Contact Us   Site Map
Airline Mismanagement

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

Part two of this series looks at the major 'value add' of a GPