cloudShadowMask: Cloud Shadow Masking for Flat Terrain

Description

Intended for interactive use, cloudShadowMask will ask the user to select a few corresponding cloud/cloudShadow pixels which will be used to estimate coordinates for a linear cloudmask shift.

Usage

cloudShadowMask(img, cm, nc = 5, shiftEstimate = NULL,
  preciseShift = NULL, quantile = 0.2, returnShift = FALSE)

Arguments

img

Raster* object containing the scene

Raster* object. Cloud mask (typically the result of cloudMask)

Integer. Number of control points. A few points (default) are fine because the final shift is estimated by coregisterImages.

shiftEstimate

NULL or numeric vector of length two (x,y). Estimated displacement of shadows in map units. If NULL, the user will be asked to select control points interactively.

preciseShift

NULL or numeric vector of length two (x,y). Use this if cloud/cloud-shadow displacement is already known, e.g. from a previous run of cloudShadowMask.

quantile

Numeric (between 0 and 1). Quantile threshold used for image co-registration. By default the 20% quantile of the total intensity (sum) of the image is used as potential shadow mask.

returnShift

Logical. Return a numeric vector containing the shift parameters. Useful if you estimate parameters on a subset of the image.

Value

Returns a RasterLayer with the cloud shadow mask (0 = shadow, NA = not-shadow).

Details

This is a very simplistic approach to cloud shadow masking (simple shift of the cloud mask). It is not image based and accuracy will suffer from clouds at different altitudes. However, just as cloudMask this is a quick and easy to use tool for Landsat data if you're just working on a few scenes and don't have fMask or CDR data at hand. Although for some test scenes it does perform surprisingly well.

Examples

Run this code

library(raster)
library(ggplot2)
## Import Landsat example subset
data(lsat)
## We have two tiny clouds in the east
ggRGB(lsat, stretch = "lin")

## Calculate cloud index
cldmsk    <- cloudMask(lsat, blue = 1, tir = 6)
ggR(cldmsk, 2, geom_raster = TRUE)

## Define threshold (re-use the previously calculated index)
## Everything above the threshold is masked
## In addition we apply a region-growing around the core cloud pixels
cldmsk_final <- cloudMask(cldmsk, threshold = 0.1, buffer = 5)

## Plot cloudmask
ggRGB(lsat, stretch = "lin") +
   ggR(cldmsk_final[[1]], ggLayer = TRUE, forceCat = TRUE, geom_raster = TRUE) +
   scale_fill_manual(values = "red", na.value = NA)
#' ## Estimate cloud shadow displacement
## Interactively (click on cloud pixels and the corresponding shadow pixels)
shadow <- cloudShadowMask(lsat, cldmsk_final, nc = 2)

## Non-interactively. Pre-defined shadow displacement estimate (shiftEstimate)
shadow <- cloudShadowMask(lsat, cldmsk_final, shiftEstimate = c(-16,-6))

## Plot
csmask <- raster::merge(cldmsk_final[[1]], shadow)
ggRGB(lsat, stretch = "lin") +
        ggR(csmask, ggLayer = TRUE, forceCat = TRUE, geom_raster = TRUE) +
        scale_fill_manual(values = c("blue", "yellow"),
        labels = c("shadow", "cloud"), na.value = NA)