Unified topological editing

[qgib]

Author Name: Paolo Cavallini (@pcav)
Original Redmine Issue: 3483

Redmine category:digitising

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Currently QGIS has two different methods for editing:

• one (optionally) pseudotopological, to be used with [[SimpleFeature]] layers (shp, postgis, spatialite etc.)
• one fully topological, limited to GRASS layers.
  IMHO it would be good to unify the two, digitizing always (optionally?) with full topology, and writing (a)topologically when on a topological format (in future also e.g. GML3-topo, [[PostGIS]]-topo), (b) non topologically when on SF.
  This would be both more powerful and clean, and less confusing for users.
  See also http://grass.osgeo.org/grass64/manuals/html64_user/wxGUI.Vector_Digitizer.html
  http://grass.osgeo.org/wiki/WxGUI_Vector_Digitizer for missing (topological) features.

[qgib]

Author Name: Sandro Santilli (@strk)

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See http://strk.keybit.net/blog/2011/10/14/postgis-topology-iso-sqlmm-complete/comment-page-1/#comment-7521 for some design ideas about PostGIS-topology specific editing toolbar.

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• assigned_to_id removed nobody
• pull_request_patch_supplied was configured as 0

[qgib]

Author Name: Luís Ferreira (Luís Ferreira)

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Some ideas [[http://strk.keybit.net/blog/2011/10/14/postgis-topology-iso-sqlmm-complete/comment-page-1/#comment-7638]]
(1) topology layer (in memory or else), where we can define the intervening layers/postgis tables;
(2) rules (overlaps, gaps, covers, must contain point, touch line, ...);
(3) a list of errors in face of choosen rules;
(4) tools to merge/subtract/create feature/mark as exception or error.

[qgib]

Author Name: Martin Dobias (@wonder-sk)

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The main issue we are facing here is that the topological models of various data sources (grass, postgis-topo, osm) differ quite a lot between each other. From what I know:

• PostGIS. Primitives: nodes, edges, faces - they correspond to the elements of a graph from graph theory: node is a point, edge is a polyline connecting two nodes, faces are formed by enclosing edges. Planar graph is enforced.

• GRASS. Primitives: point, line, boundary, centroid. Points and (poly)lines are standalone objects, boundaries are polylines that should touch at the endpoints to form areas. Centroids are special type of points that assigns a layer to the area enclosed by the boundaries. Only boundaries are required to form a planar graph (?)

• OSM. Primitives: nodes, ways. Ways are constructed strictly from nodes. There is no explicit notion of areas: a closed way is usually considered as an area, although that is not always true (e.g. roundabout is a closed way, but it represents a linear ring). The situation with polygons with holes and multipolygons is difficult: relation elements may connect individual ways. Planar graph is not enforced.

Please correct me if I am wrong.

The question is how to handle these differences:

• Use a minimal topology model that makes only few assumptions? E.g. there may be points and polylines, the editing tools would be able to edit them and individual topological models would implement the logic - decide what is valid and what is not etc.
• Build a topology abstraction that is able to handle a superset of these individual models? The individual topological models would only turn on/off some features/rules.

The former approach sounds much better

[qgib]

Author Name: Sandro Santilli (@strk)

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I'd like to avoid too much abstraction on the PostGIS side actually.
All I can tell is that for postgis topology you have these operations:

• Add an isolated node
• Move an isolated mode
• Remove an isolated node
• Add an edge connecting two existing nodes (automatically creates a face if closes a ring)
• Move an edge vertices except its endpoints
• Split an edge by a node (add a "splitting" node)
• Remove an edge (automatically removes a face if opens a ring)
• Heal two edges removing the connecting node

Note that some of the above operations (those creating new edges or faces or removing them)
also have an option to decide whether or not to save pre-existing faces/edges or always replace
them with the new ones.

Seems all very postgis-specific (or should I say ISO SQL/MM specific).
Unless someone from GRASSland sees an unification opportunity (beside icons?) I'd move this into another ticket.

[qgib]

Author Name: Sandro Santilli (@strk)

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FYI: I've started a project on github for postgis topology editing. At least will serve for brainstorming.

https://github.com/strk/qgis_pgis_topoedit/wiki

[qgib]

Author Name: Pirmin Kalberer (Pirmin Kalberer)

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• fixed_version_id was changed from Version 2.0.0 to Future Release - Nice to have

[qgib]

Author Name: Markus Neteler (Markus Neteler)

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Sandro Santilli wrote:

FYI: I've started a project on github for postgis topology editing. At least will serve for brainstorming.

https://github.com/strk/qgis_pgis_topoedit/wiki

Sandro, do you continuously develop this plugin?

[qgib]

Author Name: Sandro Santilli (@strk)

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Markus, I'm not developing it, no. But I'm maintaining it (unfortunately needed as QGIS API changes more often than I'd like).
Mind you: that plugin is postgis-specific.

[qgib]

Author Name: Giovanni Manghi (@gioman)

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• easy_fix was configured as 0

[qgib]

Author Name: Paolo Cavallini (@pcav)

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Still true in QGIS3. Unclear status: it's a lot of work, and possibly we should better concentrate in native QGIS tools.

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• status_id was changed from Open to Feedback
• description was changed from Currently QGIS has two different methods for editing:
• one (optionally) pseudotopological, to be used with [[SimpleFeature]] layers (shp, postgis, spatialite etc.)
• one fully topological, limited to GRASS layers.
  IMHO it would be good to unify the two, digitizing always (optionally?) with full topology, and writing (a)topologically when on a topological format (in future also e.g. GML3-topo, [[PostGIS]]-topo), (b) non topologically when on SF.
  This would be both more powerful and clean, and less confusing for users.
  See also http://grass.osgeo.org/grass64/manuals/html64_user/wxGUI.Vector_Digitizer.html
  http://grass.osgeo.org/wiki/WxGUI_Vector_Digitizer for missing (topological) features. to Currently QGIS has two different methods for editing:
• one (optionally) pseudotopological, to be used with [[SimpleFeature]] layers (shp, postgis, spatialite etc.)
• one fully topological, limited to GRASS layers.
  IMHO it would be good to unify the two, digitizing always (optionally?) with full topology, and writing (a)topologically when on a topological format (in future also e.g. GML3-topo, [[PostGIS]]-topo), (b) non topologically when on SF.
  This would be both more powerful and clean, and less confusing for users.
  See also http://grass.osgeo.org/grass64/manuals/html64_user/wxGUI.Vector_Digitizer.html
  http://grass.osgeo.org/wiki/WxGUI_Vector_Digitizer for missing (topological) features.

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