-First of all, I'd like to say, Im only typing with on hand so, there are bound to be alot of mistakes -
As I sent in a Pm to you, Sami, its pretty easy to do cities with with 1-2 airports, If the second one is very small.
Examples: for Seattle you have SEA and BFI, the main is Seattle and a very small amount pax go through BFI [Less than 20,000].
Another: for Portland you have three, PDX, Troutdale, and then Hillsboro. The last two are like BFI, with very few passengers. To get more accurate demand, these airports can - A: Be left alone and have indiviual route data submitted, B: Demand is generated by only the local communities. The main airports obviously have demand from their whole cities population.
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For Airports like DFW/DAL and IAH/HOU, where we have a clear main airport.... but there is still a secondary airport that has noticable service. So, as before the major airport has demand from the whole area, but this time this larger secondary airport takes demand from a larger chunk of the local area. In both cases, HOu/DAL are located closer to the city's center. So they might take demand from the county/cities/region/prefrecture/provence/etc that they are in.
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Alright, Im going to split this post into two as.... The forum does funky thing after making it longer than this, which makes it hard to type.