I've a GPS tracker that now works great, except when iPhone lost GPS connection.
When the connection is lost, the track makes one or more points to a random location, as you can see in this image:
(source: alluneed.it)
In which way can I discard these bad values?
I've thought about doing it this way:
before save my data, I can do a sum of my integer values of current latitude and longitude and compare with integer values of my old latitude and longitude; if the difference is bigger than some value, I discard my data.
Will it be a good thing?
Are there some other nice tricks to do this?
maybe i've found my fault:
in
locationManager:didFailWithError:
when i receive a kCLErrorNetwork or any other error except kCLErrorDenied i perform a stopupdatinglocation and a startupdatinglocation.
Now the app works good, i make some other test tomorrow and post here the result.
Try to check the time of the location update while the GPS has no signal (the time is provided within the location parameter if I recall correctly).
Maybe the time is some constant , and that way you can check if the difference between the current time and the location time is more then some value, to know if the location is real or not.
You can sanity check points before using them. If the accuracy level suddenly jumps up and then back down again, you can throw away that one point. If the timestamp says the user is suddenly moving at supersonic speeds to get to the new location after calculating the distance over time from any previous points, then the point is likely bogus. etc.
Related
I'm in the process of creating my first iPhone app. It is an exercise log that will allow users to use the GPS to track a run, then be able to save a map of the route as well as the time/distance and upload it to a website. A local list of runs would also be saved on the device. My question is, what is the best way to implement the saving and retrieval of the map? I recall reading somewhere that the way to do it is to have entities that have latitude and longitude attributes, and then fetch these in reverse by time when plotting the map. This would mean that each entity is a point during the run. Is there a way to store all of the coordinates in an array in one entity so that one entity would represent a whole run?
I haven't really looked at relationships since I'm new to app development, but it seems like I could use relationships to store runs? As in, have the parent entity be the run, and have one of the destinations be all the coordinate entities of that run. Does this sound correct?
Thanks!
Having run as an entity makes sense. For the waypoints along the route, suggest a relationship with a 1-to-many cardinality (that is, one run has many waypoints). The attributes of the run might include start time/date, end time/date. The waypoints attributes might be latitude, longitude, altitude, date/time. You'll probably want to experiment with how you decide to log a waypoint during the run. Maybe collect every minute, or based on moving a certain distance from the last waypoint.
The waypoint with the earliest date/time is the starting point, and the waypoint with the latest date/time is the ending point.
With the above, you can plot the route one a map, calculate speed between waypoints, average speed, total distance, and maybe some sort of difficulty factor based on altitude changes.
I'm working on a location-based app that makes use of the CLLocationManager region monitoring.
I'm using a single CLLocationManager and single delegate (which are set up in the main app delegate at startup), and I'm noticing that I often get a burst of multiple calls to my delegate (on locationManager:didExitRegion:) when exiting a monitored region -- usually two calls, but sometimes more. Has anyone else experienced this, or have any ideas what can be going wrong?
I'm instantiating the CLLocationManager as follows, in a class that is instantiated in the app delegate:
_locationManager = [[CLLocationManager alloc] init];
_locationManager.desiredAccuracy = kCLLocationAccuracyHundredMeters;
_locationManager.delegate = self;
I'm setting up region monitoring like this:
// The region instance has a radius of 300 meters
[_locationManager startMonitoringForRegion:region desiredAccuracy:1000];
As I understand from the documentation, providing the desired accuracy of 1000 means that locationManager:didExitRegion: should only be called once we're 1000 meters outside of the region.
On additional point -- as far as I've seen, I only get a burst multiple notifications if I'm in the car (and therefore travelling quite quickly). It doesn't seem to happen if I'm on a bike or on foot. Any pointers as to what I'm doing wrong (or if this is an issue that others have already encountered) are appreciated.
tl;dr: It's really quite simple - you're getting as much information as Apple can give you about the fact that you're crossing cell tower boundaries - meaning, not really good data at all.
