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To get the best use out of a GPS, you need to understand both its capabilities and its limitations.

In the third part of this series, we look at some of the weaknesses and limitations of a GPS unit, and include a bonus section on another use for your GPS.

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A Beginner's Guide to Using GPS Part 3

Errors, Inaccuracies, POIs and Speed

If the GPS appears to be telling you to turn left at the rail line, don't automatically accept its advice!

As amazingly accurate as GPS receivers can be, they still can make mistakes.

Part 3 of a 3 part introduction to GPS, as part of our broader series on GPS - see links to additional articles in the series on the right.



GPS technology is truly amazing and close to magic, but there are still limitations on what it does and how it does it.

It is only when you appreciate the limitations as well as the capabilities of GPS that you'll be able to get reliable best use from your unit.

There are too many stories of drivers who have blindly trusted the information on their GPS screen, ignoring the conflicting real world information on the road.  Never do this.  Use common sense and understand that if there is any doubt, what you see outside the car is of course more correct than what the GPS is telling you!

Points of Interest

Most GPS units come complete with an extensive range of 'Points of Interest'.  These Points of Interest (POI) can include the names and addresses and phone numbers of restaurants, gas stations, hotels, stores, and all sorts of other places and things.  It is common to find, in US GPS units, information on as many as 5 - 10 million different points of interest.

So if, for example, you're driving cross-country and want to take a break and have a meal, you can either have the unit tell you about nearby restaurants, or you can search for a restaurant by name (eg McDonalds) and you can even search for restaurants by style of cuisine.

Falling asleep at the wheel, or wanting to find a nearby Wi-Fi hotspot?  Search for the nearest Starbucks.

Wanting to go to a nearby park or playground?  Type in its name and the unit will probably find it for you.

This is all wonderful, but the reality is not as good as the promise.  Even in a unit with 10 million points of interest, the information it offers you is likely to be very very incomplete.  And because things such as restaurants or new Starbucks locations or whatever change regularly, the information ages much more quickly than the basic map information, and becomes less and less accurate and helpful with each passing month.

It can be very disappointing to make a detour off your route to go to a restaurant you've selected from your GPS' POI list only to find it has closed, or has changed and now offers different cuisine.

Treat POI data with suspicion.  If there's a phone number associated with a POI, call it before going there to confirm it still exists.  And remember that the information you are presented with is almost certainly incomplete.

Getting updated POI information is as much a reason for updating your map data as is getting the newer map information.

Location Accuracy, Errors and Ambiguities

In theory, GPS systems can be accurate to about 10' or so.  In practice, this is never achieved, because of real world imperfections, and accuracy is seldom better than 40' (although randomly it will sometimes be exact to the inch, and other times it will be even more than 40' out).

When you think about it, computing your location to within 10', using satellites that are 'floating' in space and in no fixed location, and at least 11,000 (and possibly up to 20,000 miles) away from you is staggering.  A 10' accuracy means that the position is accurate to +/- 0.00002%.

The hardest part of all to understand is how the position of the satellites, themselves, can be accurately fixed as reference points.  If the system can indeed work out your position to within 10', it must know where the satellites are to an even greater degree of precision.  Truly, GPS is close to magic.

But it isn't magic, and sometimes it doesn't work as well as you'd expect.  And the first thing to accept is that the claimed level of accuracy is seldom if ever achieved, and even units which display what they believe to be the accuracy of their location estimate are only telling you about the errors in their calculation which they know about, not about errors which they don't know about.

Although GPS units typically show your location by a single dot (or arrow or picture of a car or whatever) they really should show your location by means of a circle, with the meaning that your location is somewhere within that circle, but it can't be exactly sure where.  Years ago, GPS units would optionally display that circle, but these days the feature is seldom if ever found.  But you need to keep in your mind that although the GPS seems to show exactly where you are, it is merely showing you a possible location and there's a circle around that dot, and you could be anywhere within that circle.

To make matters worse, some of the 'clever' extra things found in most GPS units these days can actually make the unit less accurate rather than more accurate.

