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To subset the extent of a raster to a desired area, we use the 'crop' function in the 'raster' package. This is the most commonly used function to modify the extent of a raster object. The raster function takes two main arguments:

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RasterLayer object Coordinate Reference System (CRS)

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#==================================================================================
# Subset Rasters to Study Area
#==================================================================================

#----------------------------------------------------------------------------------
# Create Cookie Cutter for the Study Area
#----------------------------------------------------------------------------------
# Change Study Area spatial polygon CRS (EPSG:4326) to the Australian-specific CRS used for raster (EPSG:3577).

# Find Raster CRS
SPGC.2010q3.rl

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## class       : RasterLayer 
## dimensions  : 135159, 141481, 19122430479  (nrow, ncol, ncell)
## resolution  : 30, 30  (x, y)
## extent      : -1944645, 2299785, -4910195, -855425  (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 : C:/Users/uqbblanc/Documents/TERN/04b-DSDP_GitHub/Prep/Landscapes_AusCover-RemoteSensing/YasiEffectsonGCatMB/YasiEffectsonGCatMB_Tutorial/SPGC_2010q3.vrt 
## names       : SPGC_2010q3 
## values      : 0, 255  (min, max)

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crs(SPGC.2010q3.rl)

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## CRS arguments:
##  +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

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# Double check that this is the EPSG:3577 CRS (= to the raster's CRS)
CRS("+init=epsg:3577")

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## CRS arguments:
##  +init=epsg:3577 +proj=aea +lat_1=-18 +lat_2=-36 +lat_0=0
## +lon_0=132 +x_0=0 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0
## +units=m +no_defs

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Reproject SpatialPolygons object and create an Extent object

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# Transform the Spatial Polygon CRS (from EPSG:4326 to EPSG:3577)
StudyArea.SP.reprj = spTransform(StudyArea.SP, CRS("+init=epsg:3577"))

# Study Area Reprojected Extent
StudyArea.extent.reprj = extent(StudyArea.SP.reprj) 
StudyArea.extent.reprj

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## class       : Extent 
## xmin        : 1484019 
## xmax        : 1490198 
## ymin        : -1996975 
## ymax        : -1988144

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Crop (Spatially Subset) RasterLayer objects

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Example 1: SPGC for the Winter of 2010

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#----------------------------------------------------------------------------------
# Subset (i.e. crop) raster layer to area of interest
#----------------------------------------------------------------------------------
# Subset both Year-Season rasters
SPGC.StudyArea.2010q3.rl = crop(SPGC.2010q3.rl, StudyArea.extent.reprj)
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      : 100, 197  (min, max)

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Example 2: SPGC for the Winter of 2011

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SPGC.StudyArea.2011q3.rl = crop(SPGC.2011q3.rl, StudyArea.extent.reprj)
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      : 100, 196  (min, max)

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New Version 13

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