Author: rassta: Raster-based Spatial Stratification Algorithms

# Classification Units

#### rassta: Raster-based Spatial Stratification Algorithms

Get the data required for this vignette

    # Compressed folder with files from rassta’s installation folder
    wasoil <- system.file("exdat/wasoil.zip", package = "rassta")
    # Directory for temporary files
    o <- tempdir()
    # Copy compressed folder to directory for temporary files
    file.copy(from = wasoil, to = o)
    #> [1] TRUE

    # Extract files to subfolder
    d <- paste(o, "/rassta", sep = "")
    unzip(paste(o, "/wasoil.zip", sep = ""), exdir = d)

Classification units are first-level spatial entities that result from the stratification of a landscape factor represented by n variables(s). One example of a landscape factor is climate, which can be represented by variables such as mean annual temperature or total annual precipitation. Accordingly, climatic classification units can result from the spatial stratification of temperature and precipitation through methods such as (bio)climatic classification schemes, cluster analysis, etc. Currently, rassta allows the creation of classification units based on dimension reduction and cluster analysis.

## Dimension reduction and estimation of k

The process to create classification units starts with the reduction of the feature space represented by n variables. This low-dimension representation of the feature space is created using the self-organizing map (SOM) technique of Kohonen (1982, 1990). Subsequently, cluster analysis is performed on the reduced feature space using the partitioning around medoids (PAM) algorithm (Kaufman and Rousseeuw, 1990). The ‘optimal’ number of clusters k is estimated with the gap statistic of Tibshirani et al. (2001). The packages kohonen (Wehrens and Kruisselbrink, 2018) and cluster (Maechler et al., 2021) are imported by rassta to perform the SOM technique and the PAM clustering with gap statistic, respectively.

The following code examples demonstrate how to reduce the feature space and estimate the optimum k using four terrain variables and the function som_gap.

    # Load rassta and terra packages
    library(rassta)
    library(terra)
    # Multi-layer SpatRaster with 4 terrain variables
    var <- c("height.tif", "midslope.tif", "slope.tif", "wetness.tif")
    vardir <- paste(d, var, sep = "/")
    terr.var <- rast(vardir)
    # Plot terrain variables
    par(mfrow = c(2,2))
    nc <- c("Zissou", "Batlow", "Lajolla", "Spectral")
    for(i in 1:4){
      plot(terr.var[[i]], main = names(terr.var)[i],
           col = hcl.colors(100, nc[i]), mar = c(2, 2, 2, 3.5)
          )
    }

    # Set seed
    set.seed(963)
    # Scale variables to mean = 0 and standard deviation = 1
    terr.varscale <- scale(terr.var)
    # With terra::aggregate(), reduce spatial resolution to reduce computing time
    terr.varscale <- aggregate(terr.varscale, fact = 3)
    # Dimensionality reduction and estimation of optimum k (max k to evaluate: 9)
    terr.som <- som_gap(terr.varscale, xdim = 9, ydim = 9, K.max = 9)
    # Plot results
    plot(terr.som$SOM, main = "SOM Codes") # Self-Organizing Map's codes
    par(mar = c(4.5, 4.5, 2, 1))
    plot(terr.som$SOMgap, main = "Gap Statistic") # Gap statistic
    abline(v = terr.som$Kopt) # Optimum k

## Rasterization of the SOM and PAM outputs

Once the feature space has been reduced and the optimum k has been estimated with som_gap, rasterized versions of the SOM and PAM outputs can be created with the function som_pam. The rasterized PAM represents the final set of classification units for a landscape factor. Note that any other clustering algorithm outside of rassta can be used to create classification units from the output SOM of som_gap.

The code below demonstrates the creation of classification units with som_pam.

