识别栅格文件中的CRS

Question

I would like to identify the correct coordinate reference system for the following ASCII raster file :

class       : RasterLayer 
dimensions  : 2160, 4320, 9331200  (nrow, ncol, ncell)
resolution  : 0.0833333, 0.0833333  (x, y)
extent      : -180, 179.9999, -90, 89.99993  (xmin, xmax, ymin, ymax)
coord. ref. : NA 
data source : C:/popc_0AD.asc 
names       : popc_0AD

I tried to guess the correct projection by setting the CRS to some of the common formats and plotting it, as suggested in related posts. But I am still not sure about the correct setting. As far as I am concerned raster and related packages do not entail any function able to estimate missing CRS information. Do you have any idea what this raster file's CRS could be or how to find out?

Answer 1

The extent suggests coordinates are not projected. This seems to be the extent of Earth in degrees. Then, you may want to use EPSG 4326, which is also crs="+proj=longlat +datum=WGS84 +no_defs" :

library(raster)

r <- raster("0AD_lu/cropland0AD.asc")
projection(r) <- "+proj=longlat +datum=WGS84 +no_defs"

However, it is much better to use dataset correctly built with the coordinates reference system. It is never recommended to guess it... But I know that having clean metadata is not always possible...

Answer 2

You have

r <- raster(nrow=2160, ncol=4320, xmn=-180, xmx=179.9999, ymn=-90, ymx=89.99993, crs=NA)

Sébastien Rochette already pointed out that this is surely lon/lat and that you can set the CRS to relfect that

crs(r) <- "+proj=longlat +datum=WGS84"

It seems to me that the extent is a bit suspect. It looks like it is supposed to be a global raster, but that there has been some loss of precision. If so, you may correct that like this:

extent(r) <- c(-180, 180, -90, 90)

To get

r
#class      : RasterLayer 
#dimensions : 2160, 4320, 9331200  (nrow, ncol, ncell)
#resolution : 0.08333333, 0.08333333  (x, y)
#extent     : -180, 180, -90, 90  (xmin, xmax, ymin, ymax)
#crs        : +proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0