[Qgis-user] Detecting node valences

[Martin Bain]

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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