# 2. Preprocessing temporally explicit data

# Summary

- Overview
- Aligning rasters
- Spatiotemporal rarefaction
- Temporally explicit extraction
- Scaling rasters
- Spatiotemporal partitioning
- Generating pseudoabsences
- Outputs ready for modeling

# Overview

The preprocessing phase of TemporalModelR takes raw occurrence records and environmental rasters and produces the structured inputs that the modeling functions expect. Six functions cover this phase, and they are typically run in order:

1. `raster_align()` - reproject and mask all rasters to a common reference grid.
2. `spatiotemporal_rarefaction()` - subset occurrence points to one per pixel per time step.
3. `temporally_explicit_extraction()` - extract environmental values matched to each point’s observation time.
4. `scale_rasters()` - z-score rasters using the per-variable means and SDs from extraction. This step is optional, but should be applied for models sensitive to consistent scaling in predictor variables. If not, raw values and aligned rasters may be used.
5. `spatiotemporal_partition()` - assign points to single fold, spatial, temporal, spatiotemporal, or random cross-validation folds.
6. `generate_absences()` - produce fold-stratified pseudoabsences for presence/absence models.

This vignette walks through each step using the bundled synthetic dataset described in the About the Example Dataset vignette. Read that vignette first if you have not already, as it explains the structure of the raw rasters and occurrence points used here. We use the seasonal (compound time-step) workflow throughout, with `forest_cover` (annual), `prseas` (year-season), and `elevation` (static) as predictors. The annual workflow is identical apart from substituting `pr_ann` for `prseas` and dropping `season` from the time columns.

    library(TemporalModelR)
    library(terra)
    library(sf)

    raw_dir  <- system.file("extdata/rasters_raw", package = "TemporalModelR")

    pts_file <- system.file("extdata/points/synthetic_occurrence_points.csv",
                            package = "TemporalModelR")

    ref_file <- file.path(raw_dir, "elevation.tif")
    study_crs <- sf::st_crs(terra::rast(ref_file))

# Aligning rasters

`raster_align()` standardizes every raster in a directory to a single reference grid by reprojecting, resampling, and masking. This is essential because downstream functions assume all environmental layers share identical CRS, resolution, and extent. Even small differences are enough to misalign extraction at the pixel level. With the bundled dataset the raw rasters are already on the same grid, but we run the alignment step to show the call pattern.

    aligned_dir <- file.path(tempdir(), "rasters_aligned")

    raster_align(
      input_dir        = raw_dir,
      output_dir       = aligned_dir,
      reference_raster = ref_file,
      resample_method  = "bilinear",
      overwrite        = TRUE
    )

`resample_method = "bilinear"` is appropriate for continuous variables like elevation and precipitation. Use `"near"` for categorical rasters such as land cover classes; bilinear interpolation between class codes would incorrectly generate codes that do not exist.

We now have a directory of aligned rasters which can be used in downstream analyses.

    aligned_dir <- system.file("extdata/rasters_aligned", package = "TemporalModelR")

    list.files(aligned_dir, pattern = "\\.tif$")[1:6]
    #> [1] "elevation_Masked_Updated.tif"       "forest_cover_10_Masked_Updated.tif"
    #> [3] "forest_cover_11_Masked_Updated.tif" "forest_cover_12_Masked_Updated.tif"
    #> [5] "forest_cover_13_Masked_Updated.tif" "forest_cover_14_Masked_Updated.tif"

# Spatiotemporal rarefaction

`spatiotemporal_rarefaction()` reduces sampling bias and pseudoreplication by retaining one occurrence per pixel per unique time-step combination. The produces an only spatially rarefied table (one point per pixel, regardless of time), and when `time_cols` is provided, it additionally produces a spatiotemporally rarefied table that preserves multiple observations from the same pixel when they fall in different time steps.

This distinction matters for temporally explicit modeling. A static workflow would discard the second observation at the same pixel as redundant. A temporally explicit workflow keeps it, correctly recognizing that the environment at that pixel may have changed between the two observation times, making the second record carry unique information about the species’ niche.

    rare_dir <- file.path(tempdir(), "rarefied")

    rare_out <- spatiotemporal_rarefaction(
      points_sp        = pts_file,
      output_dir       = rare_dir,
      reference_raster = ref_file,
      time_cols        = c("year", "season"),
      xcol             = "x",
      ycol             = "y",
      points_crs       = study_crs,
      output_prefix    = "Pts_seasonal",
      verbose          = FALSE
    )

    rare_out$input_points
    #> [1] 150

    rare_out$spatial_points
    #> [1] 84

    rare_out$spatiotemporal_points
    #> [1] 150

The spatial-only count is the number of unique pixels containing at least one record, retaining only 84 points of the starting 150. The spatiotemporal count is the number of unique pixel-year-season combinations and so will typically be larger whenever the dataset contains repeat visits to the same pixel in different time steps. In this instance, all 150 starting points contain unique pixel-year-season combinations.