Snap to Road errors

Most GPS units today are programmed to 'snap to the road'.  This means that when the GPS calculates its location as being very close to a road - when the road is within its likely circle of error - it assumes you are on the road and shows its dot locator on the road, even if the road is right on the very edge of the circle of error.

Normally, this is a sensible and correct decision for the GPS unit to make.  But sometimes it will get confused - typically when there are two roads close to each other and going in the same direction, and when the accuracy is lower than normal.

This type of error can often happen in the downtown of big cities.  Just like the high-rise buildings can interfere with the quality of the music signal on your car radio, so too can they interfere with the reception of the satellite signals.  Multi-pathing, ghosting, echoes, and interference all cause problems, and quite apart from that, if you have tall buildings on both sides of the road, you can't see as much of the sky so the unit can't see as many satellites, they are not as spread out, and the calculation it makes is less accurate to start with.

So sometimes, in cases like this, the unit computes your position halfway between two streets and makes the wrong guess as to which street you are on.  You are actually on 5th St, but the unit thinks you are on 6th St.

This can also happen when you turn corners.  You might be driving along High Street and then turn left into 12th Ave, but the unit decides, wrongly, you actually turned left into 11th Ave (or 13th Ave).

Even in a suburb with a good view of the sky, you might find the unit sometimes confusing a street with a service lane, or a parking lot with the street next to it.

Which turning to take errors

Maybe you're approaching an intersection with multiple roads leading away from it, or perhaps a roundabout or traffic circle, or maybe a freeway exit with several collector/distributor lanes branching off.

Errors in the GPS unit's position calculation, and delays in its updating its data can mean that you end up choosing the wrong turn.  In particular, at 65 mph, you're covering 100 ft every second, and so if the GPS unit is 50' off in its calculation, and one second slow in updating your current position, it could be showing you 150' away from where you actually are.

Or, if you're traveling more slowly, and the GPS error is moving you ahead of where you are, the unit might show you further ahead rather than further behind.

Snapping to Roads - another problem

Here's an interesting problem.  The unit not only assumes you are driving on a nearby road, but it also assumes you are driving on the correct road for your route, as well.

So if you take the wrong turning, the unit will continue assuming you are on the right road until the difference in location between where you are and where it thinks you should be is too big for it to ignore.

So if you're not certain if you're making the right turn, wait a while for the unit to over-ride any assumptions it is making that 'of course' you did the right thing.

Routing errors

These are fairly obvious when they occur.  If you're on a new road or a road that has been diverted, and which the GPS map data doesn't know about, you're going to terribly confuse the unit.

Depending on the nature of the road, you'll either have to just generally drive in the right direction and hope that the GPS data will synch up again with the actual roads, or perhaps ignore the new better route and stick to the route in the GPS.

Traffic Data

Some of the higher end GPS units supplement their basic map information and assumptions about average traffic speeds with semi-realtime information about actual traffic information.  These are discussed in more details in our article on GPS data services.

For the purpose of this article, the key thing to appreciate is that this traffic information is not necessarily as accurate as you might assume it to be.  I've regularly had GPS units tell me that there are traffic delays ahead, only to find the road wide open and traffic flowing smoothly, and I've also come across slowdowns which the GPS has known nothing about.

Some GPS units will even calculate the number of minutes of traffic delay that you'll encounter, and will also offer to (or maybe even will automatically) re-route you to avoid traffic delays.  Treat this information with a great deal of skepticism, for two reasons.

The first reason is that the GPS does not know traffic details on every piece of road.  It probably might know about traffic on some freeways, but probably won't know about traffic conditions on the surface streets, and these conditions may be as bad or worse than on the freeway.  Secondly, remember that GPS units tend to be overly optimistic about how fast you can travel on surface streets at any time of day, let alone at rush hour.

If you're traveling on a freeway at peak rush-hour times and the GPS suggests you take a surface street route instead, keep in mind that the surface streets get congested too at peak travel times.  You may find that the GPS recommended 'better' route is actually worse than if you'd stayed on the freeway.

