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I got two sets of points.

  1. The first set has a unique ID
  2. The second set references to this ID (one-to-many).

So I merged the two sets. Now every point in the joined table has the X and Y coordinates from the related first set.

Name;ID;X;Y;X_2;Y_2;Distance

Every X and Y is unique but X_2 and Y_2 repeats from a given set.

Now I would like to calculate the distance between (X,Y) and (X_2,Y_2) in the new column for each row.

I tried distance() but I'm way too new in Python to get it working.

I also tried some tools like Distance Matrix, but I got results for all distances (or to the number defined) but not only to the distances in one row. Or between Points sharing the same ID.

1

Executed in QGIS 2.18 and QGIS 3.0.3

In the Field Calculator/Expression dialog, please try

distance(make_point("X", "Y"), make_point("X_2", "Y_2"))

Function distance() returns the minimum distance (based on spatial ref) between two geometries in projected units.

Syntax: distance(geometry a, geometry b)


Another formula which can lead to the answer is

distance(geom_from_wkt('POINT(' + "X1" || ' ' || "Y1" + ')'), geom_from_wkt('POINT(' + "X2" || ' ' || "Y2" +')'))

P.S. To check whether the calculated distances are correct, you can apply the Measure length tool, see the image below.

Measure_length_tool

References:

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