Single GPS reading help

mouser58907 · 02-01-2012, 07:45 PM

In my app I only need a single GPS reading to get their location within a few 100m or so. My solution to this was to start the standard updates, and let the GPS run for 5 seconds and terminate it with a timer.

I recently found there is a flaw in this method though. During the first run, it passes the test for isLocationServicesEnabled, then prompts the user asking if they want to allow GPS. I handled the case if the user says no fine, but if they wait longer than 5 seconds and hit yes, my GPS is already disabled and my code will never get called.

Is there a better way to organize my code?

Thanks

Duncan C · 02-01-2012, 07:54 PM

Quote:

Originally Posted by mouser58907

In my app I only need a single GPS reading to get their location within a few 100m or so. My solution to this was to start the standard updates, and let the GPS run for 5 seconds and terminate it with a timer.

I recently found there is a flaw in this method though. During the first run, it passes the test for isLocationServicesEnabled, then prompts the user asking if they want to allow GPS. I handled the case if the user says no fine, but if they wait longer than 5 seconds and hit yes, my GPS is already disabled and my code will never get called.

Is there a better way to organize my code?

Thanks

Getting a reasonable GPS reading is frankly a pain in the a$$.

Here's what you have to do:

Create a location manager and ask it to begin updates.

In the delegate method for the location manager, first check the date stamp of the locations it gives you. If they are more than a couple of seconds older than now ( [NSDate date] ), reject them. You have to do that because location services caches very stale GPS readings, and will give you a reading that could be HOURS old when you first start location updates.

Then check the horizontal accuracy reading. If it is negative, reject it (that indicates an invalid reading.)

Then make sure the accuracy reading is <= 100 meters. If the value is > 100 meters (or whatever your desired accuracy) reject it.

If you get through all those tests, stop location updates.

Bear in mind that it frequently takes 15, 30, or more seconds to get a reading that's accurate to 100 meters. The GPS in iOS devices sucks, quite frankly.

mouser58907 · 02-01-2012, 08:04 PM

Thanks, Duncan. This seems like a much cleaner solution. I appreciate your help.

RickSDK · 02-01-2012, 08:23 PM

Maybe i'm just getting lucky but my app finds the current location in less than a 10th of a second with accuracy set to 100 meters :/

Duncan C · 02-01-2012, 09:19 PM

Quote:

Originally Posted by RickSDK

Maybe i'm just getting lucky but my app finds the current location in less than a 10th of a second with accuracy set to 100 meters :/

You can set the desired accuracy to 100 meters, but that doesn't mean that's what you get. Check the things I outlined. I bet the reading you get within 1/10 second is the cached reading from the last time the GPS was active. It can be off by multiple kilometers.

I've spent weeks testing this stuff, and there is no way you are getting an accurate reading in 1/10 second. Not unless you're using a 4 or 4s and Apple has completely replaced the GPS hardware in those devices and is using a much higher grade unit (which I seriously doubt).

Try downloading the iPhone version of our geo-location app, "Save My Place" (it's only 99 cents) and watch the location and accuracy readings it displays. It shows the accuracy reported from the GPS as new readings come in. It will display the accuracy in feet or meters, or with a qualitative reading (excellent, good, fair, or poor).

Alternately, log the age and accuracy of the GPS readings you get.

Add this code to your locationManager:didUpdateToLocation:fromLocation: delegate method:

Code:

NSTimeInterval readingAge = [NSDate timeIntervalSinceReferenceDate] - 
  [[newLocation.timestamp timeIntervalSinceReferenceDate];
CLLocationAccuracy horizontalAccuracy = newlocation.horizontalAccuracy;
NSLog(@"GPS reading is %.2f seconds old, accuracy = %.2f", readingAge, horizontalAccuracy);

RickSDK · 02-01-2012, 10:19 PM

my phone is an iphone 4

Duncan C · 02-02-2012, 06:36 AM

Quote:

Originally Posted by RickSDK

my phone is an iphone 4

Like I said, I bet you a bottle of good single-malt scotch that you're not getting an accurate reading in 1/10 second after waking up the GPS. I bet you're getting an out-of-date reading that was cached. Either that, or the GPS is already running (like if you are showing the blue dot on the map.)