The traffic data can sometimes also be somewhat out of date.  The data signals sent to the GPS unit are very slow and it can take considerable time between each complete refresh of data that is sent to the unit.

And also, remember that the traffic data you are seeing on the GPS for the freeway ten miles ahead of you not only might be out of date right now, but by the time you get to the freeway, will be even more out of date.  If you're going into the rush hour time of day, with building volumes, each extra minute will see traffic getting worse, and if you're going out of the peak travel time, then hopefully each extra minute will see traffic clearing.

So far, we've never seen a good set of reliable traffic data on any GPS unit.  As promising as this concept is, it remains flawed in execution on all units, and you should treat the traffic data with great caution before basing a routing decision that 'feels wrong' to you on the recommendations from your GPS traffic service.

GPS Bonus - Calibrate Your Speedometer

The speedometer in most cars is not very accurate, and may be either under or over stating your speed, and may be doing this by different amounts at different speeds.  The wear on your tires can also make the speedometer's accuracy vary over time.

On the other hand, a GPS will calculate your speed to within about a quarter of a mile per hour (best case is about 0.1 mph accuracy, worst case scenario is about 0.5 mph accuracy).

Note that the GPS speed reading is most accurate if you are driving at a steady speed, in a straight line, and with a good sky view with plenty of satellites locked on.

Compare the speed shown on your speedometer with that shown by the GPS.  You might be surprised by the difference.  And, if there is a difference, trust the GPS speed over your speedometer.  The only time when your GPS speed may be less reliable is when you're rapidly changing speeds and directions - because the GPS speed shows the average speed you have traveled at for the last second or so, if you're changing speed or direction, it takes a while for the average speed to catch up with your instantaneous speed.

Most vehicles I have driven have a speedometer that exaggerates the speed.  That is, for example, if it says I'm driving 70 mph, maybe I'm only driving 65 mph.

If you're keen to drive as fast as you safely and legally can, it is important to know as exactly as possible what speed you're traveling at.  Many people set their speed based on the expectation that they can drive some miles an hour faster than the speed limit and not get a ticket - for example, in my state, the general belief is that State Troopers won't stop you for speeds of up to 7mph over the limit.  So, if you set your cruise control for, eg, 77 mph on a freeway with a posted 70mph limit, in theory you can cruise on by State Troopers with impunity.

But if your speedometer is under-reading your speed by even a couple of miles an hour, that 77 mph becomes 79 mph and suddenly you're getting a ticket, a fine, points on your license and an increased insurance premium.

On the other hand, if your speedometer is under-reading, you can safely notch up the speed indicated on your speedometer a bit higher and still not exceed your target speed.

This can make a difference on a long journey.  A 5% change in speed on a 400 mile journey with a target speed of 70mph can make a difference of 23 minutes in your travel time.  Or, look at it another way, if the GPS can correct your cruising speed so as to avoid a speeding ticket, you've saved yourself a huge cost in extra insurance bills.

So it pays to use your GPS to calibrate your speedometer.  The GPS truly is close to exactly 100% accurate, but your speedometer may be way wrong.


GPS receivers are amazing devices.  But they're not infallible, and the information they offer is sometimes not the best information, and sometimes is plain wrong.

They are extremely helpful tools, and if you understand their limitations as well as their capabilities, you'll get best value from using one, and be delighted with it, limitations notwithstanding.

Read more in the GPS articles series

See the links at the top right of the page to visit other articles in our extensive GPS series.

This particular article is part 3 of a three part article introducing you to GPS receivers, and what they can and can't do.  Please also visit

1.  Beginner's Guide Part 1 - How the GPS Knows Where You Are
2.  Beginner's Guide Part 2 - Maps, Routing and ETAs
3.  Beginner's Guide Part 3 - Errors, Inaccuracies, POIs, Speed

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Originally published 6 Jun 2008, last update 21 Jul 2020

You may freely reproduce or distribute this article for noncommercial purposes as long as you give credit to me as original writer.


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