why can't I get the proper altitude when mouse over


I have use the picking show case in Sandcastle. I want to show altitude changing when mouse move. Why I always get a height value under zero when camera is far. When I get closer, the value become zero or someting like 1.04265498e-9.

I don’t know how should I get the correct height.

showcase : http://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=Picking.html&label=Showcases


Please see my answer in this post:


Let me know if you have any questions,