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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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Example 1: Re-scale SPGC for the winter of year 2010

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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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## class       : RasterLayer 
## dimensions  : 295, 206, 60770  (nrow, ncol, ncell)
## resolution  : 30, 30  (x, y)
## extent      : 1484025, 1490205, -1996985, -1988135  (xmin, xmax, ymin, ymax)
## coord. ref. : +proj=aea +lat_1=-18 +lat_2=-36 +lat_0=0 +lon_0=132 +x_0=0 +y_0=0 +ellps=GRS80 +units=m +no_defs 
## data source : in memory
## names       : SPGC_2010q3 
## values      : 39.21569, 77.2549  (min, max)

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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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## class       : RasterLayer 
## dimensions  : 295, 206, 60770  (nrow, ncol, ncell)
## resolution  : 30, 30  (x, y)
## extent      : 1484025, 1490205, -1996985, -1988135  (xmin, xmax, ymin, ymax)
## coord. ref. : +proj=aea +lat_1=-18 +lat_2=-36 +lat_0=0 +lon_0=132 +x_0=0 +y_0=0 +ellps=GRS80 +units=m +no_defs 
## data source : in memory
## names       : SPGC_2011q3 
## values      : 39.21569, 76.86275  (min, max)

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Multiple-Layer Raster Calculations

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Example 1: Compute the change in Raw (in %) Seasonal Persistent Ground Cover (SPGC) fraction between the winters of 2011 and 2010

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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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## class       : RasterLayer 
## dimensions  : 295, 206, 60770  (nrow, ncol, ncell)
## resolution  : 30, 30  (x, y)
## extent      : 1484025, 1490205, -1996985, -1988135  (xmin, xmax, ymin, ymax)
## coord. ref. : +proj=aea +lat_1=-18 +lat_2=-36 +lat_0=0 +lon_0=132 +x_0=0 +y_0=0 +ellps=GRS80 +units=m +no_defs 
## data source : in memory
## names       : layer 
## values      : -7.843137, 7.058824  (min, max)

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Example 2: Compute the change in Standardised (in Standard Deviations) Seasonal Persistent Ground Cover (SPGC) fraction between the winters of 2011 and 2010

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Method 1: Compute using Raster Algebra

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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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## class       : RasterLayer 
## dimensions  : 295, 206, 60770  (nrow, ncol, ncell)
## resolution  : 30, 30  (x, y)
## extent      : 1484025, 1490205, -1996985, -1988135  (xmin, xmax, ymin, ymax)
## coord. ref. : +proj=aea +lat_1=-18 +lat_2=-36 +lat_0=0 +lon_0=132 +x_0=0 +y_0=0 +ellps=GRS80 +units=m +no_defs 
## data source : in memory
## names       : layer 
## values      : -3.245601, 2.273073  (min, max)

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Method 2: Compute using Higher Level Functions from the ‘raster’ package

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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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## class       : RasterLayer 
## dimensions  : 295, 206, 60770  (nrow, ncol, ncell)
## resolution  : 30, 30  (x, y)
## extent      : 1484025, 1490205, -1996985, -1988135  (xmin, xmax, ymin, ymax)
## coord. ref. : +proj=aea +lat_1=-18 +lat_2=-36 +lat_0=0 +lon_0=132 +x_0=0 +y_0=0 +ellps=GRS80 +units=m +no_defs 
## data source : in memory
## names       : layer 
## values      : -3.245601, 2.273073  (min, max)

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Method 3: Compute using ‘normImage’ function in ‘RStoolbox’ package

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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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## class       : RasterLayer 
## dimensions  : 295, 206, 60770  (nrow, ncol, ncell)
## resolution  : 30, 30  (x, y)
## extent      : 1484025, 1490205, -1996985, -1988135  (xmin, xmax, ymin, ymax)
## coord. ref. : +proj=aea +lat_1=-18 +lat_2=-36 +lat_0=0 +lon_0=132 +x_0=0 +y_0=0 +ellps=GRS80 +units=m +no_defs 
## data source : in memory
## names       : layer 
## values      : -3.245601, 2.273073  (min, max)

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Compare Raster resulting from the different Standarisation Methods

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all.equal(SPGC.StudyArea.StdDiffq3.rl, SPGC.StudyArea.StdDiffq3.rl.m2)

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## [1] TRUE

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all.equal(SPGC.StudyArea.StdDiffq3.rl, SPGC.StudyArea.StdDiffq3.rl.m3)

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## [1] TRUE

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Remove unwanted objects

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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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