Two CSVs are written to `output_dir`. The spatiotemporal one is what we pass forward; the spatial-only file is retained for reference and for users who want a static comparison:

    rare_out$files_created
    #> $spatiotemporal
    #> [1] "C:\\Users\\Conno\\AppData\\Local\\Temp\\Rtmp4eKei8/rarefied/Pts_seasonal_OnePerPixPerTimeStep.csv"
    #>
    #> $spatial
    #> [1] "C:\\Users\\Conno\\AppData\\Local\\Temp\\Rtmp4eKei8/rarefied/Pts_seasonal_OnePerPix.csv"

If we wanted to focus on annual variation rather than both annual and seasonal variation, we could instead run this focusing only ‘year’ as our time_col of interest. This would instead save all unique pixel-year combinations, but assume locations from the same year and location but different seasons share no new data.

    rare_dir <- file.path(tempdir(), "rarefied")

    rare_out_annual <- spatiotemporal_rarefaction(
      points_sp        = pts_file,
      output_dir       = rare_dir,
      reference_raster = ref_file,
      time_cols        = "year",
      xcol             = "x",
      ycol             = "y",
      points_crs       = study_crs,
      output_prefix    = "Pts_ann",
      verbose          = FALSE
    )

    rare_out_annual$input_points
    #> [1] 150

    rare_out_annual$spatial_points
    #> [1] 84

    rare_out_annual$spatiotemporal_points
    #> [1] 144

This still retains many more points (144) than just the spatially explicit extraction.

# Temporally explicit extraction

`temporally_explicit_extraction()` temporally aligns the raster dataset with the temporal columns of the now rarefied species occurrence dataset. For each occurrence record it extracts environmental values from the raster matched to that point’s observation time, rather than from a long-term average. The pairing between point and raster is driven by `variable_patterns`: clean variable names on the left, filename patterns with time placeholders on the right.

    ext_dir <- file.path(tempdir(), "extracted")

    ext_out <- temporally_explicit_extraction(
      points_sp           = rare_out$files_created$spatiotemporal,
      raster_dir          = aligned_dir,
      variable_patterns   = c(
        "elevation"    = "elevation",
        "forest_cover" = "forest_cover_YEAR",
        "prseas"       = "prseas_YEAR_SEASON"
      ),
      time_cols           = c("year", "season"),
      xcol                = "X",
      ycol                = "Y",
      points_crs          = study_crs,
      output_dir          = ext_dir,
      output_prefix       = "extracted_seasonal",
      save_raw            = TRUE,
      save_scaled         = TRUE,
      save_scaling_params = TRUE,
      verbose             = FALSE
    )

    head(ext_out$raw_values)
    #>   year season pixel_id elevation forest_cover prseas   x    y
    #> 1   11 Spring        5      1247        0.846    310 450 1450
    #> 2    4 Autumn        5      1247        0.856    370 450 1450
    #> 3    7 Autumn        5      1247        0.825    370 450 1450
    #> 4   13 Spring        6      1290        0.844    364 550 1450
    #> 5   15 Autumn        6      1290        0.809    370 550 1450
    #> 6    5 Autumn        6      1290        0.903    340 550 1450

A few details worth highlighting:

- `"elevation" = "elevation"` is a static pattern with no placeholder, so every record is matched to the single `elevation.tif` raster.
- `"forest_cover" = "forest_cover_YEAR"` substitutes each record’s `year` value for `YEAR`, so a record where the year column has a value of 7 is matched to the raster `forest_cover_7.tif`.
- `"prseas" = "prseas_YEAR_SEASON"` substitutes both `year` and `season`, so a record where the year column is 3 and the season column = spring is matched to the raster `prseas_3_Spring.tif`.
- We could also include `"pr_ann" = "pr_ann_YEAR"` to match with annual measures of precipitation.

