QGIS » QGIS Application

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.

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.

• 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

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.

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 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?

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.