CODEWITHSUNDEEP
rr-stars
sawari
sawari

Reputation: 43

How can I cast a data frame to a stars object with a time dimension?

Let's say I have a data frame with spatial columns (x,y), a time column (time), and a data column (value). How can I correctly coerce this to a stars object using st_as_stars?

df <- expand_grid(time=1:10,
                  x=1:10,
                  y=1:10)
df$value <- 1:1000

This is the original order of the data frame:

> head(df)
# A tibble: 6 x 4
   time     x     y value
  <int> <int> <int> <int>
1     1     1     1     1
2     1     1     2     2
3     1     1     3     3
4     1     1     4     4
5     1     1     5     5
6     1     1     6     6

Something is incorrectly specified here when I try to cast the data frame to a stars object:

s <- st_as_stars(df,
                 dimensions=st_dimensions(time=sort(unique(df$time)), 
                                          x=sort(unique(df$x)),
                                          y=sort(unique(df$y)), 
                                          point=TRUE),
                 dims=c('time','x','y'))

This stars object has problems. (1) The 'y' column was taken as an attribute.

> s
stars object with 3 dimensions and 2 attributes
attribute(s):
       y            value       
 Min.   : 1.0   Min.   :   1.0  
 1st Qu.: 3.0   1st Qu.: 250.8  
 Median : 5.5   Median : 500.5  
 Mean   : 5.5   Mean   : 500.5  
 3rd Qu.: 8.0   3rd Qu.: 750.2  
 Max.   :10.0   Max.   :1000.0  
dimension(s):
     from to offset delta refsys point values x/y
time    1 10    0.5     1     NA    NA   NULL [x]
x       1 10   10.5    -1     NA    NA   NULL [y]
y       1 10    0.5     1     NA    NA   NULL

And (2) casting the stars object back to a data frame shows that the data 'value' in not ordered as in the original data frame.

> head(as.data.frame(s))
  time  x   y y.1 value
1    1 10 0.5   1    91
2    2 10 0.5   1   191
3    3 10 0.5   1   291
4    4 10 0.5   1   391
5    5 10 0.5   1   491
6    6 10 0.5   1   591

How can I correctly cast the data frame back and forth to a stars object?

The stars object and methods are extremely useful, but I am puzzled why I can't work out this seemingly simple data conversion. Any help would be greatly appreciated.

Upvotes: 4

Views: 956

Answers (1)

Edzer Pebesma
Edzer Pebesma

Reputation: 4121

Have you tried

st_as_stars(df, dims = c("x", "y", "time"))

?

Upvotes: 4

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