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3) Interactive Delineation

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Ivan Ivanov edited this page Sep 30, 2019 · 8 revisions

Description:

Interactive delineation supports the creation of final cadastral boundaries. The interactive delineation is implemented in the open source geographic information system QGIS as BoundaryDelineation plugin. The plugin contains different functionalities to delineate parcels from imagery (Table 1, Figure 1).

Table 1: Delineation functionalities of BoundaryDelineation QGIS plugin.

Figure 1: Interactive delineation functionalities: (a) connect lines surrounding a click, or (b) a selection of lines. (c) Close endpoints of selected lines to a polygon. (d) Connect lines along least-cost-path.

Input:

  • image segmentation lines (.shp)
    • with boundary likelihood as attribute (optional)
  • RGB orthoimage raster (.tif)

Output:

  • final cadastral boundary lines (.shp)

Steps:

  1. Install QGIS & BoundaryDelineation plugin
  2. Load input data
  3. Delineate and save boundaries

1. Install QGIS & BoundaryDelineation plugin

  • Install QGIS3, which is a free and open source GIS on your computer by downloading the QGIS installer
    • instructions on how to install the software are provided on the QGIS website
  • Install BoundaryDelineation plugin from the QGIS Plugin Manager

2. Load input data

Goal:

To load input data to be used during the interactive delineation.

Input:

  • image segmentation lines (.shp)
    • with boundary likelihood as attribute (optional)
  • RGB orthoimage raster (.tif)

Output:

  • final cadastral boundary lines (.shp)

Steps:

  • Raster base map: load RGB orthoimage raster (.tif)
    • select file from drop-down list if it is already opened in QGIS or load file by clicking on ...
  • Segments layer: load image segmentation lines (.shp)
    • select file from drop-down list if it is already opened in QGIS or load file by clicking on ...
    • simplify the input lines to remove zig-zagging along pixels by setting a desired simplification tolerance
      • simplification tolerance is a value in meters by which the line is simplified based on douglas-peuker (example: if the value is 1m, the simplified lines are straight for at least 1m before changing direction)
      • simplification tolerance should align with local (cadastral accuracy) requirements and the resolution of the input image (see tooltip in plugin for example values)

  • Output: save output final cadastral boundary lines (.shp)
    • leave empty to create temporary layer (which can be saved later) or define path to save final cadastral boundary layer by clicking on ...
  • Its4land Proejct: load input data saved on the its4land platform
    • Load test data by clicking Connect|Ensoka|Ensoka|Load as segments layer
  • Add length attribute: enable to add a length attribute to the segments layer, which can be used for least-cost-path calculation of the Vertices functionality in StepII

  • Process: simplifies Segments layer into Simplified Segments ( lines) based on simplification tolerance and creates Vertices (points) at intersections of lines in Segments layer

You now have all required layers displayed in the layer panel:

  • Simplified Segments: simplified lines loaded as Segments layer (.shp)
  • Vertices: points at intersections of Simplified Segments (.shp)
  • Candidates: boundary lines that you are currently delineating (.shp)
  • Final: boundary lines that you would like to save (.shp)
  • RGB orthoimage raster (.tif) you loaded as Raster base map
  • image segmentation lines (.shp) you loaded as Segments layer

3. Delineate and save boundaries

Goal:

To interactively delineate final cadastral boundaries.

Input:

  • simplified image segmentation lines (.shp)
  • vertices at intersections of image segmentation lines (.shp)

Output:

  • final cadastral boundary lines (.shp)
  • final cadastral boundary polygons (.shp) (optional)

Steps:

The plugin offers four functionalities to delineate boundaries (Table 1).

Note:

  • Select multiple lines or points by holding Shift
  • Unselect selected features by holding Ctrl

  1. Polygons

    • What: Connect lines surrounding a click or selection of lines
    • How: Click inside polygon or select lines around which to create a polygon
    • Shortcut: Ctrl + Alt + 1

  2. Lines

    • What: Connect endpoints of selected lines to a polygon regardless of Simplified Segments
    • How: Select lines for which endpoints should be closed to a polygon
    • Shortcut: Ctrl + Alt + 2

  3. Vertices

    • What: Connect vertices along least-cost-path based on a selected attribute, e.g., boundary likelihood or line length
    • How: Select vertices to be connected along least-cost-path based attribute selected in drop-down menu (selected vertices appear yellow)
    • Shortcut: Ctrl + Alt + 3

  4. Manual

    • What: Manual delineation with the option to snap to input lines and vertices
    • How: Connect manual clicks (enable snapping to snap your manual lines to existing layers)
    • Shortcut: Ctrl + Alt + 4

After creating a Candidates boundary with one of the functionalities, chosen one of the following options:

  • Reject: Reject feature in candidates layer. Deletes feature and restarts delineation.
  • Accept: Accept feature in candidates layer. Moves feature to final layer and restarts delineation.
    • Note: if Update edits is enabled before clicking Accept, Simplified Segments will be updated according to edits you made in Candidates (e.g. Manual edits)
  • Edit: Edit feature in Candidates layer. Enables Toggle Editing.

To save the lines you added to Final, click:

  • Finish: Finish boundary delineation. Removes Candidates layer and displays Final layer only.
    • Note: if Polygonize final layer is enabled, Final lines will be transformed to polygons and displayed as Final Polygons

Get help

The help tab in the plugin provides links to:

General notes:

  • Most of the explanations in this wiki are provided in message boxes in QGIS and tooltips in the plugin
    • tooltips are little text boxes that appear when you hover with the mouse over a plugin button.

References:

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