I am confused because QFF being actual mean sea level pressure is subject to change why would we use it for mapping terrain? Does this mean the height of a given terrain changes with the change in QFF? Tried to look for an answer for this everywhere but I couldn’t find any. Can someone tell me what am I getting wrong?
Maps and charts show true elevation, meaning tape measure altitude from the sea level. The way the mapmakers and chartmakers measure elevation has changed over time. Currently, I think GPS is prevalent. The way GPS computes elevation is depend on the WGS84 Geoid earth model. This modes surface does not coincide with actual sea level but GPS takes that into account. And today in most aeronautic related industries WGS84 is becoming a standart, which also includes maps and charts.
On the other hand altimeter provides height using pressure of air. QNH and QFF are the altimeter settings used to adjust for non-standard pressure and temperature, respectively. QNH is always provided to pilots and is the most prevalent. I have never heard that the QFF is used or requested by pilots.
As I said QNH setting only corrects for non standard pressures. It really assumes ISA temperature, 15C. So as you asked what happens if the temperature is not standard. Well someone needs to compensate for that and make correction.
Colder than ISA temperatures needs to be corrected for minimum enroute altitudes and obstacle clearances. Because the aircraft will be at lower true altitude than what altimeter shows, at colder temperatures. As for hot days, it’s less risky because aircraft’s true altitude will be higher than what is shown on altimeter. Only example comes to my mind is when you’re shooting an approach that has decision altitude kind of minimums. Since the aircraft will be higher than true altitude you may end up going around during real minimum IFR situations. However you will be on the safe side at hot temperatures, in terms of obstacle clearances.
As mentioned in that article; correcting minimum sector altitudes (MSA) and minimum vectoring altitudes for non-standard temperatures are ATC’s responsibility. In other cases, such as doing a published approach, it’s pilot’s responsibility.
That same article provides an excel at the end to use on the ground. Also some pilot operating handbooks provide charts to apply for altitude adjustments for non-standard temperatures. One technique for flight planning is to find the correction for the coldest temperature for the day and apply that to whole flight. But that applies only to GA type flights really.
Not only that, but also, there is this phenomenon called geopotential altitude. It implies that 1 altimeter-measured-feet at sea level is not equal to 1 altimeter-measured-feet say at 30,000 feet. And that’s because gravity is not constant as we climb. So the actual true elevation of the obstacle will always be slightly higher than what you see on your altimeter even when you’re side by side. However it’s really a small difference (less than 1%) and not a big factor at low altitudes where obstacles mostly live. [wp reference]
I understand this much but what I’m not able to grasp is the fact that QFF would change every now and then, what happens to the calculated height of the hill then, does it also change?
You measure the pressure at the top and at the reference point and use the temperature and humidity at that moment to calculate the height, write it down and be done with it. On another day the measurements will all come out different, but unless you make an error measuring or calculating, the height should come out the same. Because it is the same, after all. The accuracy won't be that great though. Also note that you need the temperature and humidity; QFF alone is useless without those.
why would we use it for mapping terrain?
I don't think anybody would, actually. Today you'd use satellite-based altimeter—that allows creating a complete height map in several passes and there are already such datasets including at least one free one—or GNSS. But before I believe a triangulation using theodolites was always more accurate, and to make a map you needed to do that to get accurate positions anyway.