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

[Edzer Pebesma]

It now also works for multi-band rasters (fixed in sf when installed 
from github): https://github.com/r-spatial/stars/issues/320

On 9/9/20 1:10 PM, Julian M. Burgos wrote:
> 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
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> 
> 
> --
> Julian Mariano Burgos, PhD
> Hafrannsóknastofnun, rannsókna- og ráðgjafarstofnun hafs og vatna/
> Marine and Freshwater Research Institute
> Botnsjávarsviðs / Demersal Division
>    Fornubúðir 5, IS-220 Hafnarfjörður, Iceland
> www.hafogvatn.is
> Sími/Telephone : +354-5752037
> Netfang/Email: julian.burgos using hafogvatn.is
> 

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