raster summary returns different values using `terra::extract`

Question

I have a municipality polygon that looks like this:

shp_muni
class       : SpatVector 
geometry    : polygons 
dimensions  : 1, 1  (geometries, attributes)
extent      : 5.08748, 5.467728, 52.43524, 52.69111  (xmin, xmax, ymin, ymax)
coord. ref. : lon/lat WGS 84 
names       :   PC4
type        : <int>
values      :  8244

I have a raster that covers the entire world with resolution of 0.25

class       : SpatRaster 
dimensions  : 696, 1440, 1  (nrow, ncol, nlyr)
resolution  : 0.25, 0.25  (x, y)
extent      : -180, 180, -90, 84  (xmin, xmax, ymin, ymax)
coord. ref. : lon/lat WGS 84 
source      : memory 
name        : year_value 
min value   :        1 
max value   :      100

I want to calculate mean, max, and min of raster cells that fall within the municipality including the cells that intersect with the boundary of the polygon. I did this:

cells_score <- terra::extract(temp_rast, shp_muni, exact = T, cells = T, na.rm = TRUE, as.raster = F)
cells_score
    
ID year_value   cell   fraction
 1       14      180741 0.24591428
 1       14      180742 0.50551934
 1       17      182181 0.03271280
 1       16      182182 0.09146393

Using the above, I can calculate min(=14), max(=17) and mean (using the fraction as weights after re-normalising the fraction so that it adds up to 1). However, if I do the following, I get a different number for min:

terra::extract(temp_rast, shp_muni, "max", touches = T, na.rm = TRUE, as.raster = F)
ID year_value
1       17
    
terra::extract(temp_rast, shp_muni, "min", touches = T, na.rm = TRUE, as.raster = F)
ID year_value
1        0

Why I am getting different values using the two approaches?

Robert Hijmans · Accepted Answer · 2022-09-06 00:06:58Z

0

I cannot reproduce that with these example data

library(terra)
v <- vect(system.file("ex/lux.shp", package="terra"))[1,]
r <- aggregate(rast(system.file("ex/elev.tif", package="terra")), 10, mean, na.rm=TRUE)

extract(r, v, touches=T, na.rm=TRUE)[,2] |> sort()
# [1] 352.8333 404.6000 415.4900 448.0000 457.7356 460.4800 464.3478 465.9756 469.1800
#[10] 476.0698 478.1951 494.5867
 
extract(r, v, fun=min, touches=T, na.rm=TRUE)
#  ID elevation
#1  1  352.8333
extract(r, v, fun=max, touches=T, na.rm=TRUE)
#  ID elevation
#1  1  494.5867

I do note that you use different options when you are using argument "fun" (touches=TRUE instead of exact=TRUE); that might explain it. Also, extract does not have an argument as.raster. And I am not sure why the title of your question refers to summary as you are not using it.

answered Sep 6, 2022 at 0:06

Robert Hijmans

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