1

I have a set of vector lines (which represent translation vectors for georeferencing). Based on the layer CRS, how can I compute the coordinates of each line in an attribute table ?

i.e (Xmax-Xmin; Ymax-Ymin) for each line.

I have managed to get the line length with $length in the expression editor but I am struggling with other geometrical parameters.

Edit: I am looking to "store the formula" within the layer. The idea is to keep these values automatically updated with the geometry. (Auto)update should happen if a new line is added or modification of the current geometry is done.

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    About your recent edition, look at virtual fields (stores the formula but not within the layer). About the $length() function, it calculates the ellipsoidal length, use length( $geometry) instead if you want the planimetric one (probably not so). About the (Xmax-Xmin; Ymax-Ymin) example, it is assumed to be a string of characters because a pair of values can not be attributed to a single numeric field. – Gabriel De Luca Feb 11 at 1:45
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For QGIS 3.x, the formula in the field calculator is :

'(' || x_max($geometry) || '-' || x_min($geometry) || '; ' || y_max($geometry) || '-' || y_min($geometry) || ')'
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You can use the Field Calculator. Open the attribute table of your line layer and select the Open Filed Calculator icon from the toolbar. I use an OSM road layer.

enter image description here

In the Field Calculator window create a new column of decimal type, in the Geometry group of functions you can find x_min, x_max, y_min, y_max functions. To create a column with x_range of features use the next screenshot:

enter image description here

You can create a column for y_range in a similar way.

  • Thanks: that is the method I was after but I was missing $geometry to call the proper entity. – kFly Feb 10 at 20:18
  • Is there a method to automatically compute these parameters for each line created within this layer as soon as they are created ? – kFly Feb 11 at 0:23
  • or update automatically when the geometry is modified ? – kFly Feb 11 at 0:35
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    See this post gis.stackexchange.com/questions/178355/… or user virtual fields (virtual fields are calculated when you open the attribute table) which is technically simpler but slower. – Zoltan Feb 11 at 14:15
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Following J.Monticolo's answer, yet another expression is:

'(' || bounds_width($geometry) || '; ' || bounds_height($geometry) ||')'
  • kFly didn't say if (s)he want (Xmax-Xmin) as text or the result of subtraction (q.e.d. width or height). In case of text result, my solution is the good one, in the case of substraction, it's the Kazuhito's one. – J. Monticolo Feb 10 at 19:26
  • I want actually the numerical value, but thanks for underlining up this difference – kFly Feb 10 at 20:08

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