CAS is often thus shown on VFR charts (not in the UK) but it would be difficult to implement it in software.
In both cases you would need some form of certified terrain database.
It is also pretty obvious that such a CAS base cannot be enforced by ATC – because there is no way to depict that sort of thing on ATC displays. It might be implemented in some obscure software they have. In my flying around Europe I have never seen any evidence of it though. I would guess they regard some form of “sector altitude” as the CAS base in such cases.
Yes, Skydemon does this. It seems to work well – at least in reasonably flat areas.
You don’t need a ‘certified’ database for anything. I suspect they use the SRTM data. Software wise it doesn’t seem all that complicated to me – if your software can represent ground level in some form and warn about terrain, then it can easily warn about CAS at ground level +Xft.
In my experience, the places that have this type of CAS (Germany mainly), are pretty relaxed about issuing clearances through it. In France there are quite a few military corridors that are SFC to x ft AGL. Easy to avoid, of course.
