I solved thank you a lot !!!
var ghiacciai = new ol.layer.Vector({ //aggiungo i ghiacciai
altitudeMode: ‘clampToGround’, // to drape polygon on 3d
title: ‘Ghiacciai’,
visible: true,
source: new ol.source.Vector({
url: ‘data/ghiacciai.json’,
projection: ‘EPSG:3857’,
format: new ol.format.GeoJSON()
}) ,
style: myStyle_ghiacciai
});
it’s work !!!
Paolo