i use CLLocation for a app that record people's trace in map view when they are running or walking ,but i found when my device is still (not moved) ,the
- (void)locationManager:(CLLocationManager *)manager didUpdateLocations:(NSArray *)locations is also get called frequently ?
currently ,my locationmanager's desiredAccuracy is 10 meters and distanceFilter is 10.
how to deal with this situation? I have tried use big distancefilter value(like 150) ,but I found if i do this, then i can't record exactly when people is running or walking
GPS is not exact. You can move around a few feet and get the same location. Or you can sit still and get told you have moved a few feet.
You might be able to combine measurements from the accelerometer to determine if you have really moved but this would only work if the device was sitting on a table not moving.
Have you called stopUpdatingLocation after the initial startUpdatingLocation? It will keep updating location if you do not call it.
How does system know whether or not you have actually moved? It MUST fetch your location to find out. The more accurate your desired accuracy is, the more vigorously and frequently it would look. By vigorous I mean if it's suppose to use cell-tower information then it would look into more cell-towers to better triangulate. Or simply put it, the interval between its fetches would be smaller. Concluding: OS would fetch data even if you don't move.
Additionally to triangulate your position the OS (depending on your desiredAccuracy and previous movements) would use a mix of GPS, wifi, Cell-tower. And because someone may all of sudden turn off/on their wifi, or satellite that you were using to get your location has moved a bit or the satellites have changed, or a cell-tower signal may become more or less accurate due to its bandwidth limitations then your calculated location may change which triggers a callback if it's more than your distanceFilter. ( I don't believe you get callbacks for less than your distanceFilter, but I may be wrong) This likely means your distanceFilter is set to very small number which depending on your business requirements may or may not be a good choice. Concluding: your location is never ever 100% accurate
The result of periodical fetching, possibility of error and small distanceFilters lead to possible incorrect locationChanges.
Related
I need to know if the GPS device component of my users is healthy, so i want to get the location ONLY by the GPS and NOT form wifi or surrounding antennas.
I'm forcing my users to close their wifi before this operation so i definitely know that the received signal is not from the wifi (because its possible).
I tried to work with the CLLocationManager and CLLocation's horizontalAccuracy.
some people say that when the horizontalAccuracy is lower than 160 it means we have a good GPS strength, but its not perfect at all..
I also tried to create an array of [CLLocation] after calling [locationManager startUpdatingLocation], and than i saw that when i'm in a place like a basement(with no GPS signal), my array's count was lower that a situation with signal because the updates frequency is lower.. but this is not a perfect solution either..
I just want to be sure that the GPS component is definitely working and the location that i get is from the GPS.
so, any solution please?
I'm developing an iOS app that needs precision velocimeter data capturing. For that i need to know what are the maximum sample frequency of iOS devices velocimeters. Couldn't find that on devices especifications or anywhere.
Please help
So the speed gets calculated from distance and time, there is not a real frequency.
The CLLocation speed attribute gets updated every time your device is moving.
It means, even when you looking for the object every millisecond, you only get the last calculated coordinate.
Update
https://developer.apple.com/reference/corelocation/cllocationmanager/1423500-distancefilter
This distance is measured relative to the previously delivered location. Use the value kCLDistanceFilterNone to be notified of all movements. The default value of this property is kCLDistanceFilterNone.
This property is used only in conjunction with the standard location services and is not used when monitoring significant location changes.
So I think this means, that the frequency is depending on the gps signal it self.
https://en.wikipedia.org/wiki/GPS_signals
I am by no means an iOS developer, and am just hacking something basic together for myself. Sorry if this is too beginner of a question.
I'm trying to collect my own location data from my phone to my own back-end service. Looking around, I found a sample project to collect location info: https://github.com/chriseidhof/PostGPS/
From the looks of things, this project uses significant location update, so even though I changed my desired accuracy to 100 meters, the app will still only provide update when enough cell towers have changed.
Instead, I'm thinking of using geofencing (CLCircularRegion) by creating grids of 3x3 geofences with 500 meter radius (with maybe a 50 meter overlap between each region), and each time I exit the central region, a new set of 3x3 geofences gets generated.
My questions are as follows:
Is this (the deleting and re-fencing part) a correct use of geofencing on iOS?
