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Extract Main Valley Lines from dtm

extract_main_valleys.Rd

This function identifies and extracts the main valley lines based on flow accumulation values. It selects the top N valley tributaries by maximum flow accumulation, merges segments within each tributary, and returns the result with ranking information.

Usage

extract_main_valleys(valleys, dtm, nr_main = 2, metrics, plot_result = FALSE)

Arguments

valleys

An sf object containing valley line network with TRIB_ID attribute. This must be pre-extracted using extract_networks(dtm, type = "valley", metrics = metrics). This parameter is REQUIRED.

dtm

A SpatRaster object (from terra package) representing the digital elevation model. Used for spatial reference.

nr_main

Integer specifying the number of main valleys to extract. Default is 2.

metrics

A list containing pre-calculated geomorphometric metrics from calc_geomorph_metrics(). This parameter is REQUIRED. Must contain flow_acc for valley analysis.

plot_result

Logical. Should the result be plotted? Default is FALSE.

Value

An sf object containing main valley lines with attributes:

  • TRIB_ID: Tributary identifier

  • RANK: Ranking (1 = highest flow accumulation)

  • flow_acc_max: Maximum flow accumulation value for the tributary

  • geometry: LINESTRING geometry

Details

This version matches the QGIS plugin behavior by:

  1. Rasterizing valley lines to extract flow accumulation at pixel centers

  2. Removing confluence points (pixels that touch multiple TRIB_IDs)

  3. Ranking tributaries by their highest non-confluence flow accumulation point

The function follows this workflow: Rasterize valley lines to identify cells containing valleys Extract flow accumulation values at valley cell centers Create points at those cell centers with flow values Spatially join points to valley lines (with buffer = half cell size) Remove points that belong to multiple TRIB_IDs (confluence points) For each TRIB_ID, find the single point with highest flow accumulation Rank tributaries by their maximum non-confluence flow value Select top N tributaries Merge segments within each selected tributary Add RANK attribute (1 = highest flow) Preserve CRS from input valleys

Note: This method differs from simple terra::extract() by excluding confluence points where multiple tributaries meet, preventing "flow accumulation theft" by short segments at confluences.

Examples

if (FALSE) { # \dontrun{
dtm <- rast("path/to/dtm.tif")

# Calculate metrics (REQUIRED)
metrics <- calc_geomorph_metrics(dtm)

# Extract valley network (REQUIRED)
valleys <- extract_networks(dtm, type = "valley", metrics = metrics)

# Extract main valleys (QGIS-compatible method)
main_valleys <- extract_main_valleys(
  valleys = valleys,
  dtm = dtm,
  nr_main = 2,
  metrics = metrics
)
} # }