3. stars tidyverse methods

Edzer Pebesma

For a better version of the stars vignettes see https://r-spatial.github.io/stars/articles/

This vignette shows how some of the tidyverse verbs can be used on stars objects.

The stars and tidyverse packages are loaded by

library(stars)
library(dplyr)

Methods now available for class stars are

methods(class = "stars")
##  [1] $<-               %in%              Math              Ops              
##  [5] [                 [<-               [[<-              adrop            
##  [9] aggregate         aperm             as.data.frame     as.tbl_cube      
## [13] as_tibble         c                 coerce            contour          
## [17] cut               dim               dimnames          dimnames<-       
## [21] drop_units        droplevels        filter            hist             
## [25] image             initialize        is.na             merge            
## [29] mutate            plot              predict           print            
## [33] pull              select            show              slice            
## [37] slotsFromS3       split             st_apply          st_area          
## [41] st_as_sf          st_as_sfc         st_as_stars       st_bbox          
## [45] st_coordinates    st_crop           st_crs            st_crs<-         
## [49] st_dimensions     st_dimensions<-   st_extract        st_geometry      
## [53] st_interpolate_aw st_intersects     st_join           st_mosaic        
## [57] st_normalize      st_redimension    st_sample         st_set_bbox      
## [61] st_transform      st_transform_proj transmute         write_stars      
## see '?methods' for accessing help and source code

We will work with a three-band section of a landsat image:

system.file("tif/L7_ETMs.tif", package = "stars") %>%
    read_stars -> x
x
## stars object with 3 dimensions and 1 attribute
## attribute(s):
##              Min. 1st Qu. Median     Mean 3rd Qu. Max.
## L7_ETMs.tif     1      54     69 68.91242      86  255
## dimension(s):
##      from  to  offset delta                       refsys point values x/y
## x       1 349  288776  28.5 UTM Zone 25, Southern Hem... FALSE   NULL [x]
## y       1 352 9120761 -28.5 UTM Zone 25, Southern Hem... FALSE   NULL [y]
## band    1   6      NA    NA                           NA    NA   NULL

slice

slice slices a sub-array out of the cube; this is done by specifying the dimension on which to act, and the slice number.

x %>% slice(band, 6) -> x6
x6
## stars object with 2 dimensions and 1 attribute
## attribute(s):
##              Min. 1st Qu. Median     Mean 3rd Qu. Max.
## L7_ETMs.tif     1      32     60 59.97521      88  255
## dimension(s):
##   from  to  offset delta                       refsys point values x/y
## x    1 349  288776  28.5 UTM Zone 25, Southern Hem... FALSE   NULL [x]
## y    1 352 9120761 -28.5 UTM Zone 25, Southern Hem... FALSE   NULL [y]

It returns a lower-dimensional array if a single element is selected along the slice dimension.

filter

Similar to slice, filter selects on dimensions but evaluates their values rather than their index: in

x %>% filter(x > 289000, x < 291000, band > 3) -> x7
x7
## stars object with 3 dimensions and 1 attribute
## attribute(s):
##              Min. 1st Qu. Median     Mean 3rd Qu. Max.
## L7_ETMs.tif     5      54     70 71.79194      88  252
## dimension(s):
##      from  to  offset delta                       refsys point values x/y
## x       1  70  289004  28.5 UTM Zone 25, Southern Hem... FALSE   NULL [x]
## y       1 352 9120761 -28.5 UTM Zone 25, Southern Hem... FALSE   NULL [y]
## band    1   3       4     1                           NA    NA   NULL

the subarray is created based on the x coordinate values.

Note that filter converts the object to a tbl_cube, and uses the dplyr filter method for tbl_cube objects. This has the limitation that stars objects with rectilinear, curvilinear or simple feature geometries cannot be handled. For such objects, using regular [ selection or using st_crop may be an alternative.

pull

pull pulls out an array from a stars object:

x %>% pull(1) -> x8
class(x8)
## [1] "array"
dim(x8)
##    x    y band 
##  349  352    6

mutate

x %>% mutate(band2 = 2 * L7_ETMs.tif) -> x2 
x2
## stars object with 3 dimensions and 2 attributes
## attribute(s):
##              Min. 1st Qu. Median      Mean 3rd Qu. Max.
## L7_ETMs.tif     1      54     69  68.91242      86  255
## band2           2     108    138 137.82484     172  510
## dimension(s):
##      from  to  offset delta                       refsys point values x/y
## x       1 349  288776  28.5 UTM Zone 25, Southern Hem... FALSE   NULL [x]
## y       1 352 9120761 -28.5 UTM Zone 25, Southern Hem... FALSE   NULL [y]
## band    1   6      NA    NA                           NA    NA   NULL

select

select selects an attribute, or a set of attributes:

x2 %>% select(band2) -> x9
x9
## stars object with 3 dimensions and 1 attribute
## attribute(s):
##        Min. 1st Qu. Median     Mean 3rd Qu. Max.
## band2     2     108    138 137.8248     172  510
## dimension(s):
##      from  to  offset delta                       refsys point values x/y
## x       1 349  288776  28.5 UTM Zone 25, Southern Hem... FALSE   NULL [x]
## y       1 352 9120761 -28.5 UTM Zone 25, Southern Hem... FALSE   NULL [y]
## band    1   6      NA    NA                           NA    NA   NULL

geom_stars

geom_raster is a ggplot2 geom function that accepts stars objects as its data argument and

An example use is

library(ggplot2)
library(viridis)
ggplot() + 
  geom_stars(data = x) +
  coord_equal() +
  facet_wrap(~band) +
  theme_void() +
  scale_fill_viridis() +
  scale_x_discrete(expand = c(0, 0)) +
  scale_y_discrete(expand = c(0, 0))