One of the major components of the new ATC system in X-Plane v10 are the taxi layouts. We sometimes internally refer to this as the "taxi network" because what it's REALLY comprised of is a network of taxi segments attached to other segments at taxi nodes. The concept is pretty simple, a node connects two or more segments together and it's a place where a decision could be made by the ATC system in order to change directions. Every single land-based towered airport in the world has it's own taxi network.
To give you an idea of what a network looks like, here's one for Seattle (click to see it clearer)
White segments are runways, green-ish (my wife would say "seafoam") segments are taxiways, red segments are taxiways inside of an active-zone* and the white dots are the nodes. For ground operations, this is what ATC knows about and it's the ONLY thing ATC knows about. It doesn't know about buildings or obstacles in the way, it only knows about the pavement. Anytime ATC wants you to get from one point to another, it has to give you a routing composed of a connected list of these segments. The only exceptions are the start and end segments. ATC is only allowed to go "off the grid" to get you onto or off of the network. So if you're at a gate and you request a taxi to runway 34L, it first tries to figure out the fastest way to get you onto the network. It does this by calculating whether you're closest to a node or a segment on the network, choosing the closest one and then giving you a straight line to that point. Now you're on the network. The opposite happens for end points.
So that's how you get in and out of the network, but what happens along the way? It's not all that different than the GPS in your car. ATC analyzes essentially every possible routing from your start/end nodes and chooses the one with the lowest cost. Note that I said lowest cost, not shortest distance. Think about the best way to get to work in the morning. Do you take all back roads? Do you maximize highways? Do you avoid tolls? Your decisions are based on far more criteria than the mere distance. The ATC system is no different. Distance is just one of the contributing factors that goes into it's cost calculations.
Some (but not all) of the other things in the costs are segment type, segment usage, segment angle and the aircraft's mode of operation. These can be fairly complicated so I'll summarize them quickly. Segment's have different types. They can be one way or two way just like streets in a city. Obviously the cost of using a one way street depends entirely on the direction you want to go. If you want to go the legal direction, the cost is nothing out of the ordinary. If you want to go the wrong way, the cost is REALLY high. In other words...only do it if you have no other options! The final segment type is "runway". ATC tends to avoid taxiing you on the runways because that'd be terrible for airport efficiency...especially if the runway is an active runway. Another contributor to cost is whether or not the segment is already marked to be used. In other words, if another aircraft is taxiing southbound on a particular segment and you want to taxi northbound, that's a potential problem, so the cost is high. If you're going in the same direction, it's no big deal. Segment angle is important as well. ATC does it's best not to issue routings that result in steep taxi angles. 747's have trouble making 150 degree turns for instance. Lastly, there's your mode of operation. Remember how I said ATC hates using runways for taxi purposes? There's an exception. When you land, it's often beneficial to continue taxiing on the runway since you're already on it and you're already going quite fast. All of these factors contribute to the cost weighting and the route chosen for you is the one with the lowest cost at the time.
So where do these networks come from? Where does the metadata come from? Well, there are thousands of airports in the world that need these taxi networks so that ATC can provide service. It was important to us that ATC would function at any airport out of the box so Ben wrote some insanely cool algorithm that runs "on-the-fly" when you load up an airport. The algorithm looks at the pavement, erodes it inward leaving the "average" center points of all of the edges. This works pretty well but it's not without it's problems. The first problem is that it has no clue what the pavement is to be used for. Is it an apron? Is it a taxiway? Is it an access road? Is it a shoulder on a runway? Pavement is generic in X-Plane so there's no way to know. The result, as you can see in the screenshot above, is that sometimes you get routings on small access roads. Another problem is algorithmic "noise" that's causes the taxi lines to be quite zig-zagged and wavy. This is why the AI planes often look drunk when they're taxiing. Often, the segments are not "reduced" or "smoothed" as much as we'd like. You can see the entrances to each runway tend to have a "Y" shape to them because of the "fillet" on the taxiway. This is a byproduct of the erosion code. There is code in there to reduce it as much as possible and "snap" the edges to the runway but it has to be generic enough to work reasonably well at thousands of different airports. Most of the time it causes no problems but there are cases where the "Y" becomes extremely wide because the runway has a thick shoulder created with a strip of pavement. One final problem is the lack of metadata. The active-zones are stamped out around the runways proportional to their size so seldom do they line up with any painted hold-short lines. We have no clue what the taxiways are named. I know what you're thinking "Why not use the painted taxi markings and signs to aid in the algorithm's decision making". That was actually one of the first things we tried and found the data to be too sporadic to be useful and it made the algorithm very slow and complicated.
So if the algorithm has so many problems, why are we using it? Two reasons. First, the alternative is that no airports would have ATC capabilities on the ground until a user made a custom layout by hand. Second, the algorithm was created to be a stop-gap solution, not a substitute for human data! The REAL taxi networks need to come from a human! Once they are made, they'll be added to the apt.dat and distributed with the sim moving forward. This means that over time, human data will replace the algorithm's data and taxiing will get better. The taxi network generation is one of the major features of WED v1.2 which will be released in the coming weeks. Authors can create a network for their airports relatively easily. In my first time using WED, it took me only 2-3 hours to create the network and I'm completely clumsy using any kind of CAD type software. Someone who knows what they're doing could do an equally complex airport in about an hour I would guess.
The layout in the first screenshot above is an autogenerated one. That's NOT what you'll be using if you go to KSEA in the simulator and that's because I made a custom layout. If the sim sees a custom layout, it always prefers to use that. Here's what my custom layout looks like.
So you'll notice the lines are straight! There are no taxi segments through buildings anymore, there are no taxi segments on the access roads, there are no "Y" branches at the runway entrances/exits. Also, not visible from this height, the segments are lined up perfectly with the taxi paint and the active-zones perfectly coincide with the hold-short markings as well. I've also named each segment properly so that ATC can tell you exactly how to get to your destination e.g. "Taxi via Alpha Charlie Two Foxtrot Delta".
There really is no substitute for human data. It definitely results in a much more realistic and far less problematic experience.
*An active zone is a taxi segment that's protected by ATC because it's adjacent to or in some way affected by an active runway. This is how ATC knows what to protect with hold-shorts and it's also how ATC knows when it's time to talk to Ground vs. Tower.