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In R raster calculations, involving one or more raster layers, can be conducted in 3 different ways:

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Below examples of raster calculations are shown below. These examples are taken from the “Effects of Cyclone Yasi on Green Cover at Mission Beach” tutorial. Putting the code snippets in context, by looking at a broader section of the R script, could be benefitial. Boxes with grey background contain code snippets, and boxes with white background containt code (text) outputs.

Single-Layer Raster Calculations

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#==================================================================================
# Re-scale raster cell values
#==================================================================================
# The rasters cell values are in scale 0-255, so we re-scale them a 0-100 scale to represent the Green Cover Fraction in %.
# Generalized version: ((X - Xmin) * 100) / (Xmax - Xmin)
SPGC.StudyArea.2010q3.rl = SPGC.StudyArea.2010q3.rl * 100 / 255
SPGC.StudyArea.2010q3.rl

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Example 2: Re-scale SPGC for the winter of year 2011

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SPGC.StudyArea.2011q3.rl = SPGC.StudyArea.2011q3.rl * 100 / 255
SPGC.StudyArea.2011q3.rl

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#----------------------------------------------------------------------------------
# Raw Change (in %)
#----------------------------------------------------------------------------------
# We use Raster Algegra to calculate Raw Change (i.e. change in %) in SPGC

SPGC.StudyArea.Diffq3.rl = SPGC.StudyArea.2011q3.rl - SPGC.StudyArea.2010q3.rl
SPGC.StudyArea.Diffq3.rl

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# Method 1: Compute using Raster Algebra
# --------------------------------------
SPGC.StudyArea.StdDiffq3.rl = ((SPGC.StudyArea.2011q3.rl - cellStats(SPGC.StudyArea.2011q3.rl,mean)) / cellStats(SPGC.StudyArea.2011q3.rl,sd)) - 
                               ((SPGC.StudyArea.2010q3.rl - cellStats(SPGC.StudyArea.2010q3.rl,mean)) / cellStats(SPGC.StudyArea.2010q3.rl,sd))
SPGC.StudyArea.StdDiffq3.rl

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# Method 2: Compute using Higher Level Functions from the Package `raster`
# ------------------------------------------------------------------------

calc.DiffStdRaster.f = function(rla, r1b)
{
    diff.std.rl = ((r1b - cellStats(r1b, mean)) / cellStats(r1b, sd)) - ((rla - cellStats(rla, mean)) / cellStats(rla, sd))
    return(diff.std.rl)
} # calc.DiffStdRaster.f = function(rla, r1b)
SPGC.StudyArea.StdDiffq3.rl.m2 = calc.DiffStdRaster.f(SPGC.StudyArea.2010q3.rl, SPGC.StudyArea.2011q3.rl)
SPGC.StudyArea.StdDiffq3.rl.m2

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# Method 3: Compute using function `normImage` in package 'RStoolbox'
# -------------------------------------------------------------------
SPGC.StudyArea.StdDiffq3.rl.m3 = normImage(SPGC.StudyArea.2011q3.rl) - normImage(SPGC.StudyArea.2010q3.rl)
SPGC.StudyArea.StdDiffq3.rl.m3

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# Remove unnecessary (i.e. repeated) objects
#ls(pattern="SPGC.StudyArea.StdDiffq3")
rm(SPGC.StudyArea.StdDiffq3.rl.m2, SPGC.StudyArea.StdDiffq3.rl.m3)
#ls(pattern="SPGC.StudyArea.StdDiffq3")

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