modiscloud (version 0.14)

get_bitgrid_2bits: extract the value of 2 particular bits at each pixel

Description

Some MODIS cloud products have 2-bit codes, e.g. the MOD35_L2 product has a 2-bit code specifying the 4 possible outputs of the cloud-classification algorithm: confident cloudy, probable cloudy, probable clear, confident clear.

Usage

get_bitgrid_2bits(grd, bitnums)

Arguments

grd

A SpatialGridDataFrame, derived from e.g. readGDAL

bitnums

The bit you would like to extract

Value

grdr_vals_bitstrings, a list of strings for the bits in question (string, e.g. "11" or "01" or "00")

Details

Note: this means that, when interpreting the two 2-bit strings in the MODIS image, you have to reverse the bit order. This is done by default here.

References

Ackerman S, Frey R, Strabala K, Liu Y, Gumley L, Baum B and Menzel P (2010). "Discriminating clear-sky from cloud with MODIS algorithm theoretical basis document (MOD35)." MODIS Cloud Mask Team, Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin - Madison. .

GoldsmithMatzkeDawson2013

See Also

extract_bit

get_bitgrid

Examples

#######################################################
# Load a TIF
#######################################################
# Code excluded from CRAN check because it depends on modiscdata
## Not run: 
# library(devtools)
# # The modiscdata (MODIS c=cloud data=data) package is too big for CRAN (60 MB); so it is available on github:
# # https://github.com/nmatzke/modiscdata
# # If we can't get install_github() to work, try install_url():
# # install_github(repo="modiscdata", username="nnmatzke")
# install_url(url="https://github.com/nmatzke/modiscdata/archive/master.zip")
# library(modiscdata)
# tifsdir = system.file("extdata/2002tif/", package="modiscdata")
# tiffns = list.files(tifsdir, pattern=".tif", full.names=TRUE)
# tiffns
# 
# library(rgdal)	# for readGDAL
# 
# # numpixels in subset
# xdim = 538
# ydim = 538
# 
# 
# # Read the grid and the grid metadata
# coarsen_amount = 1
# xdim_new = xdim / floor(coarsen_amount)
# ydim_new = ydim / floor(coarsen_amount)
# 
# fn = tiffns[1]
# grd = readGDAL(fn, output.dim=c(ydim_new, xdim_new))
# 
# grdproj = CRS(proj4string(grd))
# grdproj
# grdbbox = attr(grd, "bbox")
# grdbbox
# 
# #######################################################
# # Extract a particular pair of bits for all the pixels in the grid
# #######################################################
# bitnum = 2
# grdr_vals_bitstrings = get_bitgrid_2bits(grd, bitnum)
# length(grdr_vals_bitstrings)
# grdr_vals_bitstrings[1:50]
# ## End(Not run)