Draw Bridges at intersections with correct angle with PyQGIS?

Question

I have a road layer and a river layer. I want to draw bridges at the intersections with correct orientation of the bridge (parallel to the road).

I have found out that you can use the "Vector->Analysis Tools->Line Intersections" to identify the intersections. But how do I identify the angle of the road layer at this exact point and automatically roate it acordingly?

I have found this rotate point along line layer, they suggested to use the following code:

from math import atan2

# get layers
bridges = QgsMapLayerRegistry.instance().mapLayersByName('bridges')[0]
trails = QgsMapLayerRegistry.instance().mapLayersByName('trails')[0]

# get fieldindex
fni = bridges.fieldNameIndex('angle')

# initializations
tolerance = 0.01  # search tolerance
bridges.startEditing()

# loop over all bridges
for bridge in bridges.getFeatures():
    x = bridge.geometry().asPoint().x()
    y = bridge.geometry().asPoint().y()

    # get the rectangular search area 
    searchRect = QgsRectangle(x - tolerance, y - tolerance,  x + tolerance, y + tolerance)

    # find trails 
    for trail in trails.getFeatures(QgsFeatureRequest().setFilterRect(searchRect)):
        # get the nearest vertex on trail and the one before and after
        pnt, v, b, a, d = trail.geometry().closestVertex(bridge.geometry().asPoint())
        p1 = trail.geometry().vertexAt(v)
        # when vertex before exists look back, otherwise look forward
        if v>-1 and b>-1:
            p2 = trail.geometry().vertexAt(b)
        elif v>-1 and a>-1:
            p2 = trail.geometry().vertexAt(a)

        # calculate azimuth
        angle = atan2(p2.x() - p1.x(), p2.y() - p1.y()) / 0.017453
        bridge[fni] = angle
        bridges.updateFeature(bridge)

# save changes and stop editing
bridges.commitChanges()

I followed the steps however I get an error for line 15 like so

File "<input>", line 15
bridges.commitChanges()
      ^
SyntaxError: invalid syntax

Line 15 is:

pnt, v, b, a, d = trail.geometry().closestVertex(bridge.geometry().asPoint())

Answer 1

When inserting a python code in QGIS, you should aways use the editor (the third icon from the right above the outout window), insert your code there and execute it from there. Single Code snippets are a bit tricky.