I've been using CLLocationManager alot these days with mixed results. I'm able to get locations from the GPS and also significant location changes when the app is running in the background.
However, I always seem to get a course of -1.0, which (according to the documentation) indicates an invalid course.
In my didUpdateToLocation method I'm simply doing the following:
double courseDegrees = newLocation.course;
Am I missing something?
EDIT: Yes, I've tried it while moving.
From the docu of CLLocationManager Class Reference:
startMonitoringSignificantLocationChanges
This interface delivers new events only when it detects changes to the
device’s associated cell towers, resulting in less frequent updates
and significantly lower power usage.
To receive a course you need GPS, and not cell tower locating. If you need course then you
have to change to use startUpdatingLocation start using
desiredAccuracy= AccuracyBestForNavigation
Related
I want to Know what does startmonitoringsignificantlocationchanges uses to deliver location, is it GPS or Wifi/Internet? My Understanding is it uses cellular or wifi,to know if device has moved significantly but what I don't know is, at time of picking location does it use GPS alone to pick up location.
My Findings are : I travelled 30 KM in City Train but did not get any locations in background and during this journey my Wifi and Cellular data was off. So please tell if Significant fails to provide locations based on GPS only.
Building location apps in the past, I assumed that GPS was only used when startUpdatingLocation() was called. After double-checking the docs, Apple doesn't explicitly state whether significantLocationChanges CAN use GPS, but it is a power-saving option, and their docs specify that it "requires the presence of cellular hardware", which makes me pretty certain that my assumption (and yours) are correct. I'd leave this up for a bit, in case there's a hardware expert who knows for sure. The docs you want: https://developer.apple.com/reference/corelocation/cllocationmanager
Like all location functionality, it uses a combination of cell, wifi, and GPS. Use of cell is primary. The idea is to keep use of GPS to a minimum, in order to prevent drain on the battery.
You certainly cannot expect this to work with the cell turned off; indeed, if your code properly checks significantLocationChangeMonitoringAvailable with the cell off, I would intuitively expect the answer to be false.
(Also note that you will not get any events unless you have a location manager with a delegate.)
As explained in this answer "Hows does Significant Location Change work?" (and in WWDC videos from 2013-2015) Significant location change monitoring uses a technique called Cell Tower Triangulation.
You can think of it as almost the same mechanism as GPS triangulation except that the cellular hardware will be used to gauge distance from nearby cell towers instead of firing the GPS hardware to get signals from actual satellites.
In my experience I've noticed that this method gives accuracy of approx. 1000m. It does not use the GPS hardware and hence is more battery efficient. It is good for coarse location updates. See WWDC 2013 session Harnessing iOS to Create Magic in Your Apps where the presenter makes a passing reference to this technology.
TL;DR:
My objective: Try all forms of getting a location and then choose the most accurate one, (try get a location for a minute then time out).
I'm a little confused about where it gets the location from, is it GPS? Cellular? or a mix of both depending on toolkit?
While getting location on Android I used both and then choose which provider based on accuracy. I do not see anything similar for iOS.
I'm using CLLocationManager which is showing good accuracy but I'm not sure where the location is from.
On iOS, location manager is built more intelligently. Relying on your configuration, it either uses GPS or WiFi/cell radios, as described in the CLLocationManager documentation:
For the services you use, configure any properties associated with
that service accurately. Core Location manages power aggressively by
turning off hardware when it is not needed. For example, setting the
desired accuracy for location events to one kilometer gives the
location manager the flexibility to turn off GPS hardware and rely
solely on the WiFi or cell radios, which can lead to significant power
savings.
For more information, read
CLLocationManager documentation.
EDIT
And, If you want the most accurate location, you should put kCLLocationAccuracyBestForNavigation for location manager's desiredAccuracy property.
If you look at a map, the radius of accuracy begins big and then gets smaller as it gets more accurate. But locationManager:didupdateLocations: doesn't get called again when that changes.
Is there something that is called on that event?
I believe that the didUpdateLocations: callback will actually be called again when accuracy is updated. According to this Apple docs on Core Location:
In addition to a location object’s timestamp, you can also use the
accuracy reported by that object to determine whether you want to
accept an event. As it receives more accurate data, the location
service may return additional events, with the accuracy values
reflecting the improvements accordingly. Throwing away less accurate
events means your app wastes less time on events that can’t be used
effectively anyway.
What sort of tests have you tried to improve accuracy? Did you try it on an iPhone? Did you set the delegate and the desiredAccuracy property properly?
If you provide the following code, you will get very frequent location updates.
self.locationManager.desiredAccuracy = kCLLocationAccuracyBestForNavigation;
But according to Apple, it is recommended to plug in the charger to the iOS device while location manager is active. This is because the battery power gets drained fast while getting this much location updates.
There are other levels of accuracy for CLLocationManager( kCLLocationAccuracyBestForNavigation, kCLLocationAccuracyBest, kCLLocationAccuracyKilometer, kCLLocationAccuracyHundredMeters etc..)
Is there something that is called on that event?
No. You continue to get the same callbacks, but the accuracy of each location is (potentially) improved. There is no callback for "accuracy improved".
The CLLocations you receive in the callback contains vertical and horizontal accuracy information.
