导入 csv 时 PyQGIS 更改底图 crs

Question

I am using PyQGIS to import a csv file with a lat and long, when doing this I am using the appropriate crs of EPSG:4326. I'm plotting this onto Google Maps.

I load my basemap, then import my CSV. The issue is that my basemap projection then changes to 4326 and I need it to remain on 3857.

I've tried importing the basemap after the CSV and moving it down in the layers, however this still changes the projections.

import requests
from PyQt5.QtGui import *
from PyQt5.QtCore import *
from qgis.core import *
from qgis.utils import iface
from qgis import core

#Use Google Street Map as QGIS basemap.
service_url = "mt1.google.com/vt/lyrs=m&x={x}&y={y}&z={z}" 
service_uri = "type=xyz&zmin=0&zmax=21&url=https://"+requests.utils.quote(service_url)
tms_layer = iface.addRasterLayer(service_uri, "GoogleSat", "wms")

#Import CSV and plot.
uri = 'file:///home/user/fred.csv?type=csv&xField=%s&yField=%s&crs=%s' % ("Site Longitude", "Site Latitude", "EPSG:4326")
layer_csv = QgsVectorLayer(uri, 'fred', 'delimitedtext')
layer_csv.isValid()
QgsProject.instance().addMapLayer(layer_csv)

I'll be the first to admit I'm a novice with QGIS!

Answer 1

It seems this has something to do with the application not refreshing properly as mentioned in this answer on gis stack. You may want to look into it for details.

To answer your question in brief, you can add QApplication.instance().processEvents() after QgsProject.instance().addMapLayer(layer_csv) and then use setCrs() to set your basemap CRS to whatever value you need. It will hold.

proj = QgsProject.instance()
proj.addMapLayer(layer_csv)

# This line makes the difference
QApplication.instance().processEvents()

# This sets the project CRS back to 3857
proj.setCrs(QgsCoordinateReferenceSystem(3857))