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)
## Loading required package: abind
## Loading required package: sf
## Linking to GEOS 3.5.0, GDAL 2.2.2, PROJ 4.8.0
library(dplyr)
##
## Attaching package: 'dplyr'
## The following objects are masked from 'package:stats':
##
## filter, lag
## The following objects are masked from 'package:base':
##
## intersect, setdiff, setequal, unionMethods now available for class stars are
methods(class = "stars")
## [1] [ [<- $<- adrop
## [5] aggregate aperm as_tibble as.data.frame
## [9] as.tbl_cube c coerce contour
## [13] cut dim dimnames dimnames<-
## [17] filter image initialize is.na
## [21] Math merge mutate Ops
## [25] plot predict print pull
## [29] select show slice slotsFromS3
## [33] split st_apply st_area st_as_sf
## [37] st_as_sfc st_as_stars st_bbox st_coordinates
## [41] st_crop st_crs st_crs<- st_dimensions
## [45] st_geometry st_interpolate_aw st_intersects st_join
## [49] st_mosaic st_normalize st_redimension st_transform_proj
## [53] st_transform write_stars
## see '?methods' for accessing help and source codeWe 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):
## L7_ETMs.tif
## Min. : 1.00
## 1st Qu.: 54.00
## Median : 69.00
## Mean : 68.91
## 3rd Qu.: 86.00
## Max. :255.00
## dimension(s):
## from to offset delta refsys point values
## x 1 349 288776 28.5 +proj=utm +zone=25 +south... FALSE NULL [x]
## y 1 352 9120761 -28.5 +proj=utm +zone=25 +south... 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):
## L7_ETMs.tif
## Min. : 1.00
## 1st Qu.: 32.00
## Median : 60.00
## Mean : 59.98
## 3rd Qu.: 88.00
## Max. :255.00
## dimension(s):
## from to offset delta refsys point values
## x 1 349 288776 28.5 +proj=utm +zone=25 +south... FALSE NULL [x]
## y 1 352 9120761 -28.5 +proj=utm +zone=25 +south... 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):
## L7_ETMs.tif
## Min. : 5.00
## 1st Qu.: 54.00
## Median : 70.00
## Mean : 71.79
## 3rd Qu.: 88.00
## Max. :252.00
## dimension(s):
## from to offset delta refsys point values
## x 1 70 289004 28.5 +proj=utm +zone=25 +south... FALSE NULL [x]
## y 1 352 9120761 -28.5 +proj=utm +zone=25 +south... FALSE NULL [y]
## band 1 3 4 1 NA NA NULLthe 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.
mutate
x %>% mutate(band2 = 2 * L7_ETMs.tif) -> x2
x2
## stars object with 3 dimensions and 2 attributes
## attribute(s):
## L7_ETMs.tif band2
## Min. : 1.00 Min. : 2.0
## 1st Qu.: 54.00 1st Qu.:108.0
## Median : 69.00 Median :138.0
## Mean : 68.91 Mean :137.8
## 3rd Qu.: 86.00 3rd Qu.:172.0
## Max. :255.00 Max. :510.0
## dimension(s):
## from to offset delta refsys point values
## x 1 349 288776 28.5 +proj=utm +zone=25 +south... FALSE NULL [x]
## y 1 352 9120761 -28.5 +proj=utm +zone=25 +south... 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):
## band2
## Min. : 2.0
## 1st Qu.:108.0
## Median :138.0
## Mean :137.8
## 3rd Qu.:172.0
## Max. :510.0
## dimension(s):
## from to offset delta refsys point values
## x 1 349 288776 28.5 +proj=utm +zone=25 +south... FALSE NULL [x]
## y 1 352 9120761 -28.5 +proj=utm +zone=25 +south... 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
- sets up the raster or vector spatial coordinates as plot dimensions, and the first attribute as the fill variable
- allows for downsampling (without choosing a suitable downsampling level)
- chooses between using
geom_raster,geom_rectandgeom_sfdepending on whether the geometry is regular, rectilinear or has vector geometries
An example use is
library(ggplot2)
library(viridis)
## Loading required package: viridisLite
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))