It is very frustrating when you build a pd, especially t2, then later find it has no effective range because of the terrain. Sometime this is extremely difficult to predict because even tiny hills that are difficult to see, even when you zoom in, can block shots.
E.g. replay ID 6677533, from about 9 minutes to 10 minutes.
My idea is some sort of overlay which would highlight areas within the t2 pd range, that could not be hit due to terrain. This would make it much easier to tell if you are picking a decent spot for a pd, just like the range overlay helps. A bit similar to the aoe damage when you ground fire strat bombers, or tmls or nukes.
Sure maybe you think "stupid noobs need to just memorize where every barely detectable hill is" but you could say the same thing about needing to "just know" the range for pd, or radars, or engi/acu build ranges. But here it's also far more helpful than, e.g. build range overlay, because it can be difficult to figure out and is not some perfectly constant value.