How to center the map on a point while their crs differ?

Question

Just starting with Python, plugins and actions in QGIS.

From an postal code api I get a position in WGS84: lat and lon.

I want my map (in WGS 84 pseudo-Mercator) to center on this point.

Since several hours I'm googling everywhere I have tried everything but to no avail.

My code now is (based on several answers on different forums since I've not seen the straight answer I need):

map_pos = QgsPoint(lon, lat)

map_pos.crs = QgsCoordinateReferenceSystem(4326)

rect = QgsRectangle(map_pos,map_pos)

self.iface.mapCanvas().setExtent(rect)

self.iface.mapCanvas().refresh()

I get this message:

rect = QgsRectangle(map_pos,map_pos) TypeError: QgsRectangle(): arguments did not match any overloaded call: overload 1: too many arguments overload 2: argument 1 has unexpected type 'QgsPoint' overload 3: argument 1 has unexpected type 'QgsPoint' overload 4: argument 1 has unexpected type 'QgsPoint' overload 5: argument 1 has unexpected type 'QgsPoint'

Question: how to center the map on a point while their crs differ? Should be very simple!

Answer 1

The source of the error is that you are passing QgsPoint objects to the QgsRectangle constructor when it is expecting QgsPointXY objects. But you are also trying to create a rectangle from two identical points.

If you want to center the canvas on a point, then I recommend using the setCenter() method with a QgsPointXY rather than using setExtent() with a QgsRectangle.

To overcome the differing CRS you can use the QgsCoordinateTransform class to transform the point from epsg:4326 to epsg:3857.

After calling setCenter() you can also set the zoom level by calling zoomScale() and passing an appropriate value.

Try the code below:

map_pos = QgsPointXY(lon, lat)
src_crs = QgsCoordinateReferenceSystem(4326)
dest_crs = QgsCoordinateReferenceSystem(3857)
xform = QgsCoordinateTransform(src_crs, dest_crs, QgsProject().instance())
map_pos = xform.transform(map_pos)
self.iface.mapCanvas().setCenter(map_pos)
# set scale if desired
self.iface.mapCanvas().zoomScale(1500000)
self.iface.mapCanvas().refresh()

Alternatively, if you still want to use setExtent(), you would have to do something like the following:

r_min = QgsPointXY(lon-0.5, lat-0.5)
r_max = QgsPointXY(lon+0.5, lat+0.5)
rect = QgsRectangle(r_min, r_max)
src_crs = QgsCoordinateReferenceSystem(4326)
dest_crs = QgsCoordinateReferenceSystem(3857)
xform = QgsCoordinateTransform(src_crs, dest_crs, QgsProject().instance())
rect = xform.transform(rect)
self.iface.mapCanvas().setExtent(rect)
self.iface.mapCanvas().refresh()