Author: rassta: Raster-based Spatial Stratification Algorithms

# Predictive Modeling Engine

#### 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)

The approach of rassta for predictive modeling of response phenomena is based on the landscape similarity to stratification units. If each stratification unit across geographic space represents a distinct landscape configuration, and if each configuration influences a phenomenon in a distinctive way, then the spatial variability of that phenomenon can be assessed across space by relating each geographic location to each distinct landscape configuration. Therefore, the more similar a geographic location is to the landscape configuration represented by a given stratification unit, then also the more similar the response of a phenomenon will be at that location to the typical response for conditions within the given stratification unit. Both continuous and categorical response variables are supported. For categorical responses, each category must be identified by an integer value.

There are three main inputs that rassta requires to perform predictive modeling of response phenomena:

- A multi-layer SpatRaster of landscape similarity to stratification units.
- A data frame with representative observations/measurements of response phenomena (multiple responses of the same type are allowed). One row (i.e., observation/measurement) per landscape similarity layer is required.
- A polygon SpatVector representing the boundaries of the area of interest. This SpatVector can be split into tiles to speed-up modeling through parallel processing.

The code below demonstrates the preparation of inputs required for the prediction of soil organic carbon (SOC; continuous response) across a landscape.

    # Load rassta and terra packages
    library(rassta)
    library(terra)
    # Single-layer SpatRaster of stratification units
    su <- rast(paste(d, "/su.tif", sep = ""))
    # SpatVector with the boundaries of the area of interest
    aoi <- vect(paste(d, "/aoi.shp", sep = "")) # Single area, no tiles
    # Multi-layer SpatRaster with spatial signatures of classification units
    clim.sig <- rast(list.files(d, pattern = "climate_", full.names = TRUE)) # For climatic units
    mat.sig <- rast(list.files(d, pattern = "material_", full.names = TRUE)) # For material units
    terr.sig <- rast(list.files(d, pattern = "terrain_", full.names = TRUE)) # For terrain units
    # Landscape similarity to stratification units
    su.ls <- similarity(su.rast = su, sig.rast = c(clim.sig, mat.sig, terr.sig),
                        su.code = list(climate = c(1, 1),
                                       material = c(2, 2),
                                       terrain = c(3, 3)
                                       )
                      )
    # SpatVector with SOC observations for stratification units
    socobs <- vect(paste(d, "/soc.shp", sep = ""))
    # Representative SOC observation for each stratification unit
    su.obs <- observation(su.rast = su, obs = socobs, col.id = 1, col.resp = 2,
                          method = "mls", ls.rast = su.ls$landsim
                          )
    # Data frame with numeric codes of stratification units and representative SOC values
    su.soc <- su.obs$su_repobs[, c("SU", "soc")]

The code below demonstrates the use of engine() to perform the prediction of SOC values.

    # Prediction of SOC across the landscape based on 3 winning stratification units
    soc <- engine(ls.rast = su.ls$landsim,
                  n.win = 3, # n highest landscape similarity values
                  su.repobs = su.soc,
                  tiles = aoi,
                  outdir = d, # engine() writes results directly on disk
                  overwrite = TRUE
                )

The output SpatRaster with predicted SOC values and the statistical evaluation of prediction performance can be plotted as demonstrated in the code below.

    # Set graphics arrangement
    par(mfrow = c(1,2))
    # Plot modeled soil organic carbon
    plot(soc, col = hcl.colors(100, "Fall", rev = TRUE),
         main = "Modeled Soil Organic Carbon (%)", mar = c(3, 1.7, 3, 2)
        )
    # Evaluate modeling performance
    evalobs <- vect(paste(d, "/soc_valid.shp", sep = "")) # independent SOC measurements
    evalmodel <- extract(soc, evalobs) # modeled SOC to independent sample locations
    names(evalmodel)[2] <- "soc_model"
    evalobs <- cbind(evalobs, evalmodel[2]) # Table with measured and modeled SOC
    eval.rmse <- sqrt(mean((evalobs$soc-evalobs$soc_model)^2)) # RMSE
    eval.mae <- mean(abs(evalobs$soc-evalobs$soc_model)) # MAE
    # Set new graphics arrangement
    par(mgp=c(1.2, 0.2, 0), mar = c(2.5, 1.7, 3.4, 1))
    # Plot measured versus modeled SOC
    plot(evalobs$soc, evalobs$soc_model, cex.axis = 0.83,
         xlab = "Measured Soil Organic Carbon (%)", ylab = "", yaxt = "n"
        )
    axis(2, at = c(3, 3.5, 4, 4.5, 5, 5.5), labels = NA)
    abline(0, 1, lty = "dashed")
    text(x = 4, y = 5.8, cex = 0.8,
         paste("Root mean squared error: ", round(eval.rmse,2), "%", sep = "")
        )
    text(x = 3.85, y = 5.6, cex = 0.8,
         paste("Mean Absolute Error: ", round(eval.mae,2), "%", sep = "")
        )

For each cell of the SpatRaster, the predicted response value is obtained through a weighted average of the representative values from the n stratification units with the highest landscape similarity values. The units with the highest landscape similarity values at a given cell are termed winning stratification units. The weight of a winning unit’s representative value is proportional to the winning unit’s landscape similarity value at the cell. For categorical responses, the modal response value of the winning stratification units replaces the weighted average.

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