Is this going to be more accurate than what I currently have?
Will this significantly negatively affect my battery life?
The problem with using geofencing is that you can't monitor more than 20 regions at a time(OS limitation). It's better to use significant location changes. It is triggered while you're in foreground or background and the location is updated if the cell tower has changed or you have opened the app while the app was not in memory. But this method will be triggered only in the intervals of 15 mins. So even if the cell tower changes rapidly, this method will be invoked only once in 15 mins.
So if you want to update the location in background keeping in mind the battery usage as well, use significant location api.
Sometimes I get strange values for speed and course in didUpdateToLocation. This probably started when I changed the desiredAccuracy setting from kCLLocationAccuracyBest to kCLLocationAccuracyBestForNavigation. The course could drift up to 90 degrees. Another problem is that the speed sometimes is set to 0 if the device is perfectly still(even if the boat/car is moving). I don't know if the problems are related or not.
I can confirm this behavior on several devices both ipad2 and ipad3.
Has anyone experienced anything similar?
/Martin
I just tried an experiment with this, using a new iPhone with iOS 7.02. I could not replicate your issues. Specifically, I was using the delegate method didUpdateLocations,
- (void)locationManager:(CLLocationManager *)manager didUpdateLocations:(NSArray *)locations
since didUpdateToLocation has been deprecated since iOS6. I don't know that the locations array is any different than the newLocation returned in the deprecated method.
I tried it both walking and driving, and got good results. Yes, the course will bounce around, and is not as instantly accurate as the compass heading, but I didn't see it vary as much as 90 degrees while moving, nor did I see speed at zero.
If you have poor signal from GPS or even WiFi, or if you are moving then you can see that kind of behavior. Maybe this is not the sort of answer you were hoping for, but as it's been over a month, I thought I'd related my experience even though it does not confirm yours.
I could post a code snippet if that would be helpful.
I want to know the accuracy and the distance filter of the low-power significant change location service (i.e if I use startMonitoringSignificantLocationChanges how much it's accurate, and what is the distance of the significant change)?
I need some experimental (non documentary) info from real time apps
I had a chance to speak with the Apple Location Engineers at WWDC this past year and this is how it was explained to me.
The significant location change is the least accurate of all the location monitoring types. It only gets its updates when there is a cell tower transition or change. This can mean a varying level of accuracy and updates based on where the user is. City area, more updates with more towers. Out of town, interstate, fewer towers and changes.
This is also the hardest location type to test for since you can't use the simulator either. I'm not sure if they have fixed it to work with the GPX files for 6.0, but the significant location change api did not work at all in the simulator prior to iOS 6.
I have tried to avoid using the signification location change for many of these reasons. Sometimes it can't be helped. I ended up using the region monitoring API's as they are far more accurate and just as good on battery life. Hope this helps.
From the Apple documentation:
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.
There doesn't appear to be much more specific information available about the exact accuracy, so I would assume you have accuracy roughly equivalent to the approximate distance between cell towers in the area that the iOS device is currently located in (which is shorter in more highly populated areas).
I had to build an app back then that uses cell tower significant location changes.
Short answer: very inaccurate.
I was clearly crossing the boundaries of my region.
From what we observe in our app, it can be a few hundred metres to a few kilometres off. Our testing was in the city area, cell towers in suburbs parellel to the train tracks and other suburban cell towers.
Pretty rough.
It was consistent most of the time. I notice that every time I was about to go into the tunnel to the underground train station, it would fire off my 3 region crossing notifications that I have setup for the CBD city area.
I'm using Xcode 4.6.2, and you can indeed simulate significant location change on this simulator.
In the iOS Simulator, the menu entries you need are Debug->Location->Freeway Drive.
Caveats (I welcome being told I'm wrong):
1. After a long while, there seem to be no more significant location change events.
2. You can only drive a pre-defined route in the general Cupertino/SF area. If all you care about is significant location change, that's fine.
Be careful, although you can access the speed property of the location got from the significant location update, it's useless! the simulator actually gives the speed but in real devices the speed is not available because location got from cell towers will not include the actual speed(unlike GPS). more the that as said before the location itself is very inaccurate it can be a few km off.
Be aware of that.
The only way to get the speed is have two cllocation and compute the speed manually