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
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
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'
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
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
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
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.
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.
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
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
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
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
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
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
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
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
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
Related Articles, etc
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6 Jun 2008, last update
26 Jun 2019
You may freely reproduce or distribute this article for noncommercial purposes as long as you give credit to me as original writer.