stars analogous of raster::aggregate

Thanks Edzer and Micha, this was very helpful.  st_warp with use_gdal=TRUE worked like a charm.

My best,

Julian


Edzer Pebesma writes:

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://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Forcid.org%2F0000-0002-1128-1325&data=02%7C01%7C%7Cb8b83838082f41ad457c08d854354350%7C8e105b94435e4303a61063620dbe162b%7C0%7C1%7C637351935026744294&sdata=K5kMOHf6dka1eNJrVRZsgDe0UmxTEhBRU40C%2FPpRXVE%3D&reserved=0

--
Julian Mariano Burgos, PhD
Hafranns?knastofnun, ranns?kna- og r??gjafarstofnun hafs og vatna/
Marine and Freshwater Research Institute
Botnsj?varsvi?s / Demersal Division
   Fornub??ir 5, IS-220 Hafnarfj?r?ur, Iceland
www.hafogvatn.is
S?mi/Telephone : +354-5752037
Netfang/Email: julian.burgos at hafogvatn.is