tracking a subway map

let's say i want to build a smartphone app that tells a user when/where to 开发者_运维问答get off a subway station. i can think of two ways of doing this:

1) using GPS and map of the subway routes, track the location of the user and notifies him when the destination is reached 2) have him press start when he gets on a train as the train starts (which may not be realistic because the user could simply forget to do this), use the known travel time from starting station to end station, and simply notifies him when the time is up.

can someone tell me if there are any other good approaches? thanks.

You may have to go with 'dead reckoning'. Basically, dead reckoning is a navigation technique that uses a confirmed starting location plus accurate velocity and time to calculate a new location. Keep in mind that velocity is itself a combination of heading and speed and that the heading must be a true heading. In an airplane, compass heading would have to be combined with wind speed and direction to get a true heading. I don't think you have to account for drift on a subway, but you will have to account for varying device orientation as the user moves around or uses the device itself. Also, just because heading is normally a compass bearing doesn't mean that it has to be. You may be able to get the job done using only the accelerometers and a timer.

Proper use of dead-reckoning also requires frequent 'resets' to known locations as you reach them so that errors can't add up too badly. For this application, I would argue that curve- and stop-detection can be used as resets. You may get false positives for 'service' stops that are too close to real stops, but those may be rare enough to ignore. In fact, if it wasn't the exit stop, it might not matter because you might still be accurate enough for the next-stop warning and if was the exit stop, it won't matter because the ride is over.

To summarize: you need to be sure that you have a good initial starting point; you need to compensate for device reorientation to get true heading; you need to know your average speed between heading changes and the time-on-heading to calculate distance-on-heading. You can improve overall accuracy by resetting at known landmarks.

Edit: I don't know if this gets you any closer to the answer, but Chris Stratton raised an interesting point about summing the accelerometer vectors. Is it possible to track the orientation of the device accurately enough to have a reliable orientation-independent gravity vector? Can you keep that out of your vector sum? Can that provide a useful acceleration along a useful orientation-independent vector? If so, then tracking the duration of the acceleration will get you an average speed for that duration and a final speed for the end of acceleration. Absent acceleration, the speed will remain constant. Putting that all together might pile inaccuracy on top of inaccuracy to the point of uselessness.

If the subway line has cellular service (some that were going to had it turned off on security concerns) you might be able to do something with network location.

You could use the accelerometer to try to detect and count station stops - but trains stop in between stations from time to time due to delays ahead. Also battery life would be reduced. EDIT: realized you won't be able to tell acceleration from deceleration as you have no idea of orientation (unless you find the compass sensor workable in that environment) - you could only see that the vector sum of the three accelerometers was greater than gravity for a period of seconds.

You could try to use the microphone to detect the sounds of train motors and brakes (some are extremely characteristic) but that has the same problem with battery life and unscheduled stops between stations. Not to mention scheduled stations bypassed for repair work.

Perhaps you should give the user a scrollable list of stations marking the journey and let them keep track.

Both of those sound like poor solutions. You almost certainly won't get a GPS trace in a subway. And the 2nd way sounds like it will be inaccurate enough to make the app useless.

Read carefully Falmarri's answer. He's completely right, although he didn't give you an answer. Let me try then...

If the routes have no many curves, it will be easier: you just have to hardcode the latitude and longitude of each station, and then, calculating the position of the user will be straightforward (just a little of math and that's it). If the routes have curves, then you have more work to do but it's basically the same.

You will probably want to use a way to know the distance between the current location and the station location. You could use some of the existing algorithms, for instance the Haversine formula.