What causes GPS devices to give inaccurate directions?

Perhaps you've heard about or experienced the strange incidents where people drive into lakes or face other dangers because their GPS directed them there, or maybe you've had a confusing route that took you the wrong way or onto a closed road.

It is projected that around 560 million GPS devices will be used worldwide by 2012, compared to 175 million in 2007 [source: Meyer]. While many rely on these devices to guide them, sometimes following these directions can be risky. GPS systems are not always accurate. To understand where things go wrong, it's essential to first understand how they work. In simple terms, GPS receivers use signals from a network of satellites and ground stations to determine your location and destination. This data is then mapped onto the maps saved in your device.

Regardless of how high-end a GPS device may be, its performance in getting you to your destination ultimately depends on the satellite network and the quality of the map data. The first issue with GPS is that satellite signals are not 100 percent precise — for example, they can't pinpoint your exact location. However, they are generally accurate within 164 to 328 feet (50 to 100 meters) [source: Corvallis Microtechnology Inc.].

The second issue is that maps tend to become outdated quickly. The routes your GPS generates are based on digital maps from a navigation company that collaborates with the device manufacturer. Without the latest maps, your GPS may fail you. Let's explore in detail the reasons why your GPS might direct you down the wrong path, from mapping errors to faulty satellite signals.

GPS Driving Directions: Led Astray by GPS

You probably wouldn't rely on an old map you found in your car’s trunk to plan the best route, so why haven't you updated the maps on your GPS device? You won't get from Point A to Point B with outdated or inaccurate information.

Every year, up to 40 percent of roads can change — including new roads, closed roads, lane modifications, and more [source: Tele Atlas]. Consider the construction happening in your own neighborhood — roads may be closed for months or years, only to open unexpectedly. These updates will eventually make it into the next digital map version, but the process isn't immediate. The major mapping companies gather data using on-the-ground technology, such as employees driving around collecting information, user feedback, and other resources to update maps reflecting the current road systems worldwide. According to a study by the Navigation Systems Research Foundation, some crucial details, like road types, can be missed by GPS systems. This becomes problematic when your bus is accidentally routed through a residential area or your car is sent down an unpaved road.

While map data may contribute to inaccuracies, the main issue lies with outdated data from users. Many handheld or standalone GPS devices can be updated for free by connecting the device or its removable media card to a computer and downloading updated maps from the provider. Regular updates ensure that your GPS device has the most current routing information available.

Even with the latest map data, your GPS device is still dependent on its satellite network. Accuracy problems can arise due to various factors, both atmospheric and terrestrial. When a satellite cannot transmit its position — a situation known as an ephemeris or orbital error — it cannot establish a link with your GPS device. Changes in atmospheric conditions, particularly in the ionosphere and troposphere, including variations in plasma activity, temperature, pressure, and humidity, can all lead to calculation and accuracy errors within the satellite network.

Poor satellite signals and signal interference are common issues, often occurring when something obstructs the line of sight between your GPS device and the satellite network. Without a strong, clear signal, your device cannot pinpoint your location accurately. Tall buildings, dense trees, mountains, and even reflective surfaces can cause these disruptions.

To ensure you always reach your destination, update your mapping software regularly and keep a physical map handy.