iPhone LocationHow apps access the iPhone location and

what they can do with it

Outline

1. How an App Accesses iPhone Location

2. Location Information Provided

3. Alternatives to Receiving Location Updates

4. Working with Location

5. How iPhone Determines its Location

1. How an App Accesses iPhone

Location

● An iPhone app never calculates the phone’s location directly

● The operating system running on the phone determines the phone’s

location and any app that wants to use this information requests it from the

phone

● The usual way for an app to get the phone’s location is for the app to

receive periodic updates from the phone with the phone’s current location

● All apps running on a phone receive the same location updates

Listening to location updates

● When an app wants to start using location information, the app requests

that the phone begin sending location updates, and the app specifies the

level of accuracy that it needs

● The options are 3 km, 1 km, 100 meters, 10 meters, “Best”, or “Best for

Navigation”

● Requesting an appropriate level of accuracy is all about minimizing battery

drain

Requesting Accuracy Levels

● At a low of accuracy (e.g. 3 km), the phone could determine its location

solely from cellular or wifi signals, which would avoid powering up the GPS

and conserve battery life

● At higher levels of accuracy (e.g. 10 m), the phone listens to all available

signals that it can use to determine its location -- which includes GPS and

iBeacons, in addition to wifi and cellular

● Apple does not say specifically say which signals are used at which levels

of accuracy

o It could depend on how good the signals are at the time the app is running

Battery Drain

Best Level of Accuracy

● An app can force the use of GPS by specifying “Best” -- this will cause the

phone to use GPS constantly to determine location as accurately as

possible

● Requesting “Best for Navigation” goes one step further and causes the

phone to use GPS at its highest level of accuracy, plus any additional

hardware that could help

o Apple doesn’t say what additional hardware is used, but this could mean using something

like the accelerometer to detect when the phone has moved just a few feet to trigger a

location update

o The accelerometer is a sensor that detects acceleration of the phone in any direction. It is

very sensitive and will pick up even the slightest movement of the phone

Location Updates

● When the phone receives a request for location updates from an app, it

powers up any necessary hardware and starts calculating its location

based on the various signals

● From then on, the phone updates the app with a new location every time

the phone moves, until the app tells the phone it doesn’t need location

information any more

o How far the phone needs to move depends on the level of accuracy requested

● When no apps are requesting location updates, the phone shuts down the

GPS

● The first time a newly-installed app asks the phone for location updates,

the phone will ask the user to confirm that the app can access the user’s

location

● If the user says no, the app will not be able to receive location updates

from the phone

● The app will also be cut off from location updates if the user later changes

his iPhone settings to revoke the app’s access to location data or turn off

Location Services entirely

User Permissions

2. Location Information Provided

More than Just Location

● When the phone sends a location update to an app, it provides some

related information along with the phone’s current location

● Depending on the status of the various signals being received by the

phone, information provided can include:

o Estimated accuracy of the location

o Elevation, with an estimated accuracy

o Speed

o Course (direction of travel)

o Heading (direction the phone is pointing; actually comes in a separate update)

● The phone reports the phone’s location as decimal latitude and longitude

coordinates

● The phone also reports an estimated horizontal accuracy of the location in

meters

o The first few locations reported to an app may not have accuracy associated with them

because the GPS is still locking on to satellites

o Once it starts being reported, accuracy typically starts off relatively inaccurate (30 meters

or more), and gets better once the phone has a GPS lock

Location Accuracy

Location Accuracy

● When the phone has a good GPS

signal, reported horizontal accuracy is

typically 5 or 10 meters

o In practice, accuracy is often better than

this -- the error is usually less than 5

meters

● When viewing a map that shows the

user’s location as a blue dot, the outer

disc shown in light blue is the phone’s

estimated accuracy of the current

location

● When the phone has a lock on 4 satellites, it can determine its elevation

o With 3 satellites, it can only determine location

● Elevation is reported in meters, along with an estimated accuracy

● Vertical accuracy tends to be in the 3-6 meter range

● Despite being nominally in the same sub-10 meter range as location

accuracy, elevation accuracy is effectively less accurate because elevation

change for most applications is much smaller than horizontal distance

traveled

Elevation

● For example, here is an example bike ride recorded with a mobile app:

● Note that the horizontal distance is measured in miles while the vertical

distance is measured in feet -- so a 5-meter error in vertical measurement

would be more significant than the same error in horizontal measurement

Elevation Accuracy

● Speed is reported by the phone in meters per second for each location

reported

o The first few locations provided by the phone may not have speed associated with them

● Speed is not simply the distance / time from one location to the next -- it is

determined by the phone itself using a process Apple does not reveal

o It represents the phone’s speed at the moment the location is provided to an app, not the

average speed since the last location update

o The phone probably calculates this speed by combining GPS location with some other

information such as accelerometer data or an analysis of the GPS signal itself

Speed

Course & Heading

● The phone reports course with every location update, which is simply the

direction from one reported location to the next

o This would be pretty easy for an app to calculate, but the phone provides it with location

updates so the app doesn’t have to do the extra work

● An app can also access the phone’s heading, which is the direction the

phone is pointing at that moment

o Heading is provided in a separate update because it is based on different hardware than

location (it uses the phone’s magnetometer, which detects the orientation of the phone

relative to the earth in 3 dimensions by sensing magnetic fields like a compass)

o Heading is not very accurate because it can be subject to interference from external

magnetic fields, and possibly even the device itself

3. Alternatives to Receiving

Location Updates

● As an alternative to receiving updates every time the phone moves, an app

can request that it receive a location update only when the phone’s

location has changed “significantly”

