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I need to create and "ID" for items within a polygon that also takes into account the name of the polygon that the item is within, so what I need is something along the lines of: poly_X_Item 1-4, poly_Y_Item 1-4, poly_Z_Item 1-4.

I understand that I can use @row_number to create the "ID"s but its just adding the poly id to the field also that im having trouble with.

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  • "Items" being what? Polygons, lines, points? Can you show a screenshot of what you have?
    – Babel
    Commented Jul 15, 2021 at 10:21
  • points. sorry my use of terminology might be a little off due to me being pretty new at all this. I cant really share screenshots or anything due to the somewhat sensitive nature of the project
    – snakey866
    Commented Jul 15, 2021 at 10:32

1 Answer 1

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You can use an expression like the one that follows, concatenating four different elements using pipes (||) to "stick" them together: text-element poly_ (line 1), the id of the polygon the points are within (line 2), the text element _item_ (line 3) and the id of the point (line 4). Use this to create a new attribute with field calculator (Replace 'polygon' with the name of your polygon layer):

'poly_' || 
array_first ( overlay_within( 'polygon', $id)) || 
'_item_' || 
$id

This preserves the unique id (point item no.) of the points. If it should start from 1 for each polygon, the expression is a bit more complicated:

'poly_' || 
array_first ( overlay_within( 'polygon', $id)) || 
'_item_' || 
to_string (array_find (array_agg (array_first (overlay_within ('polygon', $id))||'_'||-$id,array_first (overlay_within ('polygon', $id))),array_first (overlay_within ('polygon', $id))||'_'||-$id)+1)

Screenshot 1: the first expression used here directly in the label field that dynamically creates the label text: enter image description here

Screenshot 2: the second expression, numbering the items from 1 in each polygon. The number of points per polygong is varying: enter image description here

Edit

For the problem rised in the comment, use this expression:

array_first ( overlay_within( 'distribution_area', $id)) || 
'-MST12-' || 
to_string (array_find (array_agg (array_first (overlay_within ('distribution_area', $id))||
-$id,array_first (overlay_within ('distribution_area', $id))),array_first (overlay_within ('distribution_area', $id))||-$id)+1)
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  • That's absolutely fantastic thank you so much. The only thing I need to do now is work out how to have this in an actual field as opposed to just in the labels but thanks so much I wasn't even aware that the overlay within part was an option
    – snakey866
    Commented Jul 15, 2021 at 13:47
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    Use field calculator with the exact same expression to generate it as attribute. Introducing it directly in the label was just for the screenshot/demonstration puropose.
    – Babel
    Commented Jul 15, 2021 at 13:48
  • I am once again using this expression to do the same thing however it is no longer increasing the count by 1 each time for the ID of the items any ideas why this may be? My current expression is as follows: '' || array_first ( overlay_within( 'distribution_area', "name")) || '-MST12-' || to_string (array_find (array_agg (array_first (overlay_within ('distribution_area', $id))||''||-$id,array_first (overlay_within ('disdtribution_area', $id))),array_first (overlay_within ('distribution_area', $id))||''||-$id)+1)
    – snakey866
    Commented Sep 10, 2021 at 13:53

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