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To explore the cell values of raster layers several types of graphical summaries can be used, such as boxplots, and density plots. Scatter plots can be used to explore the relationships between the values of different raster layers.

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#===================================================================================
# Visualise Change in SPGC between Winter 2010 and Winter 2011: Graph Raster Values
#===================================================================================
# We already have a rasterBrick with individuals raw (in %) values (i.e. for 2010-Winter and 2011-Winter).
# We need to create an analogous rasterBrick with standarised (i.e. in SDs) values.


# Create RasterBrick object with Standardised SPGC
# ------------------------------------------------
SPGC.StudyArea.Std2010q3_2011q3.rb = brick(normImage(SPGC.StudyArea.2010q3.rl), normImage(SPGC.StudyArea.2011q3.rl))
names(SPGC.StudyArea.Std2010q3_2011q3.rb) = c("SPGC.SA.Std2010q3", "SPGC.SA.Std2011q3")       
SPGC.StudyArea.Std2010q3_2011q3.rb

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## class       : RasterBrick 
## dimensions  : 295, 206, 60770, 2  (nrow, ncol, ncell, nlayers)
## 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.SA.Std2010q3, SPGC.SA.Std2011q3 
## min values  :         -9.645614,        -10.521245 
## max values  :          3.048260,          2.860489

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#----------------------------------------------------------------------------------
# Boxplots
#----------------------------------------------------------------------------------

# Change in 'Raw' SPGC
# --------------------
RawSPGC.IndvYrs.bwplot = bwplot(SPGC.StudyArea.2010q3_2011q3.rb, main="Raw SPGCs")

# Change in Standardised SPGC
# ---------------------------
StdSPGC.IndvYrs.bwplot = bwplot(SPGC.StudyArea.Std2010q3_2011q3.rb, main="Standardised SPGCs")


#----------------------------------------------------------------------------------
# Scatter plots with loess
#----------------------------------------------------------------------------------

# Change in 'Raw' SPGC
# --------------------
RawSPGC.IndvYrs.splot = splom(SPGC.StudyArea.2010q3_2011q3.rb, plot.loess=TRUE, xlab='')

# Change in Standardised SPGC
# ---------------------------
StdSPGC.IndvYrs.splot = splom(SPGC.StudyArea.Std2010q3_2011q3.rb, main="Standardised SPGCs", plot.loess=TRUE, xlab='')


#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# Combine the 4 Plots in a single Graph
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
grid.arrange( RawSPGC.IndvYrs.bwplot, StdSPGC.IndvYrs.bwplot, 
              RawSPGC.IndvYrs.splot, StdSPGC.IndvYrs.splot,
              nrow=2, top="SPGC - Individuals Years" )

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Density plots

Method 1: Using ‘densityplot’ from the ‘rasterVis’ package

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#---------------------------------------------------------------------------------
# Density Plots
#---------------------------------------------------------------------------------

#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# Method 1: Using 'densityplot' from the package 'rasterVis`
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

# Change in 'Raw' SPGC
# --------------------
RawSPGC.IndvYrs.dp = densityplot(SPGC.StudyArea.2010q3_2011q3.rb, main="Individual Raw SPGCs")
RawSPGC.Diff.dp = densityplot(SPGC.StudyArea.Diffq3.rl, main="Change in Raw SPGCs (2010q3-2011q3)")

# Change in Standardised SPGC
# ---------------------------
StdSPGC.IndvYrs.dp = densityplot(SPGC.StudyArea.Std2010q3_2011q3.rb, main="Individual Standardised SPGCs")
StdSPGC.Diff.dp = densityplot(SPGC.StudyArea.StdDiffq3.rl, main="Change in Standardised SPGCs (2010q3-2011q3)")

# Combine 4 Plots in a Graph
# ~~~~~~~~~~~~~~~~~~~~~~~~~~
grid.arrange( RawSPGC.IndvYrs.dp, RawSPGC.Diff.dp, StdSPGC.IndvYrs.dp, StdSPGC.Diff.dp,
              nrow=2, top="SPGC: Individuals Years & Change" )

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Method 2: Using ‘ggplot’ from the ‘ggplot2’ package

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#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# Method 2: Using 'ggplot' from the package 'ggplot2`
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

# Create a DataFrame with the required values 
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# Create a DataFrame with the required values to pass to function 'ggplot' in the package 'ggplot2' 
# Required values: SPGCs for 2010q3, 2011q3, and their difference in both scales: Raw and Standardised.
RawSPGC.2010q3 = values(SPGC.StudyArea.2010q3.rl)
RawSPGC.2011q3 = values(SPGC.StudyArea.2011q3.rl)
RawSPGC.Diff = values(SPGC.StudyArea.Diffq3.rl)
StdSPGC.2010q3 = values(normImage(SPGC.StudyArea.2010q3.rl))
StdSPGC.2011q3 = values(normImage(SPGC.StudyArea.2011q3.rl))
StdSPGC.Diff = values(SPGC.StudyArea.StdDiffq3.rl)
SPGCs.df = data.frame( RawSPGC.2010q3, RawSPGC.2011q3, RawSPGC.Diff, 
                       StdSPGC.2010q3, StdSPGC.2011q3, StdSPGC.Diff )
#summary(SPGCs.df)
                               

# Change in 'Raw' SPGC
# --------------------

# Density plot of Raw SPGC values for Individual Years 
# ....................................................
# Subset Dataset and convert wide format to long format
RawSPGC.IndvYrs.dflf = melt(SPGCs.df[c("RawSPGC.2010q3","RawSPGC.2011q3")])
RawSPGC.IndvYrs.dp2 = ggplot(RawSPGC.IndvYrs.dflf, aes(x=value, fill=variable)) +
                      geom_density(alpha=0.25)
                   
# Density plot of the change in Raw SPGC values
# ....................................................
RawSPGC.Diff.dp2 = ggplot(SPGCs.df, aes(x=RawSPGC.Diff)) + 
                   geom_density(alpha=0.25,color="darkgreen", fill="lightgreen")
                   

# Change in Standardised SPGC
# ---------------------------

# Density plot of Raw SPGC values for Individual Years 
# ....................................................
# Subset Dataset and convert wide format to long format
StdSPGC.IndvYrs.dflf = melt(SPGCs.df[c("StdSPGC.2010q3","StdSPGC.2011q3")])
StdSPGC.IndvYrs.dp2 = ggplot(StdSPGC.IndvYrs.dflf, aes(x=value, fill=variable)) +
                       geom_density(alpha=0.25)
                   
# Density plot of the change in Raw SPGC values
# ....................................................
StdSPGC.Diff.dp2 = ggplot(SPGCs.df, aes(x=StdSPGC.Diff)) + 
                    geom_density(alpha=0.25,color="darkgreen", fill="lightgreen")

                    
# Combine the 4 Plots in a single Graph
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
grid.arrange( RawSPGC.IndvYrs.dp2, RawSPGC.Diff.dp2, StdSPGC.IndvYrs.dp2,   StdSPGC.Diff.dp2,
              nrow=2, top="SPGC: Individuals Years & Change" )

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