Recommended Practice: Drought monitoring using the Standardized Vegetation Index (SVI) - R script for large areas

SVI computation using chunking for large countries

Enter the path to the a folder where you have enough free space to store your MODIS data and the resulting products.

Please do not use backslashes except you are working with a MAC or Linux.

dataPath <- "D:/Peru_SVI"

Automatic download from AppEEARS

downloadList<-"D:/Peru_SVI/peru-download-list.txt"

Cloud quality masking: Replace the ‘0’ with a ‘1’ to apply MODIS pixel reliability data. If you are not sure whether you should enable this option, it is recommended to leave it as is. Check the “in detail” page for more information.

cloudmask <- 1

Change the capture of the SVI maps (jpg) according to your study area in line 244

Setting SVI image colours

colorsSVI<-c('#4C0E0D', '#E72223', '#F19069', '#F9F6C6', '#64B14B', '#04984A', '#00320E')

No need to enter more - mark the whole code (ctrl + a) and click on ‘run’ (or press ctrl + enter) **** > Installing all needed packages

---

install.packages("zoo")
install.packages("raster")
install.packages("curl")
install.packages("rgdal")

Calling packages for use

library(zoo)
library(raster)
library(curl)
library(rgdal)

Downloading Files (except ‘VI quality control’ data as they are not needed) If you downloaded your data from appEEARS beforehand, comment out the following seven lines using “#”

filesl <- scan(downloadList, what='list', sep='\n')
for (d in 1:length(filesl)){
  if (grepl("_VI_Quality_",filesl[d]) == F){
    curl_download(url=filesl[d], destfile=paste0(dataPath,"/",basename(filesl[d])),quiet = T, handle = new_handle())
  }
  print(paste0('Downloading source data (Step 1 of 4): ',round(d / length(filesl) * 100, digits=2),'%'))
}

Creating a temp-directory and setting memory limit

dir.create(paste0(dataPath,'/temp/'))
rasterOptions(tmpdir=paste0(dataPath,'/temp/'))
rasterOptions(tolerance=1)
memory.limit(80000)

Listing up all downloaded TIF-files in the data folder

rasterData <- list.files(path=dataPath, pattern='.tif$', recursive=F, ignore.case=T, full.names=T)
rasterFiles <- basename(rasterData)

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Chunk-size for processing (in Pixel)

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You can reduce these numbers if you run into RAM-Problems early on: Try reducing the numbers to 1000. If you still run into RAM-problems, reduce to 500 or 250. Only reduce the numbers if you have memory issues (“Error: cannot allocate vector of size […]”)

chwidth <- 1500
chheight <- 1500

---

Starting SVI processing

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Listing all EVI rasters and their corresponding pixel reliability data

EVIrasterData <- rasterData[grepl('EVI',rasterData)] 
EVIqc <- rasterData[grepl('pixel_reliability',rasterData)] 

Loading example image from the downloaded data

exRST <- raster(EVIrasterData[1])

exPR <- raster(EVIqc[1])

(Automatically adjusting chunk size for small scenes if needed)

initial <- raster(EVIrasterData[1])
if (as.numeric(ncol(initial)) <= chwidth || as.numeric(nrow(initial)) <= chheight){
  chwidth <- ceiling(ncol(initial)/2)
}

Parsing all DOY and Years from the filenames

DOYs <- unique(substr(basename(EVIrasterData),38,40))
YEARs <- unique(substr(basename(EVIrasterData),34,37))

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SVI: chunkwise calculation

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Creating output folders within your main directory

dir.create(paste0(dataPath,'/SVI'))
dir.create(paste0(dataPath,'/SVIjpg'))
dir.create(paste0(dataPath,'/EVI'))

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Determining chunk-shapefile

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Loading example image from the downloaded data

exRST <- raster(EVIrasterData[1])

Determining chunks

h        <- ceiling(ncol(exRST)/ceiling(ncol(exRST) / chwidth))
v        <- ceiling(nrow(exRST)/ceiling(nrow(exRST) / chheight)) 

