfc. This is passed to sf::st_crs()
output: character the desired output format, either "raster", "sf_square" (vector), or "sf_hex" (vector); default is "raster"
touches: logical. If TRUE all cells touched by the boundary will be included, and if FALSE only cells whose center point (centroid) falls within the boundary are included. Default is FALSE.

Value

A spatial grid in output format requested with resolution and crs provided

Details

For a terra::rast() format grid, each cell within the boundary is set to value 1, whereas cells outside are NA. For sf grids, the grid covers the boundary area and cells can be square (output = "sf_square") or hexagonal (output = "sf_hex"). By default only cells whose centroids fall within the boundary are included in the grid. To include cells that touch the boundary use touches = TRUE. sf::st_make_grid() is used to create sf grids. The default ordering of this grid type is from bottom to top, left to right. In contrast, the terra::rast() grid is ordered from top to bottom, left to right. To preserve consistency across the data types, we have reordered sf grids to also fill from top to bottom, left to right.

Examples

# use get_boundary() to get a polygon of Samoa's Exclusive Economic Zone
samoa_eez <- get_boundary(name = "Samoa")
#> Cache is fresh. Reading: /tmp/RtmpDB15KU/eez-d0aa43d6/eez.shp
#> (Last Modified: 2024-08-23 06:03:35.408147)
# You need a suitable coordinate reference system (crs) for your area of interest, https://projectionwizard.org is useful for this purpose. For spatial planning, equal area projections are normally best.
samoa_projection <- '+proj=laea +lon_0=-172.5 +lat_0=0 +datum=WGS84 +units=m +no_defs'
# Create a grid with 5 km (5000 m) resolution covering the `samoa_eez` in a projection specified by `crs`.
samoa_grid <- get_grid(boundary = samoa_eez, resolution = 5000, crs = samoa_projection)