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: library(stars) st1 <- read_stars(system.file("tif/L7_ETMs.tif", package = "stars")) %>% slice(band, 1) grid <- st_make_grid(st_bbox(st1) %>% st_as_sfc(), cellsize = 285) st2 <- aggregate(st1, grid, mean) But the resulting st2 object is not a raster (i.e. does not X and Y dimensions), but instead has a single dimension and contains the geometries of the grid polygons. > st1 stars object with 2 dimensions and 1 attribute attribute(s): L7_ETMs.tif Min. : 47.00 1st Qu.: 67.00 Median : 78.00 Mean : 79.15 3rd Qu.: 89.00 Max. :255.00 dimension(s): from to offset delta refsys point values x 1 349 288776 28.5 UTM Zone 25, Southern Hem... FALSE NULL [x] y 1 352 9120761 -28.5 UTM Zone 25, Southern Hem... FALSE NULL [y] > st2 stars object with 1 dimensions and 1 attribute attribute(s): L7_ETMs.tif Min. : 57.76 1st Qu.: 69.22 Median : 79.04 Mean : 79.09 3rd Qu.: 87.63 Max. :123.51 dimension(s): from to offset delta refsys point geometry 1 1260 NA NA UTM Zone 25, Southern Hem... FALSE values geometry POLYGON ((288776.3 9110729,...,...,POLYGON ((298466.3 9120704,... > So I am stuck, wondering if perhaps st_apply is the answer. What would be the best way to replicate what the raster::aggregate function does using stars? Any ideas? Many thanks, Julian -- 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.bur...@hafogvatn.is rr _______________________________________________ R-sig-Geo mailing list R-sig-Geo@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-geo