The same call generates three outputs:

    ext_out$files_created
    #> $raw
    #> [1] "C:\\Users\\Conno\\AppData\\Local\\Temp\\Rtmp4eKei8/extracted/extracted_seasonal_Raw_Values.csv"
    #>
    #> $scaled
    #> [1] "C:\\Users\\Conno\\AppData\\Local\\Temp\\Rtmp4eKei8/extracted/extracted_seasonal_Scaled_Values.csv"
    #>
    #> $scaling_params
    #> [1] "C:\\Users\\Conno\\AppData\\Local\\Temp\\Rtmp4eKei8/extracted/extracted_seasonal_Scaling_Parameters.csv"

The raw values file is what models that don’t need scaling will consume. The scaled file applies a z-score per variable using the means and SDs computed across all records. The scaling parameters file records those means and SDs so the same transformation can be applied to the prediction rasters, if necessary.

    ext_out$scaling_params
    #>       variable         mean           sd
    #> 1    elevation 1936.4933333 462.38550909
    #> 2 forest_cover    0.8641333   0.04070431
    #> 3       prseas  317.3533333  73.04610447

Z-scoring matters when an algorithm is sensitive to variable scale, such as those that rely on Euclidean distances and will misbehave if one variable’s units dwarf the others (e.g. Temperature running from -10 to 38 C compared to precipitation running from 0 to 3000 mm. Use the scaling parameters CSV when running `scale_rasters()` so the rasters and the extracted point data share the same transformation.

# Scaling rasters

`scale_rasters()` z-scores every raster in a directory using the per-variable means and SDs from extraction. The transformation is `(value - mean) / sd`, applied pixel-wise.

    scaled_dir <- file.path(tempdir(), "rasters_scaled")

    scale_rasters(
      input_dir           = aligned_dir,
      output_dir          = scaled_dir,
      scaling_params_file = ext_out$files_created$scaling_params,
      variable_patterns   = c(
        "elevation"    = "elevation",
        "forest_cover" = "forest_cover_YEAR",
        "prseas"       = "prseas_YEAR_SEASON"
      ),
      time_cols           = c("year", "season"),
      overwrite           = TRUE,
      verbose             = FALSE
    )

`variable_patterns` mirrors the extraction call. For each variable in the scaling parameters file, every matching raster in `input_dir` is z-scored with the same mean and SD, this guarantees the rasters and the points share a consistent transformation.

    scaled_dir <- system.file("extdata/rasters_scaled", package = "TemporalModelR")

    list.files(scaled_dir, pattern = "\\.tif$")[1:6]
    #> [1] "elevation_Scaled.tif"       "forest_cover_10_Scaled.tif"
    #> [3] "forest_cover_11_Scaled.tif" "forest_cover_12_Scaled.tif"
    #> [5] "forest_cover_13_Scaled.tif" "forest_cover_14_Scaled.tif"

We now have a directory of scaled rasters which we’ll use for future analyses.

# Spatiotemporal partitioning

`spatiotemporal_partition()` assigns each occurrence record to a cross-validation fold which will be used to create training and testing datasets to build and assess our temporally explicit models. The function supports five modes:

- Single fold. All points used for both training and testing. No held-out validation. Useful for a final production model after cross validated quality has been established, or when sample sizes are too small for splitting.
- Random. Folds are random shuffles with no spatial or temporal structure. Useful as a naive baseline.
- Spatial-only. Recursive k-d tree bisection produces geographically distinct contiguous regions, one per fold. Use when you want to test geographic transferability and don’t have time structure to exploit.
- Temporal-only. Each fold spans the full study area but covers a distinct slice of the time series, defined by quantile-based breaks. Use to test temporal transferability.
- Spatiotemporal. Combines the two: `n_spatial_folds` are geographically distinct folds and `n_temporal_folds` are time-restricted folds. Tests both kinds of transferability simultaneously.

We use the spatiotemporal design with two folds in each pool. The function requires the scaled (or raw) extracted points as input, a polygon defining the study area, and a single `time_cols` column (unlike the multi-column placeholders elsewhere; see `?spatiotemporal_partition` for the rationale).

    ext_scaled_file <- system.file(
      "extdata/points/extracted_seasonal_Scaled_Values.csv",
      package = "TemporalModelR"
    )

    study_area_sf <- sf::st_as_sf(sf::st_as_sfc(
      sf::st_bbox(c(xmin = 0, xmax = 3000, ymin = 0, ymax = 1500),
                  crs = study_crs)
    ))

    partition <- spatiotemporal_partition(
      reference_shapefile_path = study_area_sf,
      points_file_path         = ext_scaled_file,
      xcol                     = "x",
      ycol                     = "y",
      points_crs               = study_crs,
      time_cols                = "year",
      n_spatial_folds          = 2,
      n_temporal_folds         = 2,
      max_attempts = 10,
      max_imbalance = 0.15,
      create_plot              = TRUE,
      verbose                  = FALSE
    )

