Why do GPS systems give wrong directions?

GPS Driving Directions: Led Astray by GPS


You probably wouldn't trust an old map you unearthed from the trunk of your car to help plot the most accurate route, so why haven't you updated the maps on your GPS device? You'll never get from Point A to Point B with inaccurate or outdated information.

Each year, you can expect roads to change as much as 40 percent -- that's new roads, closed roads, lane changes, you name it [source: Tele Atlas]. Think about new developments in your own town -- construction may go on for months or years until one day the roads suddenly open. Eventually those new roads will make it into the next version of digital mapping software, but it isn't instantaneous. How does it happen? The major mapping and navigation companies use a combination of on-the-ground technology (employees driving around and collecting data), user feedback and a variety of other sources to build maps that reflect the reality of the road systems around the world. In a study by the Navigations Systems Research Foundation, it was discovered that some important information is sometimes overlooked by GPS systems, such as types of roads. This is important information to have if your bus has accidentally been routed through a residential neighborhood or your car has been detoured to an unpaved road.

While the map data is at fault for inaccuracies, users are usually at fault for outdated data. Many handheld or stand-alone devices can be updated for free by connecting them or the removable media card directly to a computer and downloading new, updated maps from the digital map provider. With frequent downloads, your GPS device will have the most current routing information.

Even with the most up-to-date mapping and navigation software, your GPS device is still at the mercy of its satellite network. Accuracy problems can arise from a variety of conditions, from atmospheric to terrestrial. When a satellite isn't able to transmit its position, a situation called an ephemeris or orbital error, it isn't able to establish a link with your GPS device. Atmospheric conditions, specifically in the ionosphere and troposphere, including variations in plasma activity, temperature, pressure and humidity, can cause calculation and accuracy errors in the satellite network.

Bad satellite signals and signal interference are some of the most common glitches and happen when something gets in the line of sight between your GPS device and the satellite network. Without a clear and strong signal, your device can't accurately establish your location. Tall buildings, dense foliage, mountains and even reflective objects can cause such a problem.

To be sure you always get from Point A to Point B, update your mapping software often and always bring a map.

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