● This will avoid powering up the GPS and will rely on cell towers and wifi

signals only -- resulting in substantially reduced power consumption

● “Significant” is not defined

o In a city with a lot of wifi networks, it could detect a change over a relatively short distance,

such as 100 meters

o Outside a city when relying on cell towers only, it might not trigger an update unless the

phone has moved 1 km or more

Significant Location Changes Only

● The usual method where an app receives an update every time the phone

moves only works while the app is still running (in the foreground or

background)

● Significant location change notifications can continue even after an app

has stopped running -- the phone will restart the app and let it run long

enough to process the location update

Location Updates When Not

Running

● Another alternative is region monitoring, where an app defines one or more

regions by specifying their center and radius

● Then the phone notifies the app whenever it enters or exits a region

● This uses wifi and cell towers only, so accuracy is similar to “significant

changes only” and regions will only work if they are fairly large

Region Monitoring

● iBeacons are small wireless sensors that can transmit data to an iPhone

using Bluetooth Low Energy (BLE)

● iBeacons allow apps to receive special promotions, coupons,

recommendations, etc. from businesses in real time when users are

nearby

● An app can display the information received from the business, or it can

make use of the knowledge that the iBeacon is nearby for navigational

purposes as a form of region monitoring

iBeacons

4. Working with Location

● When tracking the user’s path with high level of accuracy (e.g. to see how

far the user walked), any inaccuracies in location measurement will almost

always cause the path to be longer than it should:

● This is a running activity where the path recorded by the phone is shown in

red -- the user ran straight down the sidewalk, but clearly the red line is

longer

Inaccuracy Makes Distances Long

● To counteract the extra distance problem and provide a smoother path,

iPhone will “snap” location updates to a known road network when the user

is traveling above about 15 mph (on the assumption that the user is driving

or biking during that time)

● Apps have no way of knowing when this is happening -- all they see are

location updates like normal, but when the locations are later projected on

a map, it is clear when this was happening

“Snapping” to Roads

● In this example of a bike ride recorded with iPhone, you can clearly see

smooth portions of the path where the user was moving faster than the

threshold speed and jagged ones where he was moving slower

Snapped Path

● GPS requires line-of-sight communication, so anything that blocks a direct

path from the phone to a satellite can interfere with the signal

o This includes trees, mountains, buildings, car roofs, etc

● When a direct GPS signal is blocked and the user is near a large solid

object like a building, the GPS signal can arrive at the phone indirectly

after bouncing off the building

● Because GPS works by measuring very precisely how long the signal

takes to get from the satellite to the phone, when the signal takes a longer

path like this, the calculated location will be off

GPS Signal Quality

● Here, the path recorded by the

phone is quite poor where the

user was running among the

buildings, but becomes much

better where the user was getting

a better signal by the water

GPS Signal Quality

● The path recorded by the phone

can be shorter than the actual

path traveled if the phone loses

signal

● In this example, the user was

running laps around the lake and

lost signal during one of them --

causing the path to cut straight

across to the point where the

phone regained signal

Losing GPS Makes Distance Short

● It is difficult to overcome these limitations on the phone while the app is

running

● However, if the app is recording a user’s path and saving it to a server

(e.g. the way a fitness tracking app would), the server can make some

corrections after upload, such as

o Snapping to roads or other known locations

o Smoothing out jagged parts of the path

o Replacing the inaccurate elevation reported by the phone with known elevation at that

location

Post-Processing

● iPhone makes displaying maps in apps really easy

● Many common functions are built-in and require very little from the app

developer, including:

o Displaying user location on the map

o Displaying annotations and overlays on the map that zoom and scale with the map

o Centering the map on the user’s location

● Geocoding (getting latitude/longitude coordinates from an address) and

reverse geocoding (getting an address from latitude/longitude coordinates)

is also built-in and quite easy to use

Map Displays

5. How iPhone Determines its

Location

● iPhone can use 4 different types of signals to determine its location

o Cellular

o Wifi

o GPS

o Bluetooth (from iBeacons)

● The phone’s location is determined by combining one or more of these

signals

o Which signals are used depends on availability and the requested accuracy

● All of this happens behind the scenes -- any app can access the location of

the phone, but the app does not know how it was calculated

Four Types of Signals

● The location of cell towers is known to

a high degree of accuracy. Based on

the signal strength from various

towers, the phone can estimate how

far it is from each of them and

calculate the phone’s location

● This is always available when the

phone has cell coverage, but is not

very accurate since the towers can be

far away and distance based on

signal strength is not very accurate

Cellular

● The phone uses the same process to determine location using wifi signals

as it does for cellular signals, with distance estimated to the wifi hotspots

based on signal strength

● The locations of wifi hotspots have been crowdsourced for this purpose

● Using wifi is more accurate than cell towers, but the user needs to have

wifi turned on

Wifi

● When the phone detects a Bluetooth Low Energy signal from an iBeacon,

in addition to knowing that an iBeacon is nearby, the phone can use the

relative signal strength from multiple iBeacons to determine its location

using the same technique that it uses for cellular and wifi signals

● iBeacon signals only travel about 50 feet, so this location would be fairly

accurate

● This is particularly useful indoors, where GPS reception is poor

iBeacons

GPS

● GPS is the most accurate of the 4 signals, but it is slow to get started

● The phone needs to “lock on” to 3-4 satellites within its line of sight before

it can use GPS to determine location

o This can take anywhere from 15 seconds to several minutes, depending on the phone’s

view of the sky

o 3 satellites can provide location; a fourth provides elevation

o The phone uses its location calculated from other sources (e.g. cellular and wifi) to

determine which satellites are within its line of sight at the current time, speeding up this

process (this is called Assisted GPS, or A-GPS)

● Before the phone has a GPS lock, location is less accurate