QGIS TIN Polygons Assign Elevations

Introduction

This tool acts as an alternative method to using the native "MERGE" functionality in TUFLOW for 2d_zsh polygons. The TUFLOW Z shape merge option obtains elevations by triangulation of the Zpt values (Zc, Zu, Zv and Zh) not the DEM triangulation, as outlined in the TUFLOW Manual. This processing tool allows the user to explicitly assign elevation points from a DEM layer (in a 2d_zsh template) along the polygon perimeter.

It is recommended to use this tool for Quadtree models, rather than the "MERGE" functionality, where the 2d_zsh input crosses Quadtree nesting levels.

Workflow:
This tool is useful when a user has a Z shape polygon (2d_zsh) and wants to assign perimeter elevations. The workflow for this process would be:

1. The user starts with a Z shape polygon (2d_zsh) that is read into the TUFLOW Geometry control file (TGC) using the " Read GIS Z Shape == " command.
   Note: Other Z shape layers (points and lines), may also be specified in the model, using the same " Read GIS Z Shape == " command (shown in the example below).
2. Run the tool to generate a new polygon and points layer. The polygon layer will optionally have additional vertices, and the points layer will have elevations assigned.
3. The TGC command is then updated to reference the newly generated layers, instead of the original polygon. For example:
   Before:
   Read GIS Z Shape == 2d_zsh_example_polygon | 2d_zsh_example_lines | 2d_zsh_example_points  ! Topography Features
   After:
   Read GIS Z Shape == 2d_zsh_generated_polygon | 2d_zsh_generated_points | 2d_zsh_example_lines | 2d_zsh_example_points  ! Topography Features

Usage

This tool, TIN Polygons - Assign Elevations, can be found in the QGIS processing toolbox panel under TUFLOW. To open the Processing Toolbox panel, navigate to 'Processing' in the menubar and select 'Toolbox'.

Parameters

1. Polygon Layer: The polygon layer that will have elevation points created for it.
2. Selected features only: If region(s) in the polygon layer have been selected using the 'Select Features' tool, this option will become available. If checked on, the tool will only assign elevation points along the perimeter of the selected region(s).
3. Raster Layer: The raster (DEM) layer that will be used to determine the elevations of the output points.
4. Vertex Distribution Option: Determines how the vertices will be distributed along the polygon perimeter.
   • Densify: Additional vertices will be inserted along the perimeter of the polygon layer at regular intervals (defined by "Vertex Interval"). This option is recommended unless the polygon layer has a negative 'Shape_Width' value, and a sufficient number of vertices.
   • Use Existing: Elevations will be assigned to the existing vertices of the polygon layer. This option should only be used if the polygon layer has a negative 'Shape_Width' value, and a sufficient number of vertices.
5. Vertex Interval (only used for Densify): The maximum interval distance between the vertices to be created (in map units). Since each polygon segment may not be evenly divisible by this value, the tool adjusts the interval lengths as needed while ensuring that this value remains the maximum. This value should be set to the smaller of the following:
   • Half the model cell size
   • The minimum Quadtree cell size
6. Output Points Layer: Determines the location and filename of the output points layer. It is recommended to name the output layer with the prefix '2d_zsh'.
7. Output Polygon Layer: Determines the location and filename of the output polygon layer. This layer will have the output points snapped to the polygon vertices. It is recommended to name the output layer with the prefix '2d_zsh'.

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