Now the real story:
Generally, there are only a couple of ways that CoreLocation can determine one's position:
By GPS. This is highly accurate (tens of meters), but takes lots of power to use.
By WiFi. This is generally accurate, though wifi base stations can and do change, which makes it fuzzy. This method cross-references the wifi stations in the area vs some known accurate locations - so it knows when it sees "FooWifiStation" that it's associated with a particular area, measured by precise instruments, or perhaps even other phones with GPS turned on (which is controversial, we may never know if Apple uses this method)
By cell tower locations. These don't move, so it knows you're within a big fuzzy dot of coverage when you're associated with a tower. This is the least accurate, but least power-consuming, because your phone is already doing the work to stay in contact.
You can even see this if you go into the maps application "cold": you start out immediately with a big fuzzy blue dot, (at least 1km where I am), then it shrinks as it gets a wifi location fix, then it more or less disappears when GPS gets its fix. It's doing [cell tower] => [wifi] => [gps] accuracies in real time.
From my experience, the "significant location change" clause means "We'll let you know whenever you move in a big way, as long as we don't have to do any more work or expend any more power to get you that data. This means de facto that the very best you can rely on is using transitions between cell towers. Apple deliberately left this vague, because if another app uses something that has better resolution - say, you open Maps.app, and it gets a GPS fix - it is possible that you will suddenly get a great fix on location, but you can't depend on that always being the case. You've asked for a "weak reference" to location.
Now think about what happens when you are wandering about in your car: your cell phone has to manage that transition. It gets handed off, talks to multiple towers at once, that sort of thing, to manage a seamless transition which must be viable while you are having a conversation. Just that is a pretty neat feat for any little box to do. Necessarily, this will cause you to see a bit of bounce in location updates, because, to the phone, you're vibrating between cell towers at this time as it negotiates the transition.
So what the radius really means is that you're interested in data of roughly that accuracy - but the API will not guarantee that you get updates within that radius. You can think of it as Apple saying, "We're going to bucket your accuracy into one of 3 groups - but we didn't give you an enumeration, because we want to reserve the right to develop better methods of fixing your location without you having to change your code". Of course, in reality, any real app will change if they change their method of getting location, because they are annoyingly vague about this location stuff.
In other words, you will get a location fix at the cell tower with some totally made up guess as to how good that accuracy is; when you move to the next tower, you will instantly jump to its location, with a similarly fuzzy fix - does that make sense? CoreLocation is telling you you're at the cell tower with accuracy of however far the cell tower's signal reaches; as you move to another tower, you will probably get the bounciness of the handoff.
So when it comes to it, to really do a good job, you have to assume that the data is one of "great", "ok", or "bad" and use other methods - like Kalmann filters - if you really need to have a better guess at where the user is. As a zeroth-order approximation, I would just debounce the callbacks based on the time of the update, which is given, and assume that the user isn't really leaping kilometers back and forth within a few seconds, but rather, travelling in the direction of the first new update.
I think you'd be better off, using a desiredAccuracy that is smaller than the radius of 300m, i.e. kCLLocationAccuracyHundredMeters. Have you tried that?
There is no documentation about the underlying logic, but I'd assume, that desiredAccuracy can be regarded as the minimium distance you'd have to travel that the movement counts as a "border crossing".
Region monitoring is based on "significant location events", not GPS – otherwise the battery wouldn't last half a day.
If you use such a high desiredAccuracy, the system might get more than one significant location event (these seem to be generated about every 500m – also depending how many wifi networks you have in the area.
In that case, the system might compare the current location resulting from the significant change with the distance to all sides of your region.
If you're just outside 1000m from opposite side of your region, it might notify again and only stop notifying once your outside the 1000m from each side of your region.
The reason for the accuracy parameter is rather to avoid border crossings, if you are too close to the border, so you're not seeing inside-outside-inside-outside etc while traveling just outside the border...
