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)

# 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]])
# print("Write to GeoJSON: ")

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


To create a PolygonZ use the following structure:

QgsGeometry( QgsPolygon( QgsLineString( lat_lon_points ) ) )


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

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

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


enter image description here

  • Thanks for the suggestion, but I'm getting an error when I try the code: QgsPolygon(): arguments did not match any overloaded call: overload 1: too many arguments, overload 2: argument 1 has unexpected type 'QgsLineString' – mjldbr Jan 19 at 21:58
  • What is QGIS version you use? – Kadir Şahbaz Jan 20 at 7:42
  • I am using version 3.12 - it looks like I need to upgrade. Thank you for your help! I'll accept this as the answer. – mjldbr Jan 20 at 13:39

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.

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