DTM Filter: Removing Buildings and Vegetation

Accurate flood modelling requires a digital terrain model (DTM) that represents the bare-earth surface. Buildings, trees, and other vegetation can artificially obstruct water flow in urban and forested areas, creating artefacts in flood simulations. To address this, FastFlood includes a DTM filter specifically designed to remove these obstacles from elevation datasets.

Purpose

The DTM filter ensures that water flow paths are correctly represented by eliminating non-terrain features that may distort hydrological analysis. It is particularly useful in:

  • Urban environments with dense buildings

  • Forested regions with significant tree canopy

  • Areas where accurate flow routing and flood extent prediction are critical

Methodology

FastFlood implements the DTM filter using a sweeping algorithmic framework, similar to the method employed for hydrological terrain correction. Sweeping algorithms are computationally efficient and provide robust results with simple update rules.

Two main sweeping approaches are used:

  1. Isolated Obstacle Sweep Filter

    • The algorithm moves systematically across the elevation grid.

    • Each grid cell is examined and updated based on surrounding cells to remove isolated obstacles (e.g., a single building or tree).

    • The process iteratively smooths the terrain, preserving the natural slope while removing artificial spikes.

     

  2. Progressive Terrain Sweep (if applicable)

    • Larger or clustered obstacles, such as building blocks or dense forest patches, are addressed.

    • Sweeps progressively propagate corrections across the affected regions, maintaining realistic flow paths while flattening non-terrain features.

Benefits

  • Removes urban and vegetation artefacts from DEMs for more realistic flood simulations

  • Preserves natural terrain gradients and flow directions

  • Efficient implementation, leveraging FastFlood’s optimized sweeping algorithm

  • Compatible with global DEMs (SRTM, Copernicus 30m, WorldDEM) and user-supplied elevation data

Usage

The DTM filter is automatically available in FastFlood’s terrain processing tools. Users can apply the filter to imported DEMs before running simulations, ensuring accurate flow routing and flood depth predictions.