PyQGIS - modify Z-values for Polyline

Question

I have a GeoJSON file that I have loaded into QGIS as a LineString, and draped over a DEM using Drape (set Z value from raster)

I now want to round the Z-values to 4dp before exporting the modified file back to GeoJSON.

I've tried the following PyQGIS code:

layers = QgsProject.instance().mapLayersByName('Draped')
layer = layers[0]

layer.startEditing()
for f in layer.getFeatures():
    geom = f.geometry().get()
    for g in geom:
        g.setZ(round(g.z(),4))

layer.commitChanges()

The exported file still has the unrounded values, so I'm obviously missing something in the editing phase that commits the change.

Any suggestions?

Answer 1

I managed to achieve the desired outcome via a roundabout method:

1. Iterating through the points of the original geometry of each feature
2. Creating new Points, and setting the X, Y and Z values manually
3. Adding these Points to a LineString
4. Creating a new geometry from the LineString, and assigning it back to the feature

Code as follows:

    layers = QgsProject.instance().mapLayersByName('Draped')
    layer = layers[0]
    
    layer.startEditing()
    for f in layer.getFeatures():
        geom = f.geometry().constGet()
        ls = QgsLineString()
        for p in geom:
            pt = QgsPoint(0,0,0)
            pt.setX(p.x())
            pt.setY(p.y())
            pt.setZ(round(p.z(),4))
            ls.addVertex(pt)
        newgeom = QgsGeometry.fromPolyline(ls)
        layer.changeGeometry(f.id(), newgeom)
    
    layer.commitChanges()

This seems like a convoluted method to achieve a simple outcome, so improvements are welcome!

In theory, a similar approach could be used to make arbitrary modifications to the X, Y and Z values of lines (but again, it feels like there must be a better way!)