[Qgis-user] Batch selection/new feature creation

[Groene Bij]

Hi,

 

Have you considered using postgis for this?

See: https://gis.stackexchange.com/questions/17495/dissolve-or-unsplit-lines-on-common-attributes-in-postgis-or-grass

 

First step would be to add the reference id from the spreadsheet to the polyline layer.

Polyline layer with the 100m stretches, “polyline”



 

Csv with sections, “tracks”



 

So, to add ref_id to each polyline you can use:

SELECT p.*, t.ref_id

FROM polyline as p

JOIN tracks as t

ON p.ch_1 >= t.start_ch AND p.ch_1 < t.end_ch

 

Than you can dissolve all polylines with the same ref_id like in the example link.

 

You can  combine al subqueries and export the result as new layer to your qgis-canvas.

 

That leaves the ‘saves these resulting new feature to an existing layer’ part.

You can merge features from different layers into a new or existing layer.

 

Greetings,

Jeroen Hovens

 

 

Van: Qgis-user <qgis-user-bounces at lists.osgeo.org> Namens Samuel Williams
Verzonden: donderdag 17 september 2020 11:14
Aan: Qgis-user at lists.osgeo.org
Onderwerp: [Qgis-user] Batch selection/new feature creation

 

Hello,
I'm hoping that those with more experience of QIS might be able to help point me in the right direction.
I have a layer consisting of around 700 end-to-end polylines, each of which represents a 100m stretch of railway track centreline. Each polyline is attributed with an integer value which represents its starting chainage (distance in m from the start of the railway line, abbreviated ch.).
I also have a spreadsheet/csv with several hundred rows, each of which represents a different section of the railway. These sections are defined by their start and end chainage (e.g. ch. 1300 to ch. 1800 would be a 500m stretch starting 1300m from the beginning of the line).
I am looking for an efficient way to select each of the polylines associated with each row in the spreadsheet, dissolve them (they should touch end to end), and then save them as a new feature on an existing layer. Each one will need to be given a reference ID from the relevant line in the spreadsheet.
E.g.
Row 1 of spreadsheet selects all polylines with a starting chainage 1300 to 1700 (which with 100m segments selects the whole length 1300-1800),  dissolves them, populates the ID field from a field in the spreadsheet, and finally saves these resulting new feature to an existing layer.
This process would then repeat for the other rows in the spreadsheet/csv.
I’ve got a reasonable understanding of python,  but have never tried using it within qgis. Would that be the easiest route, or is there a way of using v.extract in batch mode which reads each of the required selections from the spreadsheet? From the grass manual v.extract appears to read selections from a text file, but I can't find any good examples, or detail in the manual on the required format for the text file.

Any help much appreciated!  

Kind regards,

 

Sam.

Sam Williams

Director | Sine Acoustics
m. +447969123431 <tel:%2B44%207969%20123431> | <mailto:Sam.Williams at sineenvironmental.com> Sam.Williams at sineenvironmental.com |  <http://www.sineenvironmental.com/> www.sineenvironmental.com
Sine Acoustics is a trading name of Sine Environmental Limited, company number 9609805. Postal address: 78 Old Church Road, Clevedon, North Somerset, England, BS216PT.

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