1. A concise explanation of the problem you're experiencing.
There are significant differences between STK World Terrain and Ion terrain; some of the Ion terrain looks incorrect.
Is the data that's currently being provided via the STK World Terrain server be available via Ion (or is it currently?)
2. A minimal code example. If you've found a bug, this helps us reproduce and repair it.
A sandcastle showing the problem:
The left-hand viewer is using the Ion terrain server, and the right hand viewer is using the STK World Terrain server. They're synchronized, so if you pan/zoom/etc the left hand viewer, the right hand viewer follows along. Also, there's a "Boston Logan" button which will jump you right to a representative sample
Note that just about any airport I look at has similar issues.
Very few runways are perfectly flat, but none I've ever encountered have the sorts of undulations depicted in the Ion data.
3. Context. Why do you need to do this? We might know a better way to accomplish your goal.
4. The Cesium version you're using, your operating system and browser.
Cesium 1.47, Chrome 67 on mid-2015 Macbook Pro High Sierra 10.13.5