Catchment Areas
'Catchment areas' are actually a very complex and subjective topic. There are different types of catchment areas and they are influenced by many varying factors. Firstly, catchment areas vary by destination: an individual customer residing in Middlesbrough may select MAN for a flight to Philadelphia, but NCL or LBA for a flight to Dublin. Budget-conscious passengers (eg. stag/hen parties) may bypass geographically close airports to secure a low fare from (say) STN (an example of the economic catchment area overriding the geographical catchment area).
Some customers will fly from their preferred departure airport every time, considering (when required) an en route transfer preferable to the hassle of surface travel to a less convenient UK airport. Public transport fares, car parking charges, possible overnight hotel charges etc may need to be factored in also. And every wise traveller should remember to place a value on their own precious time (you only get to use it once!).
This is why I personally prefer to change flights en route from MAN to offline destinations such as Berlin or Madrid rather than mess about getting to a glorified bus station of MOL's choosing. If MOL and his ilk want me to fly with his company he must offer me a convenient product. I flew 22 Ryanair sectors last year (all MAN); next year the figure looks set to be zero. But I will still travel by air just as much. If Ryanair picks up other customers from elsewhere in my place then good luck to them. But I will not vary my travel preferences to accommodate Ryanair's tantrums.
The truth is that airport catchment areas are the function of every individual customer. One neighbour may find a less convenient airport acceptable for their needs whilst another will always opt for their first choice gateway.
There are regular contributors on here who claim that because there is a link from Liverpool to (say) Madrid there is no case for a parallel service from Manchester. Not so. Because those customers who currently use the LPL-MAD service do not represent 100% of the market from the Greater Manchester (or wider northern) area. Many of us change flights at AMS, CDG etc. in the absence of a dedicated flight. It is a mistake to suppose that the portion of Manchester area residents who choose the LPL-MAD option represent the entirety of the available market. An airline cannot get away with telling its customers where to go (although we all know one which tries very hard to do that!).
Certainly, all destinations are not created equal. A destination such as Wroclaw may not justify multiple departure points from the North, whereas arguably a more mainstream route such as Madrid can. Each potential service must be considered on its own specific merits.
So a catchment area cannot be simply dismissed as the geographical space which falls within a distance-based boundary line drawn on a map. Neither can it be defined only as the area from which 'Airport X' is the cheapest or quickest to reach. For every individual, distance from the airport is just one factor to be considered alongside financial cost, time expended, convenience, ease of access (availability of public transport?), personal preferences and so on. One person's catchment area is another person's no-go area.
This is why we could argue about "catchment areas" on here till the cows come home. But its best not to. There is no one correct answer. So we will never agree on one!
Cheers to all. SHED.