Assign order number in which line intersect polygons

Question

Is there any tool (plugin) for QGIS to assign order (sequence) number of line intersecting through polygons?

At this moment I use a bit cumbersome solution which works but...

1. I use Qchainage plugin to generate dense points along the line with order
2. With spatial join (Vector / Data Management Tools / Join attributes by location) I assign values from points into polygons
3. I make sure all polygons have assigned some value (in all of them was point), if not I fill in manually missing values
4. I recalculate values so they are in sequence from 1 to n (n is number of polygons the line intersect) with interval 1

There is also possible solution using PostGIS Find all intersections of a LineString and a Polygon and the order in which it intersects it . Maybe it is possible to do something similar in QGIS without PostGIS?

Answer 1

It can be done with Python, but here is a solution using a virtual layer.

• Layer -> Add Layer -> Add Virtual Layer...

• In the "Query" field, use something like :

WITH r AS (
SELECT 
c.pgid AS pgid,  
pg.geometry AS geometry /*:polygon:3163*/ ,
 Line_Locate_Point(line.geometry, StartPoint(c.geometry) )  AS location 
 FROM 
(
SELECT line.id AS lineid, pg.id as pgid, ST_Intersection(line.geometry, pg.geometry) AS geometry FROM line, pg 
) AS c  
JOIN pg ON pg.id = c.pgid JOIN line ON line.id = c.lineid
ORDER BY 
Line_Locate_Point(line.geometry, StartPoint(c.geometry) ) 
)

SELECT * , 
(SELECT count(*) FROM r AS b WHERE a.location >= b.location) AS position
FROM r AS a

Replace (6-7 times for the 2 first):

• "pg" with the name of your polygons layer
• "line" with the name of your line layer
• ":polygon:3163" : the number is the SRID you are using. Replace with yours.

The result will be a new layer with your polygons and 2 new fields :

• location : the position of the first point of the intersection of the polygon and the line (grey points on the image) along the line (between 0.0 and 1.0).
• position : the order number of the polygon you are looking for.

Hope this help. I can try to clarify the query further if needed.