Creating shapefile for each chunk

split      <- aggregate(exRST,fact=c(h,v))
split[]    <- 1:ncell(split)
splitshp <- rasterToPolygons(split)
names(splitshp) <- 'shapes'
notiles <- ncell(split)

Filtervalues for quality masking: “0” and “1” in the pixel reliability bands are accepted as sufficient quality

Rank Key	Summary QA
-1	Fill/No Data
0	Good data
1	Marginal Data
2	Snow/Ice
3	Cloudy

For this analysis we will be considering good data and marginal data

goodValues <- c(0,1)

---

Masking clouds/snow and splitting data into chunks

---

for (d in 1:length(DOYs)){
  #Filtering all relevant data for this DOY
  vrasterData <- EVIrasterData[grepl(paste0(DOYs[d],'_aid'),EVIrasterData)]
  #..and their corresponding pixel reliability data
  vQC <- EVIqc[grepl(paste0(DOYs[d],'_aid'),EVIqc)]
  #Reading years of available data
  vYear <- substr(basename(vrasterData),34,37)
  for (y in 1:length(vYear)){
    viPRE <- raster(vrasterData[y])
    #Applying quality mask to each image (if masking was activated)
    if (cloudmask == 1){
      qc <- raster(vQC[y])
      viPRE <- overlay(viPRE, qc, fun = function(x, y) {
        x[!(y %in% goodValues)] <- NA
        return(x)
      })
    }
    
    #####Splitting (masked) data into Chunks 
    for(i in 1:ncell(split)){
      ex          <- extent(splitshp[splitshp$shapes==i,])
      exx         <- crop(viPRE,ex)
      writeRaster(exx,filename=paste0(dataPath,'/temp/',DOYs[d],'_',vYear[y],'_EVICHUNK',
                                      formatC(i, width=3, flag='0')),format='GTiff', overwrite=TRUE)
    }
  }
  #(Progress report)
  print(paste0('Data preparation (SVI) & masking (Step 2 of 4): ',round(d / length(DOYs)  * 100, digits=2),'%'))
}

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Applying SVI calculation for each chunk

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List all chunks

EVIchunks <- list.files(path=paste0(dataPath,'/temp/'), pattern='_EVICHUNK', recursive=F, 
                        ignore.case=T, full.names=T)
for (d in 1:length(DOYs)){
  for (t in 1:notiles){
    #Filtering relevant chunks for this specific date
    sEVIchunks <- EVIchunks[grepl(paste0(DOYs[d],'_'),EVIchunks)] 
    sEVIchunks <- sEVIchunks[grepl(paste0('_EVICHUNK',formatC(t, width=3, flag='0')),sEVIchunks)] 
    #Listing years of data available for each DOY
    vvYear <- substr(basename(sEVIchunks),5,8)
    
    if (length(sEVIchunks) > 0){
      sT <- stack(sEVIchunks)
      
      #Removing filler-values from EVI data
      #The fill value for MODIS images is -3000  
      sT[sT<(-2999)] <- NA
      
      
      evimean <- stackApply (sT, rep (1, nlayers (sT)), mean, na.rm=T) #calculating the mean for the layer stack for each individual pixel
      evisd <- stackApply (sT, rep (1, nlayers (sT)), sd, na.rm=T) 
      SVI <- ((sT-evimean)/evisd)
      
      
      ############################################
      
      #Writing SVI-chunks for each available year
      for (y in 1:length(vvYear)){
        writeRaster(SVI[[y]],filename=paste0(dataPath,'/temp/',DOYs[d],'_',vvYear[y],'_SVICHUNK',
                                             formatC(t, width=3, flag='0')),format='GTiff', overwrite=TRUE)
      }
    }
  }
  #(Progress report)
  print(paste0('SVI processing (Step 3 of 4): ',round(d / length(DOYs)  * 100, digits=2),'%'))
}

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SVI chunk merging and output

