MapQuickItem's fade in points should be documentated, and preferably settable from application code

    if (quickMap_->zoomLevel() > opacityRampMax)
        return 1.0;
    else if (quickMap_->zoomLevel() > opacityRampMin)
        return quickMap_->zoomLevel() - opacityRampMin;
    else
        return 0.0;

import QtQuick 2.12
import QtQuick.Window 2.12
import QtLocation 5.9
import QtPositioning 5.8

Window {
    visible: true
    width: 640
    height: 480

    Map {
        id: map
        anchors.fill: parent
        center: QtPositioning.coordinate(51.48, 0)
        zoomLevel: 5

        plugin: Plugin {
            id: mapPlugin
            name: "osm"
        }

        MapQuickItem {
            coordinate {
                latitude: 51.48
                longitude: 0
            }
            sourceItem: Rectangle {
                color: "red"
                width: 50
                height: 50
            }
        }
    }

    Rectangle {
        width: 50
        height: 20
        Text {
            anchors.centerIn: parent
            text: map.zoomLevel.toFixed(1)
        }
    }
}