Extent from Centroids, Projection, and Resolution?

Question

I have a file that defines a raster, although all the metadata was lost. I know the lat/long coordinates of the grid centroids, and that the grid is in a Mercator projection with a 1/12deg equatorial resolution. Is there any way to define the extent of the raster by just knowing the centroids and resolution? Currently I'm doing:

xmin = min(longitude)-1/24
xmax = max(longitude)+1/24
ymin = min(latitude)-1/24
ymax = max(latitude)+1/24

Answer 1

The extent of the raster is equal to the extent of the cell centres, expanded by half the resolution.

Here's an example:

1. Create a dummy raster with extent c(0, 1, 0, 1) and resolution c(0.1, 0.1):

   library(raster)
   r <- raster(res=0.1, xmn=0, xmx=1, ymn=0, ymx=1)
   

2. Extract cell centres:

   p <- rasterToPoints(r)
   head(p)
   
   #         x    y
   # [1,] 0.05 0.95
   # [2,] 0.15 0.95
   # [3,] 0.25 0.95
   # [4,] 0.35 0.95
   # [5,] 0.45 0.95
   # [6,] 0.55 0.95
   

3. Calculate the extent of the cell centres:

   e <- extent(p)
   

4. Expand the extent by half the resolution. Note we don't need to specify 0.5 * res(r). The + method for extent objects yields a new extent that is res(r) units longer, overall, on each axis.

   e + res(r)
   
   # class       : Extent 
   # xmin        : 1.387779e-17 
   # xmax        : 1 
   # ymin        : -5.551115e-17 
   # ymax        : 1 
   

(There's minor floating point error in the recovered extent.)

However, I don't see any problem with your approach, either:

min(p[, 'x']) - xres(r)/2
# [1] 0

max(p[, 'x']) + xres(r)/2
# [1] 1

min(p[, 'y']) - xres(r)/2
# [1] -6.938894e-17

max(p[, 'y']) + yres(r)/2
# [1] 1