Join Line Segments (road segments) SDO_AGGR_UNION/SDO_AGGR_CONCAT_LINES

Question

Hi,
I have road segments I need to join/merge in order to get the complete road axe. Unfortunately I have some segements which are not joined and the resulting road axe has a gap.
When I look at the road segements and the join result it seems to me that aggregation does not work for roads which either have segements which are disjoint (we have a few roads where single segments do not touch) or if a road has two or more "branches" (just take the letter "Y" as example for it).
(1) SDO_AGGR_CONCAT_LINES
Didn't work at all - I could not create a view using SDO_AGGR_CONCAT_LINES. As the documention mentions - not suitable if you have arcs (we have).
(2) SDO_AGGR_UNION
I tried SDO_AGGR_UNION as well although the documentation mentions - "Do not use SDO_AGGR_UNION to merge line string or multiline string geometries; instead, use the SDO_AGGR_CONCAT_LINES spatial aggregate function."
(3) SDO_AGGR_SET_UNION
I tried SDO_AGGR_SET_UNION as well. It sounded promising as the documentation doesn't mention anything related to arcs but says: "Can aggregate sets of lines..."
2 and 3 give the same result. Please have a look at the screenshot (mh, looks like I can't upload a picture...)
I would like to aggregate the segements - is there another function in Oracle I could use?
Any idea, whether SDO_AGGR_UNION / SDO_AGGR_SET_UNION are / are not suitable for my dataset?
Many thanks, Rob
My view definitions:
CREATE OR REPLACE FORCE VIEW AV_V_STRASSEN_2
AS
  SELECT l.fid,
    ln.location_name,
    SDO_AGGR_UNION(sdo.SDOAGGRTYPE(rs.geom, 0.005)) geom
  FROM lm_lo_location l,
    lm_lo_location_name ln,
    lm_lo_road_section rs
  WHERE l.fid = ln.fid_lo_location
  AND l.fid   = rs.fid_lo_location
  GROUP BY l.fid,
    ln.location_name;
    CREATE OR REPLACE FORCE VIEW AV_V_STRASSEN_3
AS
    SELECT l.fid,
    ln.location_name,
    sdo_aggr_set_union (get_geom_set ('lm_lo_road_section', 'geom','fid_lo_location = ', l.fid), .0005 ) geom
  FROM lm_lo_location l,
    lm_lo_location_name ln,
    lm_lo_road_section rs
  WHERE l.fid = ln.fid_lo_location
  AND l.fid   = rs.fid_lo_location
  GROUP BY l.fid,
    ln.location_name;

Answer 1

R o b wrote:
after some further tests I found out that the cause of the issue seems to be the application I used for viewing my road network.
I displayed the view in SQLDeveloper and with FME and both look fine - result is expected (for view 2 and 3). But initially I used a different package for checking the view visually.
Yes, that can happen sometimes. Currently I've been working with one of the biggest GIS-applications out there, and I have noticed that especially when zooming in or out rapidly gaps appear between lines (when I know for certain that there are no gaps - because I checked). It's weird that, but it can trick you into believing things are not right.
R o b wrote:
Stefan - thanks for yopur reply:
- I need the roads not road segements when displaying the data in our GIS system, most users want a road not single segements
- performance : will see, if it's not good I will create a mat. view, road network is quite static and doesn't change often
- gaps : there are no gaps in our data set (but "gaps" appeared when I displayed the view in one specific software package).
- sdo_aggr_union / sdo_aggr_set_union seem to do what I want and seem to work with our data as well
Roads do make more sense to users - that is true. If you also need segments, maybe the LRS stuff can help you there: create the necessary segments on the fly when needed? On the other hand you are right, road networks tend to stay the same for a long time, and if the do change you usually have plenty of notification so an MV might be best (also depends on the number of users and the type of users I suppose).
Glad it's working for you now!
Cheers,
Stefan