    #> Spherical geometry (s2) switched off
    #> Spherical geometry (s2) switched on

    partition$summary
    #>                        parameter          value
    #> 1                    total_folds              4
    #> 2                n_spatial_folds              2
    #> 3               n_temporal_folds              2
    #> 4               n_balanced_folds              0
    #> 5                 n_random_folds              0
    #> 6                 partition_mode spatiotemporal
    #> 7                   total_points            150
    #> 8                 points_removed              0
    #> 9               pct_rows_removed              0
    #> 10           final_imbalance_pct          14.67
    #> 11 temporal_partitioning_enabled           TRUE

The returned `points_sf` carries a `fold` column alongside the original predictor columns. Two of the four folds are geographically restricted but span the full time series; the other two overlap spatially and span the remainder of the study area but are restricted to a distinct time slice each.

For partitioning purposes, time information must be encoded in a single column based on what temporal scale is relevant for the desired partitioning. Both example workflows in this package use the coarsest time scale for partitioning `time_cols = "year"` at this step (the season is preserved in the point data for later modeling and prediction, just not used for fold construction). If both year and season are conceptually meaningful for fold breaks, encode them into a single ordered numeric column before partitioning. Alternatively, partitioning could be done across seasons to view the environmental transferability of seasonal environmental values.

# Generating pseudoabsences

`generate_absences()` produces fold-stratified pseudoabsence/background points for presence/absence modeling. Pseudoabsences measure the environmental characteristics of locations not otherwise characterized by species occurrences and are essential for GLMs, GAMs, and random forests when no ‘true absence’ points exist. The hypervolume method is presence-only and does not need them.

Three generation methods are supported:

- Random. Uniform sampling across the study area with a small buffer around presences to avoid exact overlap.
- Buffer. Sampling within a user-specified radius of presence locations. Useful when you want pseudoabsences from the same general environment as the presences, on the assumption that nearby unsampled locations are more likely to represent true absence than locations far from any survey effort.
- Environmental. Cells outside the species’ environmental tolerance envelope are identified as candidates, then k-means clustering selects a spatially representative subset. When `buffer_distance` is also supplied the environmental filtering is restricted to within that buffer, implementing the three-step approach of Senay et al. (2013).

The function distributes pseudoabsences across folds and across time steps in proportion to the presences in each fold-time combination, so the potential environmental conditions experienced by each pseudoabsence match those experienced by the presences it complements.

We use the buffer method with a 300 m radius (three pixels at the synthetic landscape’s 100 m resolution) and a 2:1 pseudoabsence-to-presence ratio:

    absences <- generate_absences(
      partition_result         = partition,
      reference_shapefile_path = study_area_sf,
      raster_dir               = scaled_dir,
      variable_patterns        = c(
        "elevation"    = "elevation",
        "forest_cover" = "forest_cover_YEAR",
        "prseas"       = "prseas_YEAR_SEASON"
      ),
      method                   = "buffer",
      buffer_distance          = 300,
      ratio                    = 2,
      time_cols                = c("year", "season"),
      create_plot              = TRUE,
      plot_by_fold = TRUE,
      verbose                  = FALSE
    )

    absences$summary
    #>   fold n_presences n_pseudoabsences ratio_achieved
    #> 1    1          37               74              2
    #> 2    2          37               74              2
    #> 3    3          43               86              2
    #> 4    4          33               66              2

Note that `variable_patterns` and `time_cols` here use both `year` and `season`. The pseudoabsences are stamped with year-season values just like the presences, and the environmental values attached to them are pulled from the matching time-specific raster. This consistency is what keeps the temporally explicit workflow intact through the model fitting step.

# Outputs ready for modeling

At this point the three key downstream inputs are in hand:

- `scaled_dir` - the directory of scaled (or aligned in `rasters_aligned`, if scaling was skipped) rasters from which environmental conditions will be projected across space and time.
- `partition` - the rarefied, fold-assigned, time explicit environment data from species occurrence points.
- `absences` - the fold-stratified pseudoabsences with matching environmental attributes.

These are passed directly to any of the four model builders covered in the following parallel vignettes:

- Modeling with a GLM
- Modeling with a GAM
- Modeling with a Random Forest
- Modeling with a Hypervolume

Once a model is fitted, predictions are projected through space and time using `generate_spatiotemporal_predictions()`, and the resulting prediction stack is summarized in the Post-processing predictions vignette.