QGIS – How to Create Lines from Points and Transfer Attributes to Line Segments

geometry-by-expressionpoints-to-lineqgis

Background:

I need to create lines from points where the point attributes are transferred to the line segments.

The screenshot below shows two point tracks. Each track's point has a unique id and an additional attribute, in this case elevation.

Here's a screenshot showing the point layer's attribute table:

Below is a screen shot showing the desired line output. Notice that each line segment now contains both of the elevation values from the two points that comprised that line segment.

The resulting line attribute table includes three new fields: line_segment, elevation_begin, and elevation_end.

Question:

I don't know how to create new lines that contain attribute values from the source points. None of the "point to line" tools or plugins offer this functionality. There used to be a plugin named "Points2Paths" that could do this, but it has been removed from the repository.

I suppose that Geometry by Expression might offer a solution, but I don't know where to begin…

Best Answer

I imagine you're right that Geometry by Expression could be used but my brain reflexively thinks of an SQL solution:

Layer|Create Layer|New Virtual Layer...

use the following for the SQL, substituting "point_track" with the name of your point layer

SELECT p1.track,p1.id AS line_segment,p1.elevation AS elevation_begin,p2.elevation AS elevation_end, make_line(p1.geometry,p2.geometry) AS geom 
FROM point_track p1 
JOIN point_track p2 ON p2.track=p1.track AND p2.id=p1.id+1

Edit: As Stu pointed out (and I neglected to mention) this query assumes sequential integer type id numbers. I think its a quirk of the SQLite engine that it also works when the numbers are in a text field. Rather than complicating the SQL, if you do happen to have non sequential ids (or text ids like a, b, c..) the simplest approach might be to use the Processing Toolbox "Add autoincrement field":

Group values by "track".
If you have numeric ids in a text field use Sort expression to_int("id")
otherwise just sort by "id"

Then adjust the query to use the newly generated sequential id field:

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[GIS] How to find the slope of road segments with point elevation data of the same linear path

Create a copy of your road. Start editing it, select line and and split into equal length segments, I’d say approximately 50 m. Convert segments to 3D shapes:

Calculate field

abs( !Shape!.lastPoint.Z- !Shape!.firstPoint.Z)/ !Shape!.length*100

Result:

Please note both roads shown are fictions. One of them made of contour line, guess which. I wonder to see how red it would be if I’ve used slopes derived from DEM as per multiple suggestions. I hope they weren’t serious

UPDATE TO ORIGINAL ANSWER:

Picture below shows terrain slope:

Obviously engineers of the past didn’t know how to use GIS to calculate road slope:

How to Get Points-Touching-a-Line Attributes Back to Lines Layer in QGIS

Although @radouxju answer is valid, I will explain it a little more detailed.

You need to make sure that the polyline feature is split exactly above the point locations.
Use Join attribute by location. Choose the split line feature at point locations as target layer - in my case I name it "exploded".
In the summary section, select "Take summary of intersecting feature". Here, instead of running the tool two times; one for Min and another time for Max, you can run it one time and choose both Min and Max.

The out file will have the following attribute:

Add new field of type string with a name "Year" to the new shapefile from step 4.
Use Field calculator, and go to update existing field. Select "Year" and write the following expression:

to_string( "MINYEAR2" ) + '-' + to_string( "MAXYEAR2" )
The final output attribute will look like this:

The final output will be like this:

Here is the output after testing your data:

The table on the left is the point data after creating a new field of type integer, and the table on the right is after joining the line with point data using step 2 mentioned above. Then I used steps 5-6 to create the final data.

Update

I tried to convert the field YEAR from string to integer using expression to_int() and it worked. So you don't need to do it manually. However, The field type should be of type Integer not Integer64. Make sure that the field length is up to 9. If you chose a field length of 10, it will be converted to Interger64, and it will not work with Interger64. Then you can follow the process from step 2-6

Here is the final output after using the expression to_int():

In the above attribute table on the left, the YEAR is of type string and the YEAR2 is of type integer converted using expression to_int(). You can see in the attribute table on the right after following steps 2-6, I got MINYEAR2 and MAXYEAR2, then converted to string back again to concatenate everything together in the field YEAR.

Best Answer

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[GIS] How to find the slope of road segments with point elevation data of the same linear path

How to Get Points-Touching-a-Line Attributes Back to Lines Layer in QGIS

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