This will convert aspect-relative-to-grid (i.e. the current aspect tifs) to aspect-relative-to-compass, which is probably what most users will want:
aspect_to_compass <- function(x) {
crds <- coordinates(x)
## calculate longitude of each grid cell in radians
gridlon <- pi/2 - atan2(crds[, 2], crds[, 1])
angle_normalise <- function(x) (x + pi) %% (2 * pi) - pi ## normalize angle in radians to range [-pi,pi)
temp <- angle_normalise(values(x) / 180 * pi) ## convert aspect to radians and normalise to -pi,pi
temp <- temp - gridlon ## adjust aspect for longitude
temp <- angle_normalise(temp) / pi * 180 ## normalize again and back to degrees
values(x) <- temp
x
}
I've run it for the 1km and 200m single-tif files (see *_aspect_compass.tif). Probably worth pre-computing at 100m as well, but then bundling this little helper into raadtools for users to apply on the fly to 8m? (Would be wise to add a check to it, to ensure that the input x is in zero-centred polar stereo, otherwise it willna work)
mdsumner
commented
3 years ago
This will convert aspect-relative-to-grid (i.e. the current aspect tifs) to aspect-relative-to-compass, which is probably what most users will want:
I've run it for the 1km and 200m single-tif files (see
*_aspect_compass.tif). Probably worth pre-computing at 100m as well, but then bundling this little helper into raadtools for users to apply on the fly to 8m? (Would be wise to add a check to it, to ensure that the inputxis in zero-centred polar stereo, otherwise it willna work)