I have a problem in Objective C (writing an app for the iPhone). I know how to use the locationManager and its delegates. Once I called startUpdatingLocation on the locationManager, it will call the delegate method didUpdateToLocation whenever the location is updated. But the problem is, that this way is now suitable for what I want to do. I have a method (in a class) which looks as follows:
#implementation SomeClass
- (someType)getDbContentsOrderByDistance
{
... //here I need the current location
return dbContents;
}
#end
Now this function selects entries from a sqlite database ordered by their distance to the users current location. That is why I need the location in this function. What I want to do in the above method is: get the location, then select the stuff from the database and return it. I know that I could just try to access locationManager.location.coordinate.longitude, but I know that there won't be anything useful inside until the first location update has arrived. And I don't want to start the location updates before (e.g. when starting the app) because that would not be very efficient if I only need the location once.
I know how to do it the way that the delegate is called as soon as a location update arrives. The problem is that I don't know how to get that updated location from there into my method above. Basically I would need to let the method 'wait' until there is the first location update and then continue execution with that location.
Thank you for your help!
The short answer is, you can't. Getting the users location is an asynchronous process, and MUST be an asynchronous process. The device has to fire up various hardware like the GPS, cell tower locator, and WiFi triangulation system, get input from those different devices, and synthesize that into a location. That takes multiple seconds to do.
Putz's suggestion of starting a timer is a good one. I would add a few things however.
Typically the first readings you get from the location manager are really bad and should be discarded. The first reading you get is usually the last location reading when the GPS was active, and will have an out-of-date timestamp. The accuracy reading in that location might appear quite good, but it's a lie. I have seen the first location reading be off by several kilometers. You need to check the timestamp on your location updates and discard any reading that is more than 1 second old.
Once you've discarded the stale readings, the first several location updates are often really bad because the GPS hasn't settled down yet. The horizontal accuracy reading (which is really a "circle of confusion", or radius of possible positions) is an absurdly large value, sometimes a kilometer or more. Again, you need to write your location manager delegate method to discard readings that are too inaccurate. I suggest discarding values with a horizontal accuracy reading of >= 100 meters. How much inaccuracy you can tolerate depends on the specific appellation, but beware of making the accuracy requirement too accurate. Sometimes the GPS refuses to settle down. If you require a 5 meter accuracy, you might not get an acceptable accuracy during the entire run of your app. (If you're in an urban environment, a building, or other area where there is a lot of interference/obstruction of GPS signals)
Once you finally do get a value that's accurate enough, save the location and set your locationAvailable flag to YES.
Note that instead of a timer you could use the notification manager to broadcast a "got a good location" message. Any object that needs location information could register for your notification, and get called when the location manager gets an acceptable location reading.
Start a timer that fires every second or so that calls the function you need the location in. check to see if the location has been set. If the location is valid then kill the timer, if not then let the timer continue and check again in one second or so.
The way of knowing if the location is valid is in the delegate method. Set a global variable like locationAvailable = false. Then when the delegate method gets called set that variable to true.
Using location property can solve the purpose
From apple documentation
location The most recently retrieved user location. (read-only)
#property(readonly, nonatomic, copy) CLLocation *location
The value of this property is nil if no location data has ever been retrieved.
In iOS 4.0 and later, this property may contain a more recent location object at launch time. Specifically, if significant location updates are running and your app is terminated, this property is updated with the most recent location data when your app is relaunched (and you create a new location manager object). This location data may be more recent than the last location event processed by your app.
It is always a good idea to check the timestamp of the location stored in this property. If the receiver is currently gathering location data, but the minimum distance filter is large, the returned location might be relatively old. If it is, you can stop the receiver and start it again to force an update.
Whenever I create a new instance of location manager I get the most recent value of location in location property and since my distance filter is not set , it has the current timestamp. This means I dont have to call startUpdatingLocation and my data is accurate. I also need this for calculating distance between user location and a place so I can immediately return true or false if it is within the range. I find this very useful as I get location within 0.006 seconds and handy and got it from apple documentation only but still I dont know if it is a best practice.
This is stated in CLLocationManager class reference:
When requesting high-accuracy location data, the initial event delivered by the location service may not have the accuracy you requested.
This is really affecting my app. How can I make sure that the location found is the one with the correct accuracy?
I tried to use the 4th or 5th update rather than first retrieved location but this is not a good solution. Any idea?
You should check the accuracy of the updates, CLLocation
contains a property horizontalAccuracy which you can use to check the accuracy.
When the CLLocation has an accuracy that you find accurate enough you use and ignore al others.
Als you should tel CLLocationManager your desired accuracy. To do this set the desiredAccuracy property in CLLocationManager.
I think you will have to live with that. That's how Apple implemented it. Getting a fine grained position takes time, just think about how long any windshield-mounted GPS devices in cars take to fix up their position.
So instead of letting your application wait for a longer time, they provide with what accuracy is available almost immediately, based on cell-towers and WiFi hotspots in the vicinity. Only when there has been a more reliable GPS fix will they call into your app again and let you know.
In the end, it is just a question of where the waiting for the fine-grained position is: In your app, where you have the chance of doing something with the more coarse-grained data you get quickly, or in their framework with no chance for apps to do anything useful in the meantime. I think, letting you decide is the better choice.
Depending on the type of app, you could have a map that automatically zooms in deeper as soon as better position data comes in, draw a smaller circle around the position you are expecting etc. For the end user, nothing's worse than waiting without getting any feedback. So even though this is probably not the answer you would have liked, I advise to make the best of it from a user's perspective.