Finding pseudo nodes in free GIS software?

Question

Software gvSIG OA Digital Edition 2010 have tools topology for finding pseudo nodes in linear geometry. I set cluster tolerance 0.00002 and maximum number of errors -10000 for 20000 link count linear geometry. But unsuccessful result.

Are there any solutions that find pseudo nodes in free GIS software?

I need to layer pseudo nodes (one solution to this problem - to use tools topology of ArcInfo, but priority for me is to use free software). Linear geometry created several users in QGIS 1.8.0 in PostGIS (v. 2.0.1) database.

Add new image: 12 linear features with three pseudo nodes in A (line 4/5), B (line 6/7), C (line 9/10). Pseudo nodes should be points instead - two linear features with intersection in one point (node) should be one linear feature (line 4/5 - line 4, ...).

Is it possible to make a request in PostGIS, which will result in a layer of pseudo nodes?

Add new image of examples pseudo nodes: if I receive for linear layer point layer pseudo nodes (blue rects) I corrected following errors in linear layer: A - add missing geometry, B - snapped line in intersection, C - remove pseudo node.

Answer 1

Here a generic soluion, that you can impĺement with PostGIS or any other OGC-compliant software.

NOTE: as I say before, a key concept in FOSS and GIS is standardization: the best solutions adopt standards, like OGC ones.

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Your problem is to "find pseudo nodes"... But I think that it is a little more, "find non-pseudo nodes and join lines of pseudo nodes". My solution can be used for both.

OGC standards offer:

• ST_Boundary(geom): to detect the nodes of the lines

• ST_Dump(geom): to put each single node in a SQL table record.

• ST_DWithin, ST_Equals, ST_SnapToGrid, ST_Snap can be used for change tolerance. I am using ST_DWithin.

We can suppose that your main problem can be specified with these objects and properties,

• there are only line segments (of a table linesegment), represented by a LINESTRING geometry... I not tested with MULTILNE, if you have geometrytype=MULTIPOINT, you can split and cast MULTILINEs with ST_Dump and ST_LineMerge;

• each line segment have a (geometry ID) gid and a (color ID) idline.

So, the first step is to obtain the nodes that comes from joining lines,

CREATE TABLE cache_bounds AS
  SELECT gid as gid_seg, (ST_Dump(ST_Boundary(the_geom))).geom AS the_geom,
         gid as color 
         -- if you not have something for "color label" of lines, use gid.
  FROM linesegment;
ALTER TABLE cache_bounds ADD column gid serial PRIMARY KEY;

CREATE TABLE cache_joinnodes AS
  -- Use your TOLERANCE instead "1" at ST_DWithin and ST_Buffer.
  SELECT *, array_length(colors,1) as ncolors FROM (
   SELECT gid, array_distinct(array_cat(a_colors,b_colors)) as colors, the_geom FROM (
    SELECT 
      a.gid, array_agg(a.color) as a_colors, array_agg(b.color) as b_colors
      , st_buffer(a.the_geom,1) as the_geom -- any one to represent the join point.
    FROM cache_bounds a, cache_bounds b 
    WHERE a.gid>b.gid AND ST_DWithin(a.the_geom,b.the_geom,1)
    -- use ST_equals(a.the_geom,b.the_geom) if no tolerance.
    GROUP BY a.gid, a.the_geom
   ) as t
  ) as t2;

NOTE: using caches because they are faster than views. Use "EXPLAIN SELECT ..." to check CPU time, it can take a long time.

Here cycles and continuous (same color) lines are detected as ncolors=1 points, and the pseudo nodes by ncolors=2 points, so, you have a layer with that points.

Your table of "good nodes" is with the original "bounding points" and without "pseudo nodes".

CREATE VIEW vw_joinnodes_full AS
  SELECT b.*, j.ncolors
  FROM cache_joinnodes j INNER JOIN cache_bounds b 
       ON j.gid=b.gid;

CREATE TABLE cache_good_nodes AS
  SELECT *  
  FROM vw_joinnodes_full 
  WHERE ncolors=1 OR ncolors>2;

-- IF NEED ... CREATE VIEW vw_correct_linesegment AS ... 

Answer 2

Refractions Research has made a Line Cleaner tool that seems to do what you want.

Line Cleaner cleanses networks by simplifying complex, cyclical, very short and zero-length geometries, and removing pseudo-nodes and insignificant vertexes. Most significantly, in the cleansing phase, it is able to ensure that feature matches can be considered automatically

The source code can be found at GitHub.

Answer 3

Non-Free solution: FME + MRF + SmartCleaner transformer

Free solution GRASS v.clean (Latest QGIS 1.8.0 with GRASS tools is easiest way to use it) and other topology cleaning tools

Answer 4

Here are steps to Find your pseudo nodes using OpenJump a free GIS.
QGIS and gvSIG have the Sextante Plugin, so these same steps should work their too,
the Spatial Join might be slightly different.
I used version 1.2 for testing.

-- save the Line endpoints
Sextante toolbox, Topology, Extract endpoints of lines -> endpt_0

-- unsplit your lines
Sextante toolbox, Tools for line layers, Join adjacent lines

-- save the Unsplit line endpoints
Sextante toolbox, Topology, extract end points of line -> endpt_1

-- the end points removed by "Join adjacent lines" are pseudo nodes

Tools, Queries, Spatial Query,
Source layer "endpt_0"
Relation "Intersects"
Mask layer "endpt_1"

enable or click Complement Result

Answer 5

Solution for finding pseudo nodes (and other topology rules) available in QGIS 1.9.0 master (2.0) - plugin "Topology Checker". Links: Topology Checker plugin on YouTube, Topology Checker plugin on GitHub

Answer 6

With PostGIS, you can use a modified version of the query to find dangles discussed in this topic, as pseudonodes are nodes that intercept 2 linestrings and dangles are nodes that intercept 1 linestring.

WITH nodes AS 
(SELECT ST_StartPoint(geom) AS pt FROM
linestring_table UNION ALL 
SELECT ST_EndPoint(geom) AS pt FROM
linestring_table) 
SELECT pt FROM nodes
GROUP BY pt HAVING count(*) = 2;