RickSDK · 02-02-2012, 03:28 PM

hmmm... the location info seems to be pretty accurate.

In fact if you open your generic iphone map app it will pinpoint your exact location (with a blue dot) in less than a second, usually accurate to within about 5 ft. So i'm not sure why an app using the same api's would take up to 30 seconds to get the same info.

maybe your phone is broke :/

Duncan C · 02-02-2012, 03:40 PM

Quote:

Originally Posted by RickSDK

hmmm... the location info seems to be pretty accurate.

In fact if you open your generic iphone map app it will pinpoint your exact location (with a blue dot) in less than a second, usually accurate to within about 5 ft. So i'm not sure why an app using the same api's would take up to 30 seconds to get the same info.

maybe your phone is broke :/

Do me a favor. Add the code I posted to your app, and then run it and watch what happens.

I bet you get an old reading to start with, and then readings with a low accuracy for quite a few seconds before the accuracy improves.

Another test:

Turn on your phone and open the maps app. Tap the "show my location" button and wait for the blue dot to show up. Zoom WAY, WAY in in the map, to the point where its almost as far as it will zoom in.

Press the home button to exit maps and turn your phone off.

Now, the next time you travel someplace in your car, fire up your phone and re-open the maps app. Watch carefully to see what the blue dot does.

apatsufas · 02-02-2012, 04:15 PM

Quote:

Originally Posted by RickSDK

hmmm... the location info seems to be pretty accurate.

In fact if you open your generic iphone map app it will pinpoint your exact location (with a blue dot) in less than a second, usually accurate to within about 5 ft. So i'm not sure why an app using the same api's would take up to 30 seconds to get the same info.

maybe your phone is broke :/

The iPhone uses besides GPS readings, information from your wifi and cellular connections (this is called A-GPS or Assisted GPS).

A-GPS uses wifi and cellular network information to get fast and accurate information about you position.

The problem is that there are conditions in wich A-GPS won't work and then you'll have to rely on the Standalone GPS which can be quite slow even up to a few minutes depending on conditions.

If you are travelling fast or if you don't have wifi and you don't have a strong cellular signal A-GPS won't work.

That's why you can't depend on it and you always should assume that you are using the Standalone GPS.

sneaky · 02-02-2012, 04:23 PM

From my own testing, and I have done a fair bit, I will have to go with Duncan on this one.

Sure, you'll get locations immediately as soon as you ask for them but they are no good. They are too old and too inaccurate. If you get an accurate hit that soon it means that the gps was running recently already. Such as you sitting at your desk testing the app without actually moving.

Gps apps need to be road tested. Build a debug viewcontroler for your app and spend a few hours driving around and collecting data. Stop the gps and open the app again 20 miles down the road, inside a building, you will not get a location in just a few sec, it will take a while.

Duncan C · 02-02-2012, 04:46 PM

Quote:

Originally Posted by sneaky

From my own testing, and I have done a fair bit, I will have to go with Duncan on this one.

Sure, you'll get locations immediately as soon as you ask for them but they are no good. They are too old and too inaccurate. If you get an accurate hit that soon it means that the gps was running recently already. Such as you sitting at your desk testing the app without actually moving.

Gps apps need to be road tested. Build a debug viewcontroler for your app and spend a few hours driving around and collecting data. Stop the gps and open the app again 20 miles down the road, inside a building, you will not get a location in just a few sec, it will take a while.

What he said.

Although to be fair, inside a building is a very GPS-hostile environment.

Duncan C · 02-02-2012, 09:08 PM

Quote:

Originally Posted by Jimmy586

Maybe i'm just getting lucky but my app finds the current location in less than a 10th of a second with accuracy set to 100 meters :/
__________________

It gives you a location, but it is probably not giving you an accurate location. To repeat, just because you ask for an accuracy of 100 meters, that doesn't mean that's what you get. Have you checked the timestamp and the horizontal accuracy that you're getting back from the location manager? And have you tested it when you somewhere not in range of a known WiFi base station?

I've tested this extensively on a variety of devices, and you do not get sub 100 meter accuracy in a tenth of a second unless it's using WiFi hotspots. Cell triangulation is not ever accurate to 100 meters, and GPS is slow to get a fix. Sometimes very slow.