**The problem**

In QGIS, it's easy to calculate areas of polygons or lengths of lines using QGIS expressions with `area ($geometry)` / `$area` or `length($geoemtry)` / `$length` respectively. However, there is no expression to measure angles based on the three points that define it.

**The question**

Given a polygon of any shape in QGIS, how can interior angle be measured apart from the manual [measuring tool (toolbar icon)][1]?

**What I tried**

What sounds quite easy at the first glance is more complicated. QGIS expressions offer a few angle-functions as:
 -  `line_interpolate_angle`
 -  `angle_at_vertex`
 -  `main_angle`
 -  `azimuth`

However, there is no `get_angle_at_vertex(geometry,index)` function so that you could calculate the angle of any polygon- or line-geometry at a certain vertex and neither an `angle (point1,point2, point3)` function.


*Given the points 1, 2 and 3: how to get the angle at point 2, formed by the line connecting these three points **on the left side** in drawing-order?*

[![enter image description here][2]][2]


  [1]: https://docs.qgis.org/3.16/en/docs/user_manual/introduction/general_tools.html#measure-length-areas-and-angles-interactively
  [2]: https://i.sstatic.net/oMkUj.png