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
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