Save x,y,z values to a GeoJSON file from QGIS

Question

I would like to save x,y,z values for polygon vertices as a geoJSON file using Python in QGIS. I've made several attempts, but all have issues. I've found that the QgsPoint object must be used as QgsPointXY does not support Z and M values. Below is a code snippet which includes various tries and their corresponding error messages/issue:

for idx in range(n):
   col = int(round((geom.vertexAt(idx).x() - extent.xMinimum()) / xres))
   row = int(round((extent.yMaximum() - geom.vertexAt(idx).y()) / yres))
  
   lat = round(self.ur_lat - (row * lat_res), 6)
   lon = round(self.ll_lon + (col * lon_res), 6)
   alt = computeAltFromLatLon     
   point=QgsPoint(lon, lat,alt)
   lat_lon_points.append(point)

# Error: type object 'QgsGeometry' has no attribute 'fromPolygonZ'
# lat_lon_poly = QgsGeometry.fromPolygonZ([lat_lon_points])
 
# Error: type object 'QgsGeometry' has no attribute 'fromPolygon'
#lat_lon_poly = QgsGeometry.fromPolygon([lat_lon_points])
 
# Error: index 0 has type 'QgsPoint' but 'QgsPointXY' is expected
#    QgsPointXY does not support Z and M values
#lat_lon_poly = QgsGeometry.fromPolygonXY([lat_lon_points])
 
# File is successfully written here, but only x,y is saved.
lat_lon_poly = QgsGeometry.fromPolyline([lat_lon_points[0], 
lat_lon_points[1],lat_lon_points[2], lat_lon_points[3]])
  
fet.setGeometry(lat_lon_poly)
 
# print("Write to GeoJSON: ")
writer.addFeature(fet)

Any suggestions for how I can write a geoJSON file with x,y,z values for each coordinate?

Answer 1

To create a PolygonZ use the following structure:

QgsGeometry( QgsPolygon( QgsLineString( lat_lon_points ) ) )

Example:

points = [QgsPoint(1,1,1), QgsPoint(1,2,2), QgsPoint(2,2,3)]

geometry = QgsGeometry( QgsPolygon( QgsLineString( points ) ) )

print(geometry)
# <QgsGeometry: PolygonZ ((1 1 1, 1 2 2, 2 2 3, 1 1 1))>) )

fet.setGeometry(geometry)

Answer 2

Instead of trying to use the QGIS objects and built-in writer, I just created the file using the json package. For the coordinates I used an array of x,y,z values. I'm not able to copy/paste the code as the computer I was working on does not have internet access.