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In the real world:

I have a underground pipeline which has just been laid.
A surveyor has come along and taken shots to the top-of-pipe at each join along that whole pipeline.

In GIS:

I have brought those surveyed points into QGIS: each point has an elevation.
I have created a polyline representing the pipeline, using the points as nodes, starting at chainage 00 and continuing until the end of the pipeline.

enter image description here

Task:

I want to see a longitudinal section of the pipeline, by selecting a particular polyline at a time.

Constraints:

  • I don't want to rasterise/rasterize anything.
  • I don't want to use a DEM.
  • I DO want to use the elevation on the points, along with the pipeline polyline to create my longitudinal section.

Experimentation:

I have tried all (or just about all) QGIS plug-ins that create a longitudinal section, with no overall joy.

  • I think I want a scatter-plot of Chainage (x-axis) and Elevation (y-axis).
  • I think I need the ability to change the X:Y ratio of the chart so it doesn't matter if the pipeline is 10 metres long or 10 kilometres long and - I can show some vertical exaggeration.

Is there a simple way to do this without the answer being "...you can just write a Python script..."?

5

I would recommand to use DataPlotly Plugin (https://github.com/ghtmtt/DataPlotly) for that. You can use the selected points or all of them. In the example below I've used the x-coordinate of each point versus the z value (elevation).

enter image description here

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  • DataPlotly did the job! Thanks, EuroJam. It could be me, but I may have found an issue with how DataPlotly draws my graph (?) My table has the fields <OBJECTID> and <ORDERID>. <OBJECTID> was generated when the survey points were exported from the Survey software into a .SHP file and is the order the survey points were captured in the field. <ORDERID> is the order I want the graph line drawn (it is different to <OBJECTID>). DataPlotly appears to draw the graph in the order the points were added to the table, which is not the order of how I want the graph to draw.
    – GS10X
    Oct 18 at 23:22
  • I deleted the first column <OBJECTID>: no change. I deleted the <ORDERID> (which became the first column): no change. The issue is the line is plotted in this order: 2 | 1 | 3 | 4 | 5 | 6 | 7 | 9 | 8 | 10 ..., and not 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10... and I think it can only be based on the order they were added to the table, regardless of any changes to the attributes. Is there a way to alter the order of records in QGIS, or is there something I have missed?
    – GS10X
    Oct 18 at 23:23
  • 1
    In DataPlotly you can select the 2 attributes for plotting. in the example above I generated a new column first with the x value of the coordinates. this can be done with the field calculator (docs.qgis.org/3.16/en/docs/user_manual/working_with_vector/…) and the expression $x or with a processing tool (northrivergeographic.com/qgis-adding-x-y-coordinate)
    – eurojam
    Oct 19 at 4:24

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