My phone’s GPS does a terrible job at telling if I’m on or under an elevated expressway. Why?


Something I noticed when driving around.

My phone’s GPS usually does a great job at telling what lane I’m on and how far I am to an intersection, but it does a terrible job at telling if I’m on or under an elevated expressway.

Why can it tell my horizontal position so much better than my vertical position?

The phenomenon is independent to the model and make of the phone as well. I’ve switched from Android to Apple, and both phones seem to be terrible at telling if I’m on an elevated expressway or on the road under it.

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The GPS feature establishes your position in three dimensions. Often with less accuracy when it comes to height.

But the problem is often that the map program doesn’t care if you are on or under a bridge. Or that the map is poorly digitalised. Or that the map program sucks at deciding which level you were on when you entered the “intersection”.

In addition to the map/nav app itself being confused, as other comments have mentioned, sometimes physical structures can interfere with the GPS signals as well. If you’re under a concrete expressway, the GPS signals might be weak and the accuracy goes down— the signals are coming from satellites in space after all.

GPS works by listening to the signal of many satellites that broadcast their own very exact time and position over radio frequency. Since the the exact position and time of each satellite is known, trigonometry can be used to calculate the GPS receiver’s exact position. The delay of the signals as they arrive through the air will tell how far away each satellite is, as the farther the distance from a satellite, the longer the delay will be.

This works great as long as the radio signal travels in a straight line, but as soon as the receiver is close to large structures such as buildings, bridges og overpasses, where some or all of the radio waves will bounce, the wave will travel further than the actual distance to reach the receiver, disturbing the calculation.

GPS software compensates for these inaccuracies by using additional information to determine where you are, such as the direction you are travelling and if there are any roads in that direction nearby it can “snap” to the nearest road.

This breaks down when multiple roads are crossing, or you enter a tunnel where the signal is weaker/very bouncy or maybe you don’t move fast enough for the software to confidently know your direction.

* GPS is the system of satellites that send their name and the time to you from space.
* Your phone has a *navigation system* and that’s what has a hard time telling if you’re on an elevated expressway.
* That navigation system uses GPS signals in addition to other things to help figure out where you are.
* As other’s have said, sometimes the GPS signal gets weak and the system tries to rely on other options for your location.
* For this reason there is a little “wiggle room” in the location when the GPS signal is weak and that can lead the navigation system to get a little confused.