In my apps, I tried many many different combinations of radius and accuracy and settled on 500m/100m and 100m/10m – those work very well in my real work within-city szenario.
(see also the excellent article series http://longweekendmobile.com/2010/07/22/iphone-background-gps-accurate-to-500-meters-not-enough-for-foot-traffic/)
I'll bet this is your issue, locationManager:didExitRegion: gets called for EVERY region that every location manager in every application on your iPhone has registered. You need to compare the region's identifier string sent as a parameter with the region's identifier string of the region you want your app to currently do something with.
When you send [_locationManager startMonitoringForRegion:region desiredAccuracy:1000]; you're creating a region, if it doesn't already exists to be monitored for. This is NOT the same as adding an observer to NSNotification Center. Apple's region object blindly sends the notification to EVERY CLLocationManager which then sends the message to the delegate.
First, the documentation does not indicate that desiredAccuracy alone determines how far out of a region you need to be, before didExitRegion event is generated. If you want to fine-tune how often events are fired, you need to use the distanceFilter as well, which determines the horizontal movement you need to have moved before an event will be fired.
If use of the distanceFilter does not work, then I would recommend the following:
If you are using NSNotificationCenter, then ensure that you have removed other classes from notification, via [[NSNotificationCenter defaultCenter] removeObserver:self]. You can optionally specify a name and object to this call.
1000 Km is a large radius, which has a high chance of intersecting with many regions in the vicinity. I would try a lower number for this parameter to see if that doesn't decrease the number of exit notifications that you are receiving. The only thing that indicates that this may not be the solution is that you did not say you are receiving blasts of didEnterRegion as well.
Lastly, I would check the identifier of the CLRegion being passed in the didExitRegionEvent, to see if you can't set up an NSDictionary of regions yourself. You'll have to set a region to the dictionary on didEnterRegion, and remove it on didExitRegion. So, on didExitRegion, all you have to do is ensure that you are interested in the region by checking that you have the region already. I would guess that CLRegion is already equipped with isEqual: and hash to allow it to be stored in a collection.
Good luck!
I found that in order to save battery you have to use monitorForSignificantLocationChange.
My solution is to filter out multiple alerts coming within 60 seconds for the same region:
-(BOOL)checkForMultipleNotifications:(GeoFenceModel*)fence
{
GeoFenceModel *tempFence = [fenceAlertsTimeStamps objectForKey:fence.geoFenceId];
if(tempFence == nil){
fence.lastAlertDate = [NSNumber numberWithDouble:[[NSDate date] timeIntervalSince1970]];
[fenceAlertsTimeStamps setObject:fence forKey:fence.geoFenceId];
NSLog(#"checkForMultipleNotifications : no Notifications found for Region : %#, Adding to Dictionary with timestamp %.1f",fence.geoFenceId,fence.lastAlertDate.doubleValue);
}
else if(([[NSDate date] timeIntervalSince1970] - fence.lastAlertDate.doubleValue) <= 60.0){
NSLog(#"checkForMultipleNotifications : Multiple region break notifications within 60 seconds, skipping this alert");
return NO;
}
return YES;
}
Sorry for being a noob here. I am not able to clearly differentiate between CLLocationManager's properties distanceFilter and desiredAccuracy.
If I want my application to give different coordinates for even small distances (say 100-200 metres) what values should i set for these properties.
Help would be greatly appreciated.
According to developer.apple.com
distanceFilter
The minimum distance (measured in meters) a device must move laterally
before an update event is generated.
That means, based on previous location event, another location update will only be received after exceeding distanceFilter value distance.
desiredAccuracy refers to how accurate your location data should be.
For example if you wish to see the exact street you're on you a high accuracy value for this parameter. (kCLLocationAccuracyBest)
If you only wish to see the approximate area (such as in which neighbourhood you're in) you'd set a lower accuracy value for this param. (kCLLocationAccuracyThreeKilometers)
Choose this to suit your needs, however be aware that the more precise you wish to be and the more often you request updates, the more power it will drain from your device.
