Fix extent of R rasters (grid_density output) to match

Question

I have two rasters that are the result of the grid_density() function from the liDR package. With those rasters I want to compare point distribution of subsets of a LAS file.
My problem is, that I get rasters with different extent (since grid_density() aligns the extent of the output raster exactly to the point cloud. If the point cloud is filtered, points at the edges might be missing and the output raster shrinks a bit.

Often, this is only on one side and by a few rows or colums. Since the rasters are generally in the same resolution, I'd rather avoid a resample(), since it is slow and requires interpolation that shouldn't be needed in most cases.

A code example:

# create reference raster with all points
dgrid_ref = grid_density(las, 2)

# filter las pointcloud and create raster
las_c1 = filter_poi(lasp, Z>=0, Z<5)
dgrid_c1  = grid_density(las_c1, 2)

las_c2 = filter_poi(lasp, Z>=5, Z<9)
dgrid_c2  = grid_density(las_c2, 2)

I tried simply overriding the extent with dgrid_c1 = setExtent(dgrid_c1, dgrid_ref, keepres = TRUE), however that leaves me with a raster that contains only NAs.

I also thought to create an "empty" raster of the reference extent and then "inserting" the subset raster, but couldn't find a way to "paste" a raster into another.

Is there an option to use a "snap raster" for grid_density() that determines the output?
Or is there a better option than to resample()? Like overwriting the extent?

Answer 1

I think you need extend here. Let's make a sample...

Full-world raster:

> rfull = raster()
> rfull[]=1:ncell(rfull)

And extract a smaller raster with the same resolution and origin, so the cells overlap perfectly:

> e = extent(-20,10,-30,0)
> rpart = crop(rfull, e)
> rpart[] = 1:ncell(rpart)

Now we have these two rasters:

> extent(rfull)
class      : Extent 
xmin       : -180 
xmax       : 180 
ymin       : -90 
ymax       : 90 
> extent(rpart)
class      : Extent 
xmin       : -20 
xmax       : 10 
ymin       : -30 
ymax       : 0 

We want to extend the small raster to the extent of the large one, by padding with NA:

> 
> rpack = extend(rpart, rfull)
> 
> extent(rpack)
class      : Extent 
xmin       : -180 
xmax       : 180 
ymin       : -90 
ymax       : 90 

Plotting plot(rpack) gives this:

where you can see the extent is padded out to the extent of the full world raster.

(Note that using extend on a big raster to the extent of a small raster results in an unchanged raster.)