const baseLayers = {
    'OpenStreetMap': osm,
    'OpenStreetMap.HOT': osmHOT
};

const overlays = {
    'IGNOrtho': IGNOrtho,
};
const layerControl = L.control.layers(baseLayers, overlays).addTo(map);

var slider = document.getElementById("myRange");
var output = document.getElementById("sliderOutput");
output.innerHTML = slider.value;

slider.oninput = function() {
    output.innerHTML = this.value;
    IGNOrtho.setOpacity(this.value/100);
}