A smartphone normally determines its geographic position using a combination of several technologies:

• GPS/GNSS satellites: The phone receives signals from multiple satellites (such as GPS, Galileo, GLONASS and BeiDou). By measuring how long the signals take to arrive, it calculates its position, typically to within a few metres outdoors.
• Mobile phone masts: It can estimate its position from the nearby cellular towers. This is less precise than satellite positioning but works indoors and when satellite signals are poor.
• Wi-Fi networks: The phone can compare nearby Wi-Fi networks with databases of known Wi-Fi locations, often providing surprisingly accurate positioning in built-up areas.
• Bluetooth: Nearby Bluetooth beacons can provide additional location information, particularly indoors.
• Sensors: Accelerometers, gyroscopes and other sensors help the phone estimate movement and refine its position between other location fixes.

The phone’s operating system normally combines these sources, selecting or weighting them according to availability and accuracy. Consequently, it can often determine one’s approximate location even when GPS itself is unavailable.

In urban areas a smartphone can establish its position with very considerable accuracy. Outside urban areas – and especially in remote areas of countryside – smartphones tend to deliver poor accuracy as they have to rely almost exclusively on satellite and cell ID data. This is an issue because smartphone aerials are not optimised to receive satellite signals, and cell ID data are not much use unless the phone can receive signals from more than one cell. It is in these circumstances that dedicated GPS receivers come into their own.

Further issues to mull over are as follows:

1. Unlike dedicated GPS units, smartphones are generally not durable or weather resistant.
2. Battery life is severely compromised when using a smartphone as a GPS. Smartphones do not have removable batteries so a USB battery pack is required for long trips. Dedicated GPS units have lower battery drain and replaceable/rechargeable batteries, and can accept readily available primary cells in an emergency.
3. Smartphones are touch screen devices so in very cold weather one has to remove one’s outer glove to operate them. Touchscreen gloves are of course available but these are generally lightweight and unsuitable for very low temperatures or where there is substantial wind chill. Most (though certainly not all) GPS units have conventional buttons to control their features.

As a matter of interest, Garmin claim that their receivers are positionally accurate to within 15 meters (49 feet) 95% of the time and that users will see accuracy within 5 to 10 meters (16 to 33 feet) under normal conditions. As regards elevation, Garmin claim that a calibrated GPS relying on satellite data alone will deliver accuracy of +/- 122m (400 feet) given a strong GPS signal. Those devices fitted with a barometric altimeter should, with proper calibration, report elevation readings ranging from minus 600 to plus 9000m (roughly minus 2,000 to plus 30,000 feet) to within +/- 15m (+/- 50 feet).