QGIS – Connecting Points to Lines in Order Using QGIS

pointpoints-to-linepolyline-creationqgisshortest path

I have a set of points and I want to create lines from them. Points belonging to one line have unique "UniqueID" to be grouped by. I try to use QGIS's "Points to path algorithm". What I don't have is an order field, so the problem is that it connects the points in random order.

The picture contains an example of a line with 6 points. The QGIS Points to path algorithm connects the points in the following order: 1,3,2,6,4,5 and creates visually a good looking line, but with the length longer than the line between point 1 and 6. So I would like to know how to establish the order for the points to be added when creating a line. The points need to be connected from beginning to end in order i.e. 1,2,3,4,5,6.

The attribute table is shown on the picture. Each point has unique "fid" and a field called "UniqueID" that groups the points to one line i.e. points that belong to the same line have the same "UniqueID". The name of the attribute comes from the data I have received so can't really make it better.

How can I introduce an order so that I can have points connected by shortest path? The end goal is to calculate the length of lines, so if that could be done easily on the lines created from the unordered points, it is also a good solution to the issue.

Best Answer

If you are looking for a simple line that is going in only one direction, a straightforward way to solve your problem is to use the derivated geometry attributes in order to join the points in the correct order.

Using the Points to path algorithm, add the geometry attribute in Order by expression.

For example, $X will join the points form the left to the right, while $Y will join the points from south to north.

You can use the UniqueIDfield as Path group expression in order to create lines between points with the same unique ID.

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[GIS] Drawing lines between points in QGIS

One way to do this would be using a "Virtual Layer".

Create "X" and "Y" fields in your point layer containing the x and y coordinates
Create a constant field with the same value
Save a copy of the point layer
Add a virtual layer joining by the constant value of both layers (original and copy) using the following expression:

SELECT *
FROM points
JOIN points_copy
    ON points.constant = points_copy.constant

This will create a Virtual Layer that will contain every combination of X,Y origin and destination coordinates. In my example I used 29 points thus resulting in a virtual layer of 841 points (29²).

Just for symbolization purposes the point layer can be symbolized into lines using a geometry generator expression:

make_line(make_point("cx", "cy"), make_point("cx:1", "cy:1"))

To make a permanent layer I recommend using the tool XY to line from the plugin Shape Tools.

QGIS Points to Line – Creating Line Connecting Polygons/Points Based on Their Location

There's a Minimum Spanning Tree plugin for QGIS:

If you don't see it in the plugins system make sure you have the settings to show experimental plugins.

Start it from the Plugins menu and the "Minimum Spanning Tree" entry. Using the "Automatic" tab:

I set a simple polygon layer as the "Vector Layer" and got out a line shapefile.

It seems to use the centroids of the polygons for the distance ("weight") algorithm rather than the distance of nearest approach between polygons. The lines connect centroids.

The attribute table of the lines doesn't show what polygons each line comes from so you may have to do some more overlaying and intersection to get that. First use "Extract Vertices" to get two points for each line connection, and then use "Intersection" of those points over the polygons to get another point layer with the attributes of the polygon at each point. I get this:

that tells me that the line with id=0 connects the polygon with i equal to 9 to the one with i equal to 12. The next line, with id=1, connect polygon 9 to polygon 6, and so on for the complete tree.

Note it only seems to work on Shapefiles rather than loaded layers. Its also "experimental" and I managed to break it once or twice. I don't understand the difference between the "Vector" and the "Automatic" tab sections, for example, but the "Automatic" one seemed to work for my simple case. Hmm the "Vector" tab says the input "must be linestring" so I'm not sure what that is doing. For polygons, "Automatic" does the job.

The MST plugin doesn't seem to expose itself as a Processing algorithm so you won't be able to build all this into a single Processing step. Perhaps worth suggesting as an enhancement on their github code page.

There's an article here: https://www.sciencedirect.com/science/article/pii/S2352711020301771

Best Answer

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