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The default view CRS for spatial data

Source: R/view-crs.R
view_crs.Rd

The CRS view() draws in when crs is not given. The project is polar first, and a view in the data's own CRS is preferred to a reprojection, so the rule is:

Usage

view_crs(x)

Arguments

x

An sf or sfc object, a 'terra' SpatRaster or SpatVector, or a list of them.

Value

A CRS for aobcore::scene(): an "authority:code" string such as "EPSG:3031", or, when the data's CRS has no code, the sf crs object (for sf data) or its WKT (for 'terra' data).

Details

  1. A projected CRS is kept: the data are drawn in their own CRS.

  2. Lon/lat data lying entirely south of 40S (the bounding box's northern edge at or below -40) are drawn in EPSG:3031, Antarctic Polar Stereographic. This covers Antarctica, the Southern Ocean south of the Subtropical Front, and circumpolar data such as a coastline south of 40S, all of which a lon/lat view tears apart at the pole.

  3. Lon/lat data lying entirely north of 60N are drawn in EPSG:3413, NSIDC Sea Ice Polar Stereographic North. The northern threshold is stricter than the southern one because much mid-latitude land lies between 40N and 60N, where a polar view turns the map away from north-up; south of 40S there is little land other than Antarctica.

  4. Other lon/lat data are drawn in their own lon/lat CRS, flat (plate carree), which the scene spec allows. Web Mercator is not used: it cannot show the poles, and a tiled Mercator basemap is a non-goal.

For a raster the bounding box is the grid's extent, so a lon/lat grid from 90S to 40S, whose northern edge is at 40S, is drawn in EPSG:3031.

For a list of objects (see view-layers) the rule is applied once to the whole list: the CRS of the first object with a projected CRS is kept; when every object is in lon/lat, rules 2 to 4 apply to their combined latitude range, and rule 4 keeps the first object's CRS.

Data with no CRS is an error: set one first (for example sf::st_set_crs() or terra::crs<-), or pass crs to view().

Examples

coast <- sf::st_read(system.file("extdata", "coastline_south_40s.geojson",
                                 package = "aobcore"), quiet = TRUE)
view_crs(coast)
#> [1] "EPSG:3031"
nc <- sf::st_read(system.file("shape", "nc.shp", package = "sf"), quiet = TRUE)
view_crs(nc)
#> [1] "EPSG:4267"