function sfc2xy drops other dimensions to attributes

[higgicd]

Hi @edzer et al., I have been putting some time into learning this wonderful package and finding it very useful. One thing I have encountered is when switching between an sfc geometry to xy raster cells, that the other dimensions get dropped back to attributes.

For example:

# read nc sample file
f <- system.file("nc/reduced.nc", package = "stars")
reduced_nc <- read_ncdf(f)
reduced_nc

stars object with 4 dimensions and 4 attributes
attribute(s):
               Min. 1st Qu. Median       Mean 3rd Qu.  Max.  NA's
sst [°C]      -1.80   -0.03 13.655 12.9940841 24.8125 32.97  4448
anom [°C]     -7.95   -0.58 -0.080 -0.1847324  0.2100  2.99  4449
err [°C]       0.11    0.16  0.270  0.2626872  0.3200  0.84  4448
ice [percent]  0.01    0.47  0.920  0.7178118  0.9600  1.00 13266
dimension(s):
     from  to offset delta         refsys         values x/y
lon     1 180     -1     2 WGS 84 (CRS84)           NULL [x]
lat     1  90    -90     2 WGS 84 (CRS84)           NULL [y]
zlev    1   1     NA    NA             NA              0    
time    1   1     NA    NA        POSIXct 1981-12-31 UTC 

Converting the cells to points works as expected, maintaining zlev and time as dimensions:

reduced_nc_points <- reduced_nc |> stars::st_xy2sfc(as_points = TRUE)
reduced_nc_points

stars object with 3 dimensions and 4 attributes
attribute(s):
               Min. 1st Qu. Median       Mean 3rd Qu.  Max. NA's
sst [°C]      -1.80   -0.03 13.655 12.9940841 24.8125 32.97    8
anom [°C]     -7.95   -0.58 -0.080 -0.1847324  0.2100  2.99    9
err [°C]       0.11    0.16  0.270  0.2626872  0.3200  0.84    8
ice [percent]  0.01    0.47  0.920  0.7178118  0.9600  1.00 8826
dimension(s):
         from    to         refsys point                             values
geometry    1 11760 WGS 84 (CRS84)  TRUE POINT (164 -79),...,POINT (358 89)
zlev        1     1             NA    NA                                  0
time        1     1        POSIXct    NA                     1981-12-31 UTC

However, going back the other direction moves zlev and time to attributes:

reduced_nc_rast <- reduced_nc_points |> stars::st_sfc2xy()
reduced_nc_rast

stars object with 2 dimensions and 6 attributes
attribute(s):
              Min.      1st Qu.        Median          Mean      3rd Qu.         Max.  NA's
zlev          0.00         0.00         0.000  0.000000e+00 0.000000e+00         0.00  3540
time  378604800.00 378604800.00 378604800.000  3.786048e+08 3.786048e+08 378604800.00  3540
sst          -1.80        -0.03        13.655  1.299408e+01 2.481250e+01        32.97  3548
anom         -7.95        -0.58        -0.080 -1.847324e-01 2.100000e-01         2.99  3549
err           0.11         0.16         0.270  2.626872e-01 3.200000e-01         0.84  3548
ice           0.01         0.47         0.920  7.178118e-01 9.600000e-01         1.00 12366
dimension(s):
  from  to offset delta         refsys x/y
X    1 180     -1     2 WGS 84 (CRS84) [x]
Y    1  85     90    -2 WGS 84 (CRS84) [y]

Just sparking my curiosity about whether that is expected behaviour, if I am missing something in the options for st_sfc2xy, or how one might put the time and zlev dimensions back. Thanks everyone.

[edzer]

Thanks; example(st_xy2sfc) now has your example working

[higgicd]

That is excellent, a quick fix! For anyone coming across this issue in the future, passing the dims argument through the ... in the updated function now maintains the other dimensions:

reduced_nc_rast <- reduced_nc_points |> st_sfc2xy(dims=c("X", "Y", "zlev", "time"))
reduced_nc_rast

stars object with 4 dimensions and 4 attributes
attribute(s):
       Min. 1st Qu. Median       Mean 3rd Qu.  Max.  NA's
sst   -1.80   -0.03 13.655 12.9940841 24.8125 32.97  3548
anom  -7.95   -0.58 -0.080 -0.1847324  0.2100  2.99  3549
err    0.11    0.16  0.270  0.2626872  0.3200  0.84  3548
ice    0.01    0.47  0.920  0.7178118  0.9600  1.00 12366
dimension(s):
     from  to offset delta         refsys         values x/y
X       1 180     -1     2 WGS 84 (CRS84)           NULL [x]
Y       1  85     90    -2 WGS 84 (CRS84)           NULL [y]
zlev    1   1     NA    NA             NA         [0,NA)    
time    1   1     NA    NA        POSIXct 1981-12-31 UTC