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I have two raster layers with the same extent, CRS, and resolution, and a shapefile consisting of a number of polygons. I would like to calculate the Pearson correlation between the two rasters within each polygon, and write such values within a new attribute for all the polygons.

I work with QGIS and R.

  • You can convert the polygon to Raster then make the correlation using ERDAS Imagine (I can do this on ERDAS but on QGIS i can't because i'm a beginner of this software ) Jamal chaaouan GIS English – Jamal CHAAOUAN Aug 12 '18 at 9:24
  • Use raster::extract to retrieve the raster values for each polygon and then use apply or mapply to calculate the correlations. – Jeffrey Evans Dec 20 '18 at 4:14
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I find this answer: Considering that all layers are identical in extent and resolution you could stack them all together and compute correlation among stacked objects simply with:

library(raster) 
myfolder <- 'D:/myfolder' 
r_path <- file.path(myfolder, grep(".tif$", 
list.files(file.path(myfolder), 
all.files = F), 
ignore.case = TRUE, value = TRUE)) 
mystack <- raster::stack(r_path) 
raster::layerStats(mystack, 'pearson')

done!

  • But this does not consider the polygons, at all. It just produces a global figure. – Jackk Aug 13 '18 at 9:44
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I am testing this solution. Please correct me if you spot mistakes.

summaryshapes<-list.files(pattern = "\\.shp$")

shapes<-lapply(summaryshapes, readogr)

crop_masked<-list()

crop_masked[i]<-for (i in 1:length(shapes)) {
  raster1_cropped <- crop(raster1, extent(shapes[[i]]))
  raster2_cropped<- crop(raster2, extent(shapes[[i]]))
}

correlationfunction<-function(X){
b[X] <- stack(raster1_cropped[X],raster2_cropped[X])
corre[X] <- sampleRegular(b[X], 100000)
cor.test(corre[X][,1], corre[,2])
}

corr<-lapply(names(shapes), correlationfunction)

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