Dear Julian, you can do that by passing a non-NA value as no_data_value,
e.g. by
ag3 <- st_warp(src = s1,
dest = st_as_stars(st_bbox(s1), dx = 1, dy = 1),
use_gdal = TRUE,
method = "average",
no_data_value = -9999)
On 16/07/2021 12:17, Julian M. Burgos wrote:
Dear list,
My apologies for posting this again. I am not sure if it went through the
first time.
I am having a bit of trouble using st_warp() to aggregate star objects with the option
"use_gdal = TRUE". I am using the "average" method, and I want cells with NA
values to be ignored when computing the averages. To demonstrate, I will make a small raster for
testing, and will include cells with NA values:
#-------------------------------------------------
library(raster)
# Create raster for testing
myfile <- "~/myraster.tif"
myraster <- raster(xmn = 0, ymn = 0, xmx = 10, ymx = 10, resolution = 0.1,
crs = 4326, vals = sample(1:20, size = 10000, replace =
TRUE))
myraster[sample(1:10000, 200)] <- NA
writeRaster(myraster, filename = myfile, overwrite = TRUE)
#-------------------------------------------------
With the raster package, I can aggregate the cells ignoring the NA values like
this:
r1 <- raster(myfile)
ag1 <- raster::aggregate(x = raster(myfile), fact = 10,
fun = mean, na.rm = TRUE)
...which is what I want. I can also aggregate the cells including the NAs,
like this:
ag2 <- raster::aggregate(x = raster(myfile), fact = 10,
fun = mean, na.rm = FALSE)
.... which produces a raster with a lots of gaps. Now, to do a similar
aggregation with the stars package I can do this:
s1 <- read_stars(myfile)
ag3 <- st_warp(src = s1,
dest = st_as_stars(st_bbox(s1), dx = 1, dy = 1),
use_gdal = TRUE,
method = "average")
The resulting raster (ag3) has gaps like ag2, which means that the NAs are not being ignored when
computing the averages. With use_gdal_TRUE and method="average" we are using gdalwarp
with the average resampling method, which according to the documentation "computes the
weighted average of all non-NODATA contributing pixels.". So I guess the problem is that the
NAs are not being recognized as NODATA pixels.
Does anybody know how to solve this?
Many thanks,
Julian
--
Edzer Pebesma
Institute for Geoinformatics
Heisenbergstrasse 2, 48151 Muenster, Germany
Phone: +49 251 8333081
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