This document describes the HydrOffice BRESS (2.3). For the project page, go here.

2.2. Landforms

This tool mainly classifies bathymetric DTM based on landform type, (optionally) calculates pattern-based statistics, and creates area kernels (connected grid nodes with the same landform type).

2.2.1. How To Use?

  • Select the Landforms tab (Fig. 2.6) on the bottom of the BRESS interface.

  • To change the Settings for Landforms v3:

    • Click the Unlock button, and click OK to the dialogue.

    • Set:
      • The inner and outer radii of the search annulus.

      • The flatness angle and (if the Apply correction for extended forms is flagged) the flatness distance.

      • When the Adaptive flatness is flagged, the Outer multiplier for the outer radius and the adopted Percentile (in ascending order) to estimate the flatness angle.

      • The kind of landform classification in the Landform classification list. The default is a 6-type look-up table (Fig. 2.12).

    • If required, select the following optional flags:
      • Search distance in meters: When flagged, the values in the search annulus are evaluated as meters. Otherwise, number of nodes.

      • Adaptive flatness: When flagged, the flatness angle is estimated node by node, using the Outer multiplier and the Percentile fields.

      • Delta angle for openness: When flagged, a pattern direction is evaluated positive (or negative) when the delta between the zenith and nadir angles is larger than the selected flatness angle.

      • Extended-form correction: When flagged, the flatness distance is used as the limit at which to increase the threshold height to evaluate a positive (or negative) direction.

      • Grid-edge correction: When activated, all the nodes with less than eight valid directions are ignored.

    • The tool can provide the following kinds of outputs (and they can be both saved as ASCII grids or plotted):
  • To reset the Parameters to the default initial values, click the Reset button.

  • In Execution, click Landforms v2.

landforms tab

Fig. 2.6 The Landforms tab.




2.2.2. How Does It Work?

2.2.2.1. Local Ternary Patterns

The first step of this tool is to calculate the Local Ternary Pattern (LTP) for each node and in the eight surrounding major directions (Fig. 2.7). The openness of each direction is evaluated by comparing the zenith and nadir angles against the flatness angle (Fig. 2.8).

local ternary pattern

Fig. 2.7 Example of a LTP of a given node.

zenith and nadir angles

Fig. 2.8 Example of a profile looking at just one of the eight directions.

2.2.2.2. Bathymorphons

The possible 6,561 values of LTP are reduced to 498 bathymorphons by removing the duplications after rotating and mirroring the patterns.

2.2.2.3. Landforms

The landform classification is obtained by counting the number of positive and negative directions. The number of positive and negative directions are then used to look up in one of the four following tables:

  • a 10-type landform-classification table (FL: Flat, PK: Peak, RI: Ridge, SH: Shoulder, CV: Convex Slope, SL: Slope, CN: Concave Slope, FS: Footslope, VL: Valley, PT: Pit) (Fig. 2.9).

six types lut

Fig. 2.9 The 10-type landform classification table. For more details, see (Jasiewicz et al., 2013).

  • a simplified 6-type landform-classification table (FL, RI, SH, SL, FS, VL) (Fig. 2.10).

six types lut

Fig. 2.10 The 6-type landform classification table. For more details, see (Masetti et al., 2018).

  • a simplified 5-type landform-classification table (PK, FL, RI, SL, VL) (Fig. 2.11).

five types lut

Fig. 2.11 The 5-type landform classification table.

  • a simplified 4-type landform-classification table (FL, RI, SL, VL) (Fig. 2.12).

four types lut

Fig. 2.12 The 4-type landform classification table.

2.2.2.4. Statistical layers

During the calculation of the local ternary patterns, the neighborhood of each grid node is analyzed in the main eight surrounding directions. The analysis requires the calculation of the local height, that is the relative elevation of the neighborhood nodes compared to the node being analyzed.

In each direction, the analysis identifies a node (within the search annulus) that is used to evaluate the openness of the visible neighborhood. The patterns polygon is constructed by connecting these nodes.

During the analysis, a number of statistics are calculated for each grid node:

  • Local Ternary Pattern (see Local Ternary Patterns).

  • Bathymorphon (see Bathymorphons).

  • Valid Patterns: the number of valid LTPs (that is, number of directions along which the openness can be evaluated).

  • Positives: the number of directions with zenith angle larger than the selected flatness threshold.

  • Negatives: the number of directions with nadir angle larger than the selected flatness threshold.

  • Average Height: the average height of the visible neighborhood.

  • Maximum Delta: the maximum elevation delta (that is, the absolute value of the height) of the visible neighborhood.

  • Height Range: the height range of the visible neighborhood.

  • Height Variance: the height variance (calculated using the Average Height as mean value) of the visible neighborhood.

  • Average Azimuth: the average orientation of the patterns polygon.

  • Elongation Ratio: the ratio between the maximum and the minimum dimensions of the patterns polygon.

  • Maximum Width: the maximum dimension (x- vs. y-direction) of the patterns polygon.

  • Area Ratio: the ratio between the area of the patterns polygon and its maximum possible extension (based on the outer search radius).

2.2.2.5. Area kernels

An area kernel is created by connecting all the adjacent nodes (Fig. 2.13) that have the same landform classification.

area kernel

Fig. 2.13 An example of adjacent nodes classified as “FL” (flat) and thus clusted into the same area kernel.