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rgeospatialraster

How to compute r2 from two rasters in R?

I have two rasters, and I would like to see a corelation between the two, and obtain a r2.

 TOTAL2
class      : RasterLayer 
dimensions : 2803, 5303, 14864309  (nrow, ncol, ncell)
resolution : 0.008333333, 0.008333333  (x, y)
extent     : 60.85, 105.0417, 15.95833, 39.31667  (xmin, xmax, ymin, ymax)
crs        : +proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0 
source     : memory
names      : layer 
values     : 0, 400  (min, max)

> lpjENLF$VegCX2X0.7
class      : RasterLayer 
dimensions : 2803, 5303, 14864309  (nrow, ncol, ncell)
resolution : 0.008333333, 0.008333333  (x, y)
extent     : 60.85, 105.0417, 15.95833, 39.31667  (xmin, xmax, ymin, ymax)
crs        : +proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0 
source     : memory
names      : VegCX2X0.7 
values     : 1.874989e-05, 350  (min, max)

How can I compute an r2 value between these two rasters? I have tried to convert both of the rasters into dataframes, but both of the dataframes return as NA. Then I apply, na.rm=T and try to find the r2 but the length of the dataframes for both rasters become different. The second solution I tried was stacking both of the rasters and applying this code: 

layerStats(rasterstack,'pearson')

however I obtain :

$`pearson correlation coefficient`
           VegCX2X0.7 layer
VegCX2X0.7         NA    NA
layer              NA    NA

$mean
VegCX2X0.7      layer 
        NA         NA
Solution

  • Option 1: You can use na.rm in layerStats

    layerStats(rasterstack, 'pearson', na.rm=T)

    Option 2: You can first extract the values from the raster objects and apply the build in function cor. With this function you should add the argument use="complete.obs" to get it working withNA` values.

    cor(values(TOTAL2), values(lpjENLF$VegCX2X0.7), use="complete.obs", method = 'pearson')