[Qgis-user] Linking coordinates between layers

[Martin Bain]

Hi Simon,
One way would be to add the line layer as a virtual layer with the query text:

SELECT make_line(A.geometry,B.geometry) AS geom, st_distance(A.geometry,B.geometry) AS dist
FROM LayerA AS A, LayerB AS B

Just substitute LayerA and LayerB with the names of your layers.

This would create a dynamic layer with one attribute "dist" - the distance between the points, and the geometry would be a line connecting the two points.  Whenever you move a point the line layer will update, if you labelled the line layer with the distance attribute, the label position and text would also update.

It gets quite funky if you have multiple points in each layer - there are lines drawn between all points.  If there are m points in layer A and n points in layer B there will be m*n points in the dynamic layer (a "cross join" in SQL parlance).  I imagine performance would be dreadful on layers with many points - but not an issue for your use case.

Cheers,
Martin.

-----Original Message-----
From: Qgis-user [mailto:qgis-user-bounces at lists.osgeo.org] On Behalf Of simonc8
Sent: Friday, 13 October 2017 3:22 AM
To: qgis-user at lists.osgeo.org
Subject: [Qgis-user] Linking coordinates between layers

I have two point layers each with a single point in them. I want to draw a line joining the points to use to measure the distance apart. I have found various plug-ins which allow this, but is there a way I can actually link the point coordinates to the line layer so that if I move one of the points the line will update automatically?

Grateful for advice.



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