I would like to extract values from Corine Land Cover (CLC) 2018 raster.


r <- raster('U2018_CLC2018_V2020_20u1.tif')
#Warning message:
#In showSRID(uprojargs, format = "PROJ", multiline = "NO", prefer_proj = prefer_proj) :
#  Discarded datum Unknown based on GRS80 ellipsoid in Proj4 definition

I got a Warning message, probably due to a lack of a +datum parameter in the proj4 string. The Coordinate Reference System (CRS) for the CLC 2018 is EPSG:3035 (ETRS89, LAEA). The crs from the raster 'r' is :

#CRS arguments:
# +proj=laea +lat_0=52 +lon_0=10 +x_0=4321000 +y_0=3210000 +ellps=GRS80 +units=m +no_defs 

Then I load my 'data' file that is a 'data.frame' with 419389 rows and 15 columns including the 'Latitude' and 'Longitude'. I then run these few lines

lats <- c(data$Latitude)
lons <- c(data$Longitude) 
coords <- data.frame(x=lons,y=lats)
points <- SpatialPoints(coords, proj4string = r@crs) 
points <- na.omit(object = points)
values <- extract(r, points)

#class       : SpatialPoints 
#features    : 419389 
#extent      : -8.06205, 2.39726, 49.18919, 60.59815  (xmin, xmax, ymin, ymax)
#crs         : +proj=laea +lat_0=52 +lon_0=10 +x_0=4321000 +y_0=3210000 +ellps=GRS80 +units=m +no_defs

However, the 'values' return NAs.

I guess the problem is the projection, so I tried to reproject the raster but it doesn't work.

projection(r) <- crs("+proj=laea +lat_0=52 +lon_0=10 +x_0=4321000 +y_0=3210000 +ellps=GRS80 +datum=WGS84 +units=m +no_defs")

You are mixing up the difference between setting a crs (declaration) of a spatial dataset and creating a new spatial dataset with a different crs (transformation, reprojection).

You need to correctly declare the crs of your points. You set them to the crs of r, but that is wrong --- at least the variable names suggest it should instead be longitude/latitude, so you can do:

points <- SpatialPoints(coords, proj4string =CRS("+proj=longlat +datum=WGS84"))

Since this crs does not match that of the raster data, you need to transform the points (transforming point data is fast and does not degenerate the data; that is not true for transforming raster data)

pp <- spTransform(points, "EPSG:3035")

And then you should be able to use extract.

  • Thank you very much! It works, and I got where I was wrong. Apr 5 '21 at 10:37

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