Hi Sheila,
I haven't done much with viewsheds in MapInfo, so my comments comments aren't specific to the software.
I do radio propagation simulations for ground based systems. Happy to jump in & speak to viewshed interpretation & the radio propagation pieces of your post.
So, a viewshed has been described as a powerful light placed at a point in three dimensional space, with the viewshed being the terrain that's 'lit up,' right? So it's easy to envision the difference between a viewshed from a light on the ground (lots of mountain shadowing) compared to a light in the air - in your case 915m above the highest local peak (shadowing still happens but depends on the aircraft's line-of-sight). You'd expect to be able to see almost all of the mountain if you're directly above it, but only part of a mountain that's 5km away, even from 2000m altitude.
For radio system planning, viewsheds are useful for estimating areas of radio coverage, assuming radios are on or near the ground (including towers). Useful in that it tells us if we put a transmitter here, we should be able to put receivers there (but not over there, in the shadow of the mountain!) -- I might run viewsheds from a Caernarton base station to Snowdonia if I was working out how to get a certain part of the Park talking back to Caernarton. Viewsheds are, however, less useful when one end or the other is far above local terrain, as is the case with ground-to-air or air-to-ground simulations.
I can't say how aviation communications are simulated, but a viewshed feels like it might be the wrong tool. You could maybe configure it for an 'air-to-ground' viewshed, but that's only a view from one point in space, and how often is your aircraft hovering at that point (:-?). Likewise, there's theoretically a 'ground-to-air' viewshed from a tower to, say, a planar surface 2000m AMSL over the area of interest - in this case Snowdonia Park - but that's a) not how a viewshed tool works, generally, and b) a pretty abstract concept that's not particularly intuitive on a map (as opposed to a 3D view).
To your experience of seeing (visual line-of-sight) all of Snowdonia while communicating with the base in Caernarton (radio line-of-sight), this suggests you're simulating an 'air-to-ground' viewshed. Is this the goal?
To your point of getting different results using mean sea level vs above sea level, this is what you'd expect. Again, imagine a light in Caernarfon at sea level. How much of the surrounding terrain will it illuminate? If ground elevation at your base station is 10m above sea level and your base station is a 20m tower, place your light 30m above sea level and you're asking: how much terrain does my tower illuminate? Now put that light 2000m above sea level, still in Caernarfon, and you're asking: how much terrain does my aircraft over Caernarfon illuminate?
This next bit may be off topic, but if there's an 'earth curvature' setting in your viewshed tool where you have a '4/3' option, this is specific to radio propagation simulation. The nature of radio wave propagation is such that the radio horizon is actually a little farther than the visual horizon. Depending on radio frequency and environment, we expect areas where you can't see the base station but your radios can still hear each other.
Whew, that's a lot of me talking! Apologies for that. Hope there's something useful there for you.
I'd be interested to see screenshots too, if you're willing to share.
Jeff Haynes
RF Planning Engineer
