[R-sig-Geo] stars analogous of raster::aggregate

[Edzer Pebesma]

On 9/8/20 8:47 PM, Micha Silver wrote:
> 
> On 08/09/2020 19:33, Julian M. Burgos wrote:
>> Dear all,
>>
>> The raster package has the raster::aggregate function that can be used to reduce the resolution of a raster by aggregating cells by a specific factor. For example, this reduces the resolution of the L7_ETMs.tif raster by a factor of 10:
>>
>> library(raster)
>>
>> rst1 <-  raster(system.file("tif/L7_ETMs.tif", package = "stars"))
>>
>> rst2 <- aggregate(rst1, fact = 10, fun = mean)
>>
>>> res(rst1)
>> [1] 28.5 28.5
>>
>>> res(rst2)
>> [1] 285 285
>>
>> I am trying to do the same thing with a stars object.  The stars package has the stars::aggregate function, but for spatial aggregation it takes an object of class sf or sfc, so it is meant to be used for aggregation over polygons.  I could do something like this:
> 
> Probably st_warp() is what you want:
> 
> 
> l7_file = system.file("tif/L7_ETMs.tif", package = "stars")
> l7 = read_stars(l7_file)
> 
> l7_lowres = st_warp(src = l7, cellsize = c(90, 90), crs = st_crs(l7))
> 
> 
> stars::st_dimensions(l7)
>       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
> 
> 
> stars::st_dimensions(l7_lowres)
>       from  to  offset delta                       refsys point values
> x       1 111  288776    90 +proj=utm +zone=25 +south...    NA NULL [x]
> y       1 112 9120761   -90 +proj=utm +zone=25 +south...    NA NULL [y]
> band    1   6      NA    NA                           NA    NA NULL

Yes; if you want to get similar behaviour as raster, choose a cell size 
that is an exact multiple of the origin's cell size, use_gdal = TRUE, 
and method = "average"; this currently seems to only work for single 
band rasters; along these lines:

library(stars)
# Loading required package: abind
# Loading required package: sf
# Linking to GEOS 3.8.0, GDAL 3.0.4, PROJ 7.0.0
l7_file = system.file("tif/L7_ETMs.tif", package = "stars")
l7 = read_stars(l7_file)
(dest = st_as_stars(st_bbox(l7), dx = 90, dy = 90))
# stars object with 2 dimensions and 1 attribute
# attribute(s):
#     values
#  Min.   :0
#  1st Qu.:0
#  Median :0
#  Mean   :0
#  3rd Qu.:0
#  Max.   :0
# dimension(s):
#   from  to  offset delta                       refsys point values x/y
# x    1 111  288776    90 UTM Zone 25, Southern Hem...    NA   NULL [x]
# y    1 112 9120761   -90 UTM Zone 25, Southern Hem...    NA   NULL [y]
(l7_lowres = st_warp(src = l7[,,,1], dest = dest, use_gdal = TRUE, 
method = "average"))
# stars object with 2 dimensions and 1 attribute
# attribute(s):
#  file14bb5c7321dc.tif
#  Min.   : 56.81
#  1st Qu.: 68.75
#  Median : 79.00
#  Mean   : 79.25
#  3rd Qu.: 88.62
#  Max.   :207.44
# dimension(s):
#   from  to  offset delta                       refsys point values x/y
# x    1 111  288776    90 UTM Zone 25, Southern Hem... FALSE   NULL [x]
# y    1 112 9120761   -90 UTM Zone 25, Southern Hem... FALSE   NULL [y]

> 
> 
> -- 
> Micha Silver
> Ben Gurion Univ.
> Sde Boker, Remote Sensing Lab
> cell: +972-523-665918
> https://orcid.org/0000-0002-1128-1325
> 
> 
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> 

-- 
Edzer Pebesma
Institute for Geoinformatics
Heisenbergstrasse 2, 48149 Muenster, Germany
Phone: +49 251 8333081