Sorting line features into route order in attribute table using QGIS?

Question

I have a several line features stored as shapefiles, obviously constructed of several connecting segment.

I wish to export the attribute tables of each feature as a CSV to use in a separate spreadsheet. However, this requires the comprising segments being ordered in the order which they comprise the line (i.e. the bottom left segment first and the top right segment last). Is there an algorithm or tool I could use to achieve this? Ideally it would assign each segment an ID based on its location in the line feature, incrementing from the start of the line (0) to the end.

I have calculated the start/end points of each line segment but unfortunately cannot simply sort by these coordinates as the lines do not run in a continuous direction.

Expanded information: My data is a section of a road network stored as a single polyline. This line is comprised contains a number of features, each with information of their length and average travel time (see picture):

I actually have several lines, each one representing a direction of travel across a route in the road network.

I have exported the line features to a CSV so that I can compare the travel times across the routes with those produced with a traffic model. However, in order to compare like-for-like I need each feature to be in order as they form the line. Currently, each feature in the attribute table corresponds to a random section of the route - I wish to sort these features so that they appear in the order which they form the line.

Answer 1

Following the above advice by @StevenKay this small experiment worked for me. Please note this is tested on 1 set of line segments.

Overall this goes like this:

1. Join line segments into one line by Join Multiple Lines. This sorts the order and direction of segments.
2. Split the line again into segments. (Explode lines)
3. Look up the id field of the original line segments (refFunctions)
4. Join attribute tables, using the id fields.

---

First, I have this collection of random line segments

By Join Multiple Lines plugin, segments are combined into single line. Most importantly it becomes continuous and orderly (see comments by Steven Kay). At the same time attribute table loses rows except for the first one.

Then I split the line by Explode lines in the processing toolbox.

As I already have refFunctions plugin, just opened the attribute table of Exploded and created a new field "original" by expression geomequals('original_lines', 'id').

Above picture looks exactly same as the original random line segments. Their difference is only the order of segments.

Now I can Join the attribute table of original layer and this sorted layer, to get the final product.

Answer 2

you could use the GRASS tool v.split.vert in Processing

This can break a linestring out into individual segments.

Here's an example line I've drawn, with 5 segments. This one starts in Edinburgh and goes counter-clockwise.

Choose your line layer, then apply v.split.vert. Make sure you set the "Maximum number of vertices in segment" to 2.

Now, each of your individual line segments is a separate line feature. If your lines have any other attributes, these should be duplicated for the individual line segments. They should appear in the same order that the line was traced.

Open this generated layer for editing.

Add 4 columns, called start_x, start_y, end_x, end_y. These should all be of type real, width 12, precision 6.

Use the following functions for each of your new fields. You can apply these in the Attribute Table,

start_x : $x_at(0)
start_y : $y_at(0)
end_x : $x_at(1)
end_y : $y_at(1)

Once these fields have been added, and the layer saved, you can save your layer as CSV format.

You should now have your start/end points for each line segment on its own row, as shown here in LibreOffice...

If you want to number individual segments within each line, this can be done using some of the neat new aggregation functions

To add this,

• make sure you have a field (e.g. fid) which is unique for each of your original lines (1, 2, 3...)
• add an integer field called unid which uses $rownumber. This is unique for each line segment in the split layer.
• add an integer field called segnumber

The segnumber field should use...

1 + unid - minimum("unid","fid")

That will number your line segments within each line...

Answer 3

1. Make centroids for each line (Vector geometry>Centroids)
2. Use Network analysis>Shortest path (layer to point)
3. In an attribute table sort by cost column
4. You can use Vector table>Add autoicremental field