First an algorithmic description how this solution works. Afterwards the implementation in QGIS, using expressions to create new geometries.
You can create for each vertex of your polygons the closest point on the line representing the street: make a perpendicular line on the streetline going through each vertex and where this connecting line crosses the streetline, create a point. Like this, you project the polygon's vertices on the street line. You than just have to connect all projected points based a common attribute that groups together points/vertices belonging to the same polygon.
Implementation in QGIS
Preparation of the data: On the polygon layer, create new attribute
nearest_line (if using Shapefiles:
nearest - Shapefiles have a limit of 10 characters in fieldnames) using this expression:
array_to_string ( overlay_nearest( 'line',$id)). This assigns the streetline the polygon should be projected to (if you have more than one streetline). Also make sure you have a unique field 'id' in the attributes of the polygons. Using a field 'fid' creates some issues if you want to save the vertices in the next step as Geopackage (you have to delete the
fid field before as it will be no longer unique: every vertex will have the
fid of the polygon it belongs to). So just create a new field 'id' and copy the
fid value there.
Extract vertices of you polygons (
menu Vector / Geometry tools), name the created point layer
Project these vertices to the line using the following expression with
Menu / Processing / Toolbox / Geometry by expression to get an output layer
As you might notice, one of the vertices in every polygon is a duplicate. That's because a polygon to be valid, it must be closed. That means that start- and endpoint must be identical (same coordinates).
- Now you can connect the projected vertices with a line using the
points to path tool. Use the
id field to sort and group (see screenshot) to get a separate feature (line) for each polygon. In the screenshot, the projected line can be seen in red:
Using your data, I re-created the solution. As you can see, in some places a manual decision has to be taken as to where a polygon should belong to. See polygon 4455 in the next screenshot: it is projected to the road running east-west because the polygon intersects this road and thus is assigned to this line.
Depending on what you use the data for, you would possibly prefer this polygon to be projected to the road in the east, parallel to it. I checked the output and there are very few such cases. You could manually change the field
nearest_line you created automatically in step 1 for this polygon and assign the roadline you want it be projected to.
For large datasets, of course you could refine the automatization process and add additional criteria as for which line the polygon should be projected to.