Distance between point and line layers & spatial index issue

Question

I am trying to compute on QGIS the distance from some clusters (ie points) to their closest road (ie lines); no need for all combinations, just the distance from each cluster to the closest road. To do so, I saw on a forum (on this post) that a good method was to transform lines into points, which I did, and then to compute the distance between both point layers. Nonetheless, when trying to do the last step, I get this error :

"/Applications/QGIS.app/Contents/MacOS/../Resources/python/plugins/processing/tools/vector.py", line 211, in spatialindex

idx = QgsSpatialIndex(layer.getFeatures(request))

Exception: unknown"

I am using a MAC. I tried on both QGIS 2.18.2 and QGIS 3.0.3, and got the same error. I tried, as advised on a page, to export geometric attributes using MMQGIS, and then re-importing them on QGIS, but same error. I also heard that it could be useful to check the validity of points ; nonetheless as I am using all the major roads network in Africa, I get millions of points, and checking them would take hours and hours ... I tried on QGIS3 to compute the distance between points and line centers (which does not give me the answer I want, as it is not the closest distance, but I did so to check if it was working this way), and it worked well, so I guess the issue lays in the transformation of my line layer into points

Has someone faced this issue of spatial index error, and knows what is means exactly? Is there a way to correct the points, to get rid of those not correct if this is the issue ?

Answer 1

(1) Collect your lines into single Multipart object.

1. Open the attribute table of your line layer.
2. Create a new field to group all lines. I assume you create a field group with all values set to '1'.
3. Run Singleparts to Multipart tool (in Vector | Geometry Tool).
4. Open the attribute table of newly created layer Multipart and check if the record (row) is only one in this layer; and there is a group field (it's value is '1' apparently).

(2) Calculate distance

1. Open the attribute table of your point layer.
2. Create a new field (integer or decimal) with following expression:

---

 distance(
    closest_point(geometry(get_feature('Multipart', 'group', '1')), $geometry), $geometry
 )

Answer 2

From QGIS 3.22 you can use the Shortest line between features algorithm that does exactly what you want:

This is the result from a point and a line layer:

On the other hand, you can use the QgsSpatialIndex API to get what you want:

project = QgsProject.instance()

source = project.mapLayersByName('points')[0]
destination = project.mapLayersByName('streets')[0]

index = QgsSpatialIndex(QgsSpatialIndex.FlagStoreFeatureGeometries)
index.addFeatures(destination.getFeatures(), QgsFeatureSink.FastInsert)

features = []

for in_feature in source.getFeatures():

    igeom = in_feature.geometry()

    # 1 is to get only the first and closest feature
    nearest_list = index.nearestNeighbor(igeom, 1)

    request.setFilterFids(nearest_list)

    for dest_feature in destination.getFeatures(request):

        jgeom = dest_feature.geometry()

        shortest_line = igeom.shortestLine(jgeom)
        feature = QgsFeature()

        attrs = in_feature.attributes()
        attrs.extend(dest_feature.attributes())

        feature.setAttributes(attrs)
        feature.setGeometry(shortest_line)
        
        features.append(feature)

where features il the list with all the shortest lines between each point and the closest line. Then you can write all these features to a new vector layer.

Be aware if your source and destination layers have different CRS!