In QGIS, I have a polygon layer with square polygons in it. The polyogons are of same size but different orientation. I want to create a set of maps using the atlas feature. My aim is to rotate the map accordingly to the orientation of the polyogns.

The "item properties" offer the possibility to insert an expression for the map rotation. If I had a rotation-field in the property table of the poygons layer I'd be able to automate map rotation accordingly, I suppose. So my question is: is it possible to calculate the rotation of quadratic polyon features relative to the N-S (or W-E) axis?

Here I found a similar question; however, not being familiar with PostGIS, I want to accomplish this in QGIS.

  • You could write a python function to get the four coordinate points and compute the angle. Care to offer some sample data? – Spacedman Apr 10 '17 at 16:41
  • Unfortunately, I am not familiar with python at all. I am trying to do this with the azimuth function in the field calculator. However, I do not really understand how to tell the programm to always use the first two nodes of each polygon and calculate the azimuth... – yenats Apr 10 '17 at 16:51
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    (180/pi())* atan2($y_at(1)-$y_at(0), $x_at(1)-$x_at(0)) should get you an angle for the first two points. – Spacedman Apr 10 '17 at 17:03
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    Or this for azimuth: 180/pi() * azimuth(make_point($x_at(0), $y_at(0)), make_point($x_at(1),$y_at(1))) (which is probably just 90 degrees away from the previous example) – Spacedman Apr 10 '17 at 17:08
  • I'll add them as answer. – Spacedman Apr 10 '17 at 17:09

You can use $x_at(n) to get the n-th x coordinate. Then you can either use atan2 with the coordinate differences:

(180/pi())* atan2($y_at(1)-$y_at(0), $x_at(1)-$x_at(0)) 

or azimuth by making points:

180/pi() * azimuth(make_point($x_at(0), $y_at(0)), 

The 180/pi() gets you degrees. I think the only difference between these two is the angle for zero, and maybe the direction (clockwise or anti-clockwise...).

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    there's also a degrees() function to convert radians to degrees – Steven Kay Apr 10 '17 at 21:21

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