【问题原因】面实体PolygonGraphics接口perPositionHeight，该参数用于指定是否使用每个位置的高度，默认是false，意思是不使用高度，为贴地。
【解决办法】代码里加上这句perPositionHeight:true，让它不贴地。具体代码如下：
var polygon = viewer.entities.add({
polygon: {
hierarchy: {
positions: [new Cesium.Cartesian3(290254.5148736448, 5637924.074937166, 2971777.4768239637),
new Cesium.Cartesian3(286432.14053509803, 5640804.651089405, 2966391.9275969476),
new Cesium.Cartesian3(283512.2666752818, 5640502.494127799, 2969385.3689191523),
new Cesium.Cartesian3(287036.36797237827, 5636827.146248645, 2974105.8904601005)
]
},
material: Cesium.Color.BLUE.withAlpha(0.5),
outline: true,
outlineWidth: 5,
outlineColor: Cesium.Color.YELLOW,
perPositionHeight:true
}
});
viewer.zoomTo(polygon);
