Center generated atlas map to features of different layer

Question

I got a problem with setting an Atlas. I have a polygon layer with around 100 polygons, and a line layer.

What i need to do is to generate a map for each polygon and show on the fixed scale (1:500) those specified lines.

I configured atlas to generate those maps based on polygon layer, but when polygon is large, lines are not visible because atlas centering map to the centre of polygon. Is there a way to generate map centered to those lines?

Im using a "inverted polygons" as a style of Polygon layer becouse im showing only one specific polygon that im reffering to on atlas using "$id = @atlas_featureid" in expression.

Got some screenshots to show what is in my mind:

Screen 1 - Nicely generated map

Screen 2 - Map generated in 1:1000 scale showing the problem and the boundary i would like to show while in 1:500

Screen 3 - Badly generated map

Answer 1

I don't know if I understand right your problematic, but for me, if I reword : you want to focus your atlas page on the section of the line layer wich is in the green polygons at 1:500 fixed.

You should try this :

1. Go to Vector ‣ Geoprocessing Tools‣Intersection
2. Set the process like that :

Parameters :

• You Input layer is your line layer

• Overlay layer is your polygons layer

• Input fields to keep : inclunding you can set the attributes to your new table, and from both tables, it could be useful depending on the result, and ordering it.

For more details for this process check this : https://docs.qgis.org/3.4/en/docs/user_manual/processing_algs/qgis/vectoroverlay.html#intersection

You obtain a new layer : layer_line_interstec

1. The result should be only the part of the object from the layer line which are in the green polygons. Moreover, if the polygons haven't always a line inside it will save you page in the atlas.
2. Then focus your atlas on your layer_line_interstec and set it invisible, fixed scale.

What do you think?

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I got another problem with that. On screen 1 you can see that there is only 1 visible polygon (green) because i use "inverted polygons", thanks for that i din't know about this function! as a style of Polygon. I should write that in the main post. That's why i used a polygon layer as my focus on atlas. Is there a way to also show only that one single polygon that I'm referring with my lines?

To show only a polygon crossed by a unique line you have to :

1. Have a single identification for yours polygons e.g. ID
2. In your intersection process you will join this ID in the layer_line_interstec, like this each segment of your layer have the ID from the polygon intersected by the line.
3. Then set your style for your polygon layer like "ID" =attribute (@atlas_feature, 'ID')