enter image description hereI am using Extract vertices in QGIS 3. The table in result shows angles. What do those angles mean?

  • 2
    How about you post some screenshots, of both the table and the corresponding geometries? – Erik May 13 at 12:55
  • @Erik added. Also I label new generated points – Stepan Khachatryan May 13 at 13:10

Most of the tool have a description on the right side of the window (if not visible click on the arrow in the upper right corner)

In your case the description read :

Extract vertices

This algorithm takes a line or polygon layer and generates a point layer with points representing the vertices in the input lines or polygons. The attributes associated to each point are the same ones associated to the line or polygon that the point belongs to.

Additional fields are added to the point indicating the vertex index (beginning at 0), the vertex’s part and its index within the part (as well as its ring for polygons), distance along original geometry and bisector angle of vertex for original geometry.

So it seem that the angle is the

bisector angle of vertex for original geometry

enter image description here


I did compare the result with the "angle_at-vertex" expression and I get the same angle for each vertex than those from the Extract vertices tool.

The ""angle_at-vertex" description read :

Returns the bisector angle (average angle) to the geometry for a specified vertex on a linestring geometry. Angles are in degrees clockwise from north.

My understanding is that the angle you get at vertex n is the angle between North and the line geometry between vertex n and n+1 in degrees clockwise from north.

Your data doesn't seem to really follow that, could it be related to projection ?

  • 1
    Dear @J.R I read it. It seems the first point (79.675) is the angle between the north arrow and first line. But the second point (93.566) is not the angle between the first and second lines. – Stepan Khachatryan May 13 at 13:17
  • That's true, and vertex 2 and 6 have similar value (around 115 and 111) but the angle are definitely not similar..., so no idea of what's going on here – J.R May 13 at 13:52

Angle value on point is average angle of lines before and after vertex.

First point has no "before line" so its actual angle of line after vertex. Last point has no "after line" so its actual angle of line before vertex.

Angle is in degrees clockwise from north to line geometry.

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