QGIS sum selected line lengths

Question

I need a tool which displays the sum of selected lines as I go in QGIS. Something like the Measure line-tool. I´m new to PyQGIS. The logic seems quite simple, I just don´t know where to start. I guess this could be a good starting point https://gis.stackexchange.com/a/211162/161290?

Answer 1

A possible solution is using the Field Calculator to create a label that show the sum of the selected lines.

In the lines layer create a label using the expression

array_sum(aggregate(
 layer:= 'line',
 aggregate:='array_agg',
 expression:=$length,
 filter:=is_selected()))

NB: To use array_sum in the oldest QGIS versions you need to install the plugin arrayPlus.

This will create a label with the sum of the selected lines. It will appear in all the lines.

Since QGIS 3.10 in the label is integrated a Geometry Generator: you can use it to move the label to the centre of the screen or in a preferred position.

In the Layer Styling > Label Panel go to the Geometry Generator and use the variable

@map_extent_center and select the Point / Multipoint geometry option

this will move a single label to the centre of you Canvas.

If you want you can play with colour , background and offset to move your label. This is an example integrating the original expression with a text.

Here the modified expression:

concat( 'Total length: ',
array_sum(aggregate(
 layer:= 'line',
 aggregate:='array_agg',
 expression:=(format_number($length,3)),
 filter:=is_selected()))
,' m')