    # Create reference SpatRaster
    rastref <- aggregate(terr.var[[1]], fact = 3)
    # Rasterization of terrain SOM grid and terrain PAM clustering
    terr.sompam <- som_pam(ref.rast = rastref, kohsom = terr.som$SOM,
                           k = terr.som$Kopt
                          )
    # Plot results
    plot(terr.sompam$sompam.rast[[1]], main = "Terrain Self-Organizing Map",
         mar = c(1.5, 1.3, 1.5, 3.3), col = hcl.colors(100, "spectral", rev = TRUE)
        )
    plot(terr.sompam$sompam.rast[[2]], main = "Terrain Classification Units",
         type = "classes", col = hcl.colors(terr.som$Kopt, "spectral", rev = TRUE),
         mar = c(1.5, 2, 1.5, 2.5)
        )

### Reclassification

Note that using terra, the numeric IDs of classification units can be reassigned according to the interpretation of each unit. The assignment of new IDs for classes is commonly referred to as ‘reclassification’ in the geospatial analysis literature. In the next code example, the numeric ID of the terrain classification units are reassigned based on the variable height. Terrain classes at higher relative positions (e.g., summits) will have values closer to one. Conversely, terrain classes at lower relative positions (e.g., stream channels) will have values closer to 6.

    # Extract rasterized PAM solution
    terr.cu <- terr.sompam$sompam.rast[[2]]
    # With terra::zonal(), unit-wise mean value of terrain height
    terrh <- aggregate(terr.var$height, fact = 3) # Match spatial resolution
    terr.stat <- zonal(terrh, terr.cu, fun = mean)
    # Order numeric IDs based on terrain height (descending)
    terr.stat <- terr.stat[order(terr.stat$height, decreasing = TRUE), ]
    # Column with original numeric IDs
    terr.stat$CU <- seq(1, terr.som$Kopt)
    # With terra::classify(), reclassify numeric IDs
    terr.cu <- classify(terr.cu, terr.stat[, c(1,3)])
    # Plot original and reclassified terrain classification units
    plot(terr.sompam$sompam.rast[[2]], main = "Terrain Classification Units",
         type = "classes", col = hcl.colors(terr.som$Kopt, "spectral", rev = TRUE),
         mar = c(1.5, 2, 1.5, 2.5)
        )
    plot(terr.cu, type = "classes", col = hcl.colors(terr.som$Kopt, "spectral"),
         main = "Reclassified Terrain Classification Units",
         mar = c(1.5, 2, 1.5, 2.5)
        )

Clean files from temporary directory

    unlink(c(paste(o, "/wasoil.zip", sep = ""), d), recursive = TRUE)

## References

- B.A. Fuentes, M.J. Dorantes, and J.R. Tipton. rassta: Raster-based Spatial Stratification Algorithms. EarthArXiv, 2021. https://doi.org/10.31223/X50S57

- L. Kaufman and P. Rousseeuw. Finding groups in data: an introduction to cluster analysis. John Wiley & Sons, 1990. https://doi.org/10.1002/9780470316801

- T. Kohonen. Self-organized formation of topologically correct feature maps. Biological Cybernetics, 43 (1):59–69, 1982. https://doi.org/10.1007/bf00337288

- T. Kohonen. The self-organizing map. Proceedings of the IEEE, 1990. https://doi.org/10.1016/s0925-2312(98)00030-7

- M. Maechler, P. Rousseeuw, A. Struyf, M. Hubert, and K. Hornik. cluster: Cluster Analysis Basics and Extensions. R package version 2.1.2, 2021. https://CRAN.R-project.org/package=cluster

- R. Tibshirani, G. Walther, and T. Hastie. Estimating the number of clusters in a data set via the gap statistic. Journal of the Royal Statistical Society: Series B (Statistical Methodology), 63(2):411–423, 2001. https://doi.org/10.1111/1467-9868.00293

- R. Wehrens and J. Kruisselbrink. Flexible self-organizing maps in kohonen 3.0. Journal of Statistical Software, 87(1):1–18, 2018. https://doi.org/10.18637/jss.v087.i07