Topographic shadows

Detects and removes self-shadowed pixels (i.e., pixels having aspect angles oriented away from the sun) and cast shadows (i.e, pixels facing towards the direction of the sun but where sunlight is blocked by neighbor topographic features such as mountains). Self-shadowed pixels are detected when the angle between the sun and the vector normal to the grid cell's surface is higher than \(\frac{\pi}{2}\). Cast shadows are calculated using the cast_shadows() function.

Usage

shadow_removal(dem, SZA, SAA, mask = TRUE)

Arguments

dem

SpatRaster. Digital elevation model representing terrain elevation on a regular grid.

SZA

SpatRaster or numeric. Solar zenith angle in degrees.

SAA

SpatRaster or numeric. Solar azimuth angle in degrees.

mask

logical. If TRUE, shadow pixels are masked.

Value

Returns a SpatRaster with topographic shadows removed.

See also

cast_shadows(), hill_shade()

Examples

library(terra)
dem <- system.file("extdata/athabasca_dem.tif", package = "SatRbedo")
dem <- terra::rast(dem)
SZA <- 49
SAA <- 165
msk <- shadow_removal(dem, SZA, SAA, mask = TRUE)
plot(msk, col = "blue")


# Overlay an RGB composite with the shadow masks
blue <- terra::rast(system.file("extdata/athabasca_B02_20200911.tif", package = "SatRbedo"))
green <- terra::rast(system.file("extdata/athabasca_B03_20200911.tif", package = "SatRbedo"))
red <- terra::rast(system.file("extdata/athabasca_B04_20200911.tif", package = "SatRbedo"))
r <- c(stretch(blue), stretch(green), stretch(red))
RGB(r) <- c(3, 2, 1)
plot(r)
msk <- shadow_removal(dem, SZA, SAA, mask = FALSE)
plot(msk, col = grey(0:1), alpha = 0.2, add = TRUE)