Hope this helps,
Vlad
distanceFilter - this is minimal distance which device should pass from previous location which was passed to delegate with ...didUpdateToLocation:... method. And as soon as distance reached location service will invoke ...didUpdateToLocation... again and so on.
desiredAccuracy - tells to location service how accurate coordinate you want and this is minimal location error radius. If value is very low (ex. 5) radio will try to use GPS hardware and will keep powering it up hardly to make it give most accurate location. If value is large,than system may decide to use data which was retrieved from WiFi hotspots location triangulation.
I'd like to know (from a high level view) what would be required to take a pdf floor plan of a building and determine where exactly you are on that floor plan using GPS coordinates? In addition to location, the user would be presented with a "turn by turn" directions to another point on the map, navigating down hallways, between cubicles, etc.
Use case: an iPhone app that determined a user's location and guided them to a conference room or person's office in the building.
I realize that this is by no means trivial, but any help is appreciated. Thanks!
It's an interesting problem. When you're using Core Location, you're not necessarily using GPS. Using WiFi and cell tower triangulation, you can get pretty good location results. So from Core Location you get a latitude and longitude fix. (You might also get altitude info, since GPS data is 3-dimensional. You also will get an accuracy value.)
So you have lat and lon. You need to map these coordinates to the PDF plan's coordinates. Assuming that the plan is aligned with the latitude and longitude lines, and that you have a lat-long fix for one of the points on the plan, you need to calculate the x-axis scale and y-axis scale. Then it's some calculations to map the lat-long to x-y coordinates on the PDF plan.
GPS may not be accurate enough for this purpose, especially indoors. Assuming errors on
the order of 10 meters, you'll have difficulty determining which floor the user is on.
Here's a neat (?) idea that might work: can you post some "You are here" placards
at various locations around the building? You could label each one with a unique,
machine-readable location code (maybe a QR code or something similar), then take an
image using the camera, have your app read that image and interpret the location code,
and use that instead of GPS to determine the start location.
GPS inside? That's your first -- and biggest -- hurdle.
Next hurdle is knowing the GPS coordinates of at least three points on that PDF to define the plane of of your map in the real world. (The PDF will need to be to scale, of course.)
So that gives you where you are on the PDF. Now you'll need to figure out some way to determine where you can walk (or where you can't) to get directions.
I'd like to use reliable locations, even on an old iphone. However, many readings (particularly from cell towers) are too inaccurate. I think.
When I plot my position + accuracy radius (or look at google maps app), I notice the center of the estimated circle is generally close to my physical location. I'm guessing that if I cut the "accuracy" number in half, I'll still be in the circle 99% of the time.
I believe this is a probabilistic game - the location manager is trying to provide an estimate that's correct 99.99999% of the time, so they give a deliberately wide margin. Any thoughts/info?
The CoreLocation framework gives you the radius of the circle for every CLLocation you get using the horizontalAccuracy/verticalAccuracy properties. You can specify to the CLLocationManager a desiredAccuracy property that use these types:
kCLLocationAccuracyNearestTenMeters, kCLLocationAccuracyHundredMeters, kCLLocationAccuracyKilometer, kCLLocationAccuracyThreeKilometers;
So you get notifications when you get inside your desired range. That said, when you use the CLLocationManager the first event is given to you ASAP, and then the proceeding events are the ones that satisfy your conditions.
When you're using CoreLocation, you're getting back "answers" that get better and better. I've noticed that the "best" answer is almost always accurate to within 100m, so theoretically you could probably cut down on the "buffer" that you're normally given. The only way to really know, though, and this is what I would do, is to test test test. Find iphones and ipods from all generations and see what types of accuracies you're getting and what types of results you're getting. In a lot of ways, it depends on the type of app you're making, but if you want to deliver sensitive or important information based on where the user is, you should really wait for the framework to give you a nearly exact location.