Naming polylines in sequences from left to right (west to east) using QGIS

Question

I would like to give each of these polylines an "ID" e.g. 1,2,3,4 etc. from left to right (West to East) or more accurately off a bearing of 125°.

How can I achieve this in QGIS?

Data: https://we.tl/t-SzWkhwRO0D

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The example I have shown is a small sample, I have thousands of polylines. Unfortunately manual inputting would takes a while! — Curtis, Mar 23 '21 at 09:02
How to give an ID for the 2 "lines" after the selected one on your screenshot ? If you virtually extend them, they overlap so, same distance from "start". Same ID ? — J. Monticolo, Mar 23 '21 at 09:04
Hello J.Monticolo, yes ideally these would be the same ID or maybe even 16a and 16b? but 16 for both would be fine — Curtis, Mar 23 '21 at 09:07
You could try an SQL approach as used here: https://gis.stackexchange.com/questions/317604/vector-grid-in-qgis3-can-i-change-the-order-of-row-number/317621#317621 — MrXsquared, Mar 23 '21 at 15:00

score 3 · Answer 1 · answered Mar 23 '21 at 09:38

3

You can try the 'Add autoincremental field' with x(centroid($geometry)) expression for a 'Sort expression', see image below

answered Mar 23 '21 at 09:38

Taras

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Thanks Taras, this worked! I fear for more complicated datasets it will require little more work but it's a good start. Thanks! – Curtis Mar 23 '21 at 10:22
But will not work very well with a bearing of 180° ! – J. Monticolo Mar 23 '21 at 10:52

J. Monticolo · Answer 2 · 2021-03-23T11:14:55.443

EDIT : delete centroid layer creation and add it in the QGIS expression

The best solution I could find is :

Create a line that has a perpendicular orientation with the "lines" and with a length greater than the "line" layer extend;
Get this line coordinates and / or get the WKT : 'LineString(181748.27971816 823480.02355376, 182605.02880682 822909.74994162)';
In the "line" layer, create a new field with this expression by replacing the line WKT :

with_variable(
  'line',
  geom_from_wkt(
    'LineString(181748.27971816 823480.02355376, 182605.02880682 822909.74994162)' -- replace your WKT lineString here
  ),
  with_variable(
    'pts_on_line',
    aggregate(
      @layer,
      'array_agg',
      round(line_locate_point(@line, centroid($geometry)), 3)
    ), -- array of distance of features centroids projected on the line
    array_find(
      array_sort(@pts_on_line), -- sort array by distance
      round(line_locate_point(@line, centroid($geometry)), 3) -- find the current feature centroid projection distance in the array
    ) -- to start number at 1, you can add here : + 1
))

The above expression will number in order the "line" features. Unfortunately, for same "distance" lines, it will number it with different numbers (centroids coordinates imprecision due to not perfect "lines" in the base layer).

This works great, thanks. Only question is how can I get it starting it at 1 and not 0? — Curtis, Mar 23 '21 at 10:51
Look at the expression code, I put a comment to add a + 1 to start at 1. — J. Monticolo, Mar 23 '21 at 10:53
I updated the code to directly create a field in your "line" layer. — J. Monticolo, Mar 23 '21 at 11:15

Spacedman · Answer 3 · 2021-03-23T14:56:04.383

Outline algorithm:

Get the X and Y coordinates of the centroids of each feature.
Use a rotation matrix to rotate the centroids by 125 degrees.
Order by X coordinate (assuming I've got the rotation the right way round) and label 1:N

Depending on how you do this (python function, manual clicking of menu options) you then have to relate those points back to the line features, so you may need to keep a separate ID column to match them.

Seeing someone use the Autoincremental field has inspired me to try that. By putting just the Y coordinate of the rotation matrix in the sort expression you can do this. Use:

 x(centroid($geometry)) *sin(pi()*125/180) + y(centroid($geometry)) * cos(pi()*125/180)

which is the Y coordinate of the centroid rotated 125 degrees.

and then you get this:

the labels here aren't consecutive because the segments above are interweaved between them. Here they are:

Naming polylines in sequences from left to right (west to east) using QGIS

3 Answers3