[QGIS-Developer] line_substring start- and end distance

[Simon Gröchenig]

Hi all,

I have a question concerning the line_substring and recently updated 
$length expressions. I have a linestring (EPSG:4326) and a relative 
start- and end-offset (in %) and I want to create the sub linestring. 
The start- and end distances at the line_substring expression should be 
set in projection units (e.g. degrees in EPSG:4326). With the previous 
behaviour of $length, I multiplied the relative offset (in %) with the 
(planimetric) length to calculate those distances. See the following 
example:

geom_to_wkt(line_substring( geom_from_wkt('LINESTRING(13 46, 15 46)'),  
0.4* $length, 0.6* $length))  // $length => 2.0

results in

LINESTRING(13.8 46, 14.2 46)

With the latest release (3.6.0-1 und 3.4.5-1), the $length expression 
correctly calculates the ellipsoidal length 
(https://issues.qgis.org/issues/19355) and I cannot use the above 
expression. Is there a way to use relative distances in the 
line_substring function? Or is there an alternative to calculate the 
planimetric length (without implementing my own function)?

Thanks for your help

Simon

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