[Qgis-user] Detecting node valences

[phi73 at gmx.de]

Hello Martin,

yes you are right.

The terminus „Node valence“ is from topology and is defined as the number of
outgoing and incoming lines by a node.
You are example is correct:
a) a T Intersection has val=3
b) a Cross val = 4
c) a „one-way-street“ the end/starting point has val = 1


So i need to run it of the level of database as i assume.

Thanks for your reply so far.

If somebody knows to do it more „user friendly“ i would be happy to learn it.
I have some rudimentary knowledge of running SQL but last time used 15y ago ;-)

Thanks to all of you. It is a great group to be honest.

Pat


On 13. Apr. 2017, 04:44 +0200, Martin Bain <Martin.Bain at lismore.nsw.gov.au>, wrote:
> Hi Pat,
> I saw your previous question went unanswered.  I’m not familiar with the term valence, I’m guessing you want the number of lines that meet at a point.  If you had like a roads layer then three streets would meet at a T intersection – the node would have a valence of three, for a cross intersection the valence would be four.
>
> I’ll start the ball rolling by offering a possible solution, then maybe some bigger brained folks might give a better way of doing it.
>
> There’s probably a sequence of steps you could do to get your end result, maybe something in the processing toolbox?  Being a bit of a database guy, I reached for the DB Manager and came up with the SQL below which can be loaded as a point layer.  THIS QUERY ASSUMES YOUR LINES ONLY MEET AT THE ENDS, IF ONE LINE INTERSECTS ANOTHER IN THE MIDDLE IT WONT BE COUNTED.
>
> The inner select statement returns a set of points. It is a UNION query of two parts, the first part selects each line’s start point and its X and Y coordinates and the second part does the same for the end point of each line.
>
> The outer select statement groups the matching points together and gives a count.  The last line in the query “HAVING VALENCE>2”  filters out any results with a valence less than three, you can adjust the number for your needs or leave it out altogether and filter the rows in QGIS.
>
> Cheers,
> Martin.
>
> SELECT GEOMX,GEOMY, GEOM, COUNT(*) AS VALENCE
> FROM
> (
> SELECT "LCC_WATER_PIPES"."Namecompon",
> "LCC_WATER_PIPES"."MI_PRINX",start_point(
> "LCC_WATER_PIPES"."geometry") AS GEOM ,
> X(start_point("LCC_WATER_PIPES"."geometry")) AS GEOMX,
> Y(start_point("LCC_WATER_PIPES"."geometry")) AS GEOMY
> FROM "LCC_WATER_PIPES"
> UNION
> SELECT "LCC_WATER_PIPES"."Namecompon",
> "LCC_WATER_PIPES"."MI_PRINX",end_point(
> "LCC_WATER_PIPES"."geometry") AS GEOM ,
> X(end_point("LCC_WATER_PIPES"."geometry")) AS GEOMX,
> Y(end_point("LCC_WATER_PIPES"."geometry")) AS GEOMY
> FROM "LCC_WATER_PIPES"
> )
> GROUP BY GEOMX, GEOMY, GEOM
> HAVING VALENCE>2
>
>
>
>
> select A.MI_PRINX,B.MI_PRINX ,intersectION(A.geometry,B.geometry) AS GEOM
> from LCC_WATER_PIPES as A, LCC_WATER_PIPES as B
> WHERE A.MI_PRINX<>B.MI_PRINX
> AND intersects(A.geometry,B.geometry)=1
>
> From: Qgis-user [mailto:qgis-user-bounces at lists.osgeo.org] On Behalf Of phi73 at gmx.de
> Sent: Wednesday, 12 April 2017 11:03 PM
> To: QGIS User List <qgis-user at lists.osgeo.org>
> Subject: [Qgis-user] Detecting node valences
>
> Hello to all,
>
> is there a way i QGIS to run a select over a map to get
> all the nodes with a specific node valence?
>
> If not i would be happy about a shortchanged reply too :)
>
> thanks
> pat
>
>
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