That is an old StackOverflow post, and mixes several questions, which complicates any answer.
There is “a way to create terrain-hugging objects” that works well using POINTS or POLYGONS, but NOT the polygon boundaries, i.e. NOT POLYLINES.
There are workarounds for polylines using “corridor geometry”, but that is not a final solution, and I was not satisfied with the results, although I did not spend a lot of time working on that before I gave up.
Back to your question, re “terrain-hugging objects”, what type of geometry?
What type of “external data source” are you looking at? Can you provide an example?
I have had good luck using GeoJsonDataSource, and loading POINT or POLYGON geometry that contains NO Z VALUES.
That is the key - you must populate the JSON with X-Y coordinate pairs - you cannot use X-Y-Z coordinates.
You CANNOT provide the Z (height) values for your polygon boundary vertices.
Cesium will automatically use whatever height the 3D terrain faces have, when draping the geometry over the terrain.
If you are primarily interested in the polygon boundaries, I cannot give you much help.
In cadastral mapping, the property lines (polylines) that make up the parcel polygon boundaries are very important.
If there is a good Cesium solution for displaying those, I have not seen it.
-Jon “Left Gully”