QGIS3 Separately rotate the orientation of individual composer atlas maps?

Question

Is it possible to rotate each individual atlas detail map separately?

In creating an atlas for a long linear feature (rivers) to be output in engineering-style pages (11 x 17), it is nice to include as much and as consistent of an amount of the linear feature in each separate detail map as possible. Having the map oriented to N is not a particular priority. I understand how to rotate my whole canvas in the main app, and how to rotate a map rectangle within composer, but neither of these addresses the goal exactly.

My use case example is below. Each yellow polygon covers a 15 miles section of river. I want to rotate each atlas panel to 'flat' according to the bottom of the polygon, so that in each panel the visible river feature extent will be as-wide horizontally as possible.

Answer 1

I believe I achieved what you want by doing these steps:

1. Create a line shapefile with two attributes id and length
2. Split this file by a maximum length based on the length of the line (in my case 5000 m resulting in 6 segments). I update the id (= row_number) and length (length($geometry)): Split
3. Perform a Oriented minimum bounding box on the Split layer: Bounding boxes
4. Delete all the attributes but id and length in Bounding boxes layer
5. Perform a Force right-hand-rule on Bounding boxes: Reoriented
6. Perform a Oriented minimum bounding box on the Reoriented layer: Bounding boxes (remove the first layer called Bounding boxes and Reoriented to keep track of the layers in use)

7. Calculate a new attribute in the Bounding boxes layer called "rotation" based on the angle, width and height of the bounding boxes:

   case when width < height then 90-angle when angle = 0 then 0 else 180-angle end

Create a new print layout. Add a map (Map 1) showing the line-layer. Use Bounding boxes as Coverage layer in the Atlas, and let Map 1 be controlled by the Atlas. Use no Background (uncheck). Add this Expression (data defined override) for the Rotation:

attribute(@atlas_feature, 'rotation')

The Map 1 frame will now rotate while the current atlas feature always will be horizontal.

In addition I added this expression for the map scale to ensure no scale larger than 1:45000:

case when @map_scale < 45000 then 45000 else @map_scale end

NB! Remember to save the temporary layers! Otherwise they will be deleted when closing QGIS.