CODEWITHSUNDEEP
rr-rasterr-sf
Anthony W
Anthony W

Reputation: 1327

Extracting percentage distribution of values within a raster, from a polygon subset

I have a raster and sf polygon as follows:

library(raster)
libary(sf)

# Create raster r
r = raster(ncol=1000, nrow=1000, xmn=0, xmx=1000, ymn=0, ymx=1000)
values(r) = round(runif(ncell(r),1,10))

# Create sf polygon
poly_sf = st_sfc(st_polygon(list(cbind(c(0,10,50,100,0),c(0,70,300,500,0)))))

The raster contains cells of values between 1 and 10. I'd like to be able to generate a dataframe that contains the overall percentage of total cells for each value, within a subset of raster cells generated by the polygon poly_sf. I've looked at exactextractr but haven't figured out how to achieve what I want using that package.

Upvotes: 2

Views: 1220

Answers (1)

dc37
dc37

Reputation: 16178

You can have the use of mask function from raster package but you need to convert your polygon into an sf object:

library(raster)
library(sf)

# Create raster r
r = raster(ncol=1000, nrow=1000, xmn=0, xmx=1000, ymn=0, ymx=1000)
values(r) = round(runif(ncell(r),1,10))

# Create sf polygon
poly_sf = st_sfc(st_polygon(list(cbind(c(0,10,50,100,0),c(0,70,300,500,0)))))
p2 <- st_as_sf(poly_sf)

# Plot the raster object:
plot(r)

enter image description here

You can create a mask using the mask function:

plot(mask(r, p2))

enter image description here

So, to extract values from this masked object, you can use mask function and count for proportions of each values using table:

# Subset the polyfon from the SF object: 
subset_ra <- mask(r, p2)

# Calculate the porportion of each value
df <- as.data.frame(table(as.matrix(subset_ra)))
df$Percent <- df$Freq / sum(df$Freq) * 100

   Var1 Freq   Percent
1     1  154  5.517736
2     2  329 11.787890
3     3  287 10.283053
4     4  290 10.390541
5     5  325 11.644572
6     6  305 10.927983
7     7  319 11.429595
8     8  312 11.178789
9     9  315 11.286277
10   10  155  5.553565

Does it answer your question ?

Upvotes: 3

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