QGIS Field Calculator – Adding Up Line Length for Lines That Pass Through Polygon

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I have three lines as per the below picture. The layer is called 'line'. A is 60m, B is 30m and C is 90m. I have a polygon layer consisting of 1 square. The layer is called 'polygon'.

I want to make a calculated field called "ttl_length" in the polygon layer which is the total length of all lines that pass through the polygon. In other words, the total should be 180m.

I am currently using the below-calculated field:

aggregate(layer:='line',
      aggregate:='sum',
      expression:= "line_length" ,
      filter:=intersects(geometry(@parent), $geometry) 
      )

The above will only give me the sum of the length of lines actually inside the square. I need the length of all the lines (eg. 180m). How can I do that?

Best Answer

An easier way to achieve that is to use overlay_intersects() to create an array of the lengths of all intersecting lines, then calculate the sum with array_sum():

array_sum (overlay_intersects ('line', length($geometry)))

To calculate ellipsoidal length, you could also use $length instead of length($geometry).

Related Solutions

QGIS – How to Create a Dynamic Label for a Polygon Layer Summing Line Length and Other Attributes

You can use the overlay_intersects() function to grab aggregates from another layer.

For example to count the number of features from your Tree Line layer intersecting the polygon:

array_length(overlay_intersects('tree_line_layer',"tree_count_field"))

To sum the number of trees from the Tree Line features intersecting the polygon:

array_sum(overlay_intersects('tree_line_layer',"tree_count_field"))

To sum the length of Tree Line features intersecting the polygon:

array_sum(overlay_intersects('tree_line_layer',$length))

You can use this in the label expression for your polygon and it will dynamically update:

QGIS Attributes – Summing Length of Lines of a Specific Category from Another Layer

First solution: use `overlay_intersects()`

I recommend using overlay_intersects() function (available since QGIS 3.16). It is easier to handle when it involves (as here) spatial relationship (take into consideration only overlapping pistes).

Use the follwong expression. You get an array of all aa_piste_length values from layer pistes, filtered by the value defined at the end of line 5. Use array_sum() to get the sum:

array_sum(
    overlay_intersects (
        'pistes',
        aa_piste_length,
        filter:=aa_piste_difficulty='beginner'
    )
)

Second solution: use `aggregate()`

Just for curiosity since you asked how to use aggregate() function. The expression argument is simply the field name whose values you want to sum. The filter, howver, has to take into consideration intersection features only as well as category values. The expression looks like this:

aggregate(
    layer:= 'pistes',
    aggregate:= 'sum',
    expression:= "aa_piste_length",
    filter:=intersects (geometry(@parent), $geometry) and "aa_piste_difficulty"='beginner' 
)

Edit

See help of the aggregate() function how the optional filter expression to limit the features used for calculating the aggregate works:

Fields and geometry are from the features on the joined layer. The source feature can be accessed with the variable @parent.

So here, we calculate the intersection of the geometry of the current (=joined) layer - $geometry - with the geometry of the source feature - geometry (@parent)

Remark: in QGIS versions 3.28 and later, $geometry is replaced by @geometry.

Best Answer

Related Solutions

QGIS – How to Create a Dynamic Label for a Polygon Layer Summing Line Length and Other Attributes

QGIS Attributes – Summing Length of Lines of a Specific Category from Another Layer

First solution: use overlay_intersects()

Second solution: use aggregate()

Related Question

First solution: use `overlay_intersects()`

Second solution: use `aggregate()`