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Listing all created chunks

SVIchunks <- list.files(path=paste0(dataPath,'/temp/'), pattern='_SVICHUNK', recursive=F, ignore.case=T, 
                        full.names=T)
#Looping through each available DOY_YEAR combination
for (y in 1:length(YEARs)){
  for (d in 1:length(DOYs)){
    #Listing relevant chunks (EVI, SVI) for this specific date
    sSVIchunks <- SVIchunks[grepl(paste0(DOYs[d],'_',YEARs[y],'_'),SVIchunks)]
    sEVIchunks <- EVIchunks[grepl(paste0(DOYs[d],'_',YEARs[y],'_'),EVIchunks)] 
    #Creating raster-list of the EVI chunks
    if (length(sEVIchunks) > 0){
      if (length(sEVIchunks) > 1){
        sMos <- list()
        for (o in 1:length(sEVIchunks)){
          sMos <- append(sMos, raster(sEVIchunks[o]))
        }
        #Mosaicking the EVI chunks
        sMos$fun = mean
        Mos <- do.call(mosaic, sMos)
        Mos <- Mos*0.0001
        #Output of mosaicked, scaled EVI image
        writeRaster(Mos,filename=paste0(dataPath,'/EVI/',DOYs[d],'_',YEARs[y],'_EVI'),format='GTiff', 
                    overwrite=TRUE)
      }else{
        #Fail-safe in case only one chunk is available
        exp <- raster(sEVIchunks[1])
        exp <- exp*0.0001
        writeRaster(exp,filename=paste0(dataPath,'/EVI/',DOYs[d],'_',YEARs[y],'_EVI'),format='GTiff', 
                    overwrite=TRUE)
      }
    }
    #Creating raster-list of the SVI chunks
    if (length(sSVIchunks) > 0){
      if (length(sSVIchunks) > 1){
        sMos <- list()
        for (o in 1:length(sSVIchunks)){
          sMos <- append(sMos, raster(sSVIchunks[o]))
        }
        #Mosaicking the SVI chunks
        sMos$fun = mean
        Mos <- do.call(mosaic, sMos)
        
        #Plotting JPG of SVI
        
        stdev <- cellStats(Mos, stat='sd')#calculating the standard deviation for each layer of the raster data,
                                          #returning as many values as number of input layers
        stdev2 <- 2*stdev #2 standard deviations, returning as many values as number of input layers
        stdev15 <- 1.5*stdev #1.5 standard deviations, returning as many values as number of input layers
        
        breaksSVI <- c(-4, -stdev2, -stdev15, -stdev, stdev, stdev15, stdev2, 4) 
        #definition of the breaks of the resulting maps based on statistics
        #(standard deviation), returning as many values as number of input layers
        #times 6 (i.e. 6 values for each of the 15 layers) plus 2
        #(the -4 in the beginning and 4 in the end)
        
        l <- nlayers(Mos)
        x <- seq(2, 6*l+1, l)
      
        
        jpeg(filename=paste0(dataPath,'/SVIjpg/',DOYs[d],"_",YEARs[y],".jpg",sep=""), quality = 100)
        
        
        plot(Mos, #zlim=c(0,100), 
             breaks=c(-4, breaksSVI[x], 4),                       #sets the breaks as defined above
             col=colorsSVI,                                       #sets the colors as defined above
             main=paste("SVI"," (EVI)"," Peru ",DOYs[d]," ",YEARs[y],sep=""))#automizes the title of the plot
        
        
        dev.off()
        
        #Output: final SVI mosaic
        writeRaster(Mos,filename=paste0(dataPath,'/SVI/',DOYs[d],'_',YEARs[y],'_SVI'),format='GTiff', 
                    overwrite=TRUE)
      }else{
        #Fail-safe in case only one chunk is available
        writeRaster(raster(sSVIchunks[1]),filename=paste0(dataPath,'/SVI/',DOYs[d],'_',YEARs[y],'_SVI'),
                    format='GTiff', overwrite=TRUE)
      }
    }
    print(paste0('EVI output & SVI output (Step 4 of 4): ',round((d+(length(DOYs)*(y-1)))/(length(DOYs)*length(YEARs))*100, digits=2),'%')) 
  }
}

Removing temp files to free space

#tmp <- list.files(path=paste0(dataPath,'/temp/'), recursive=F, ignore.case=T, full.names=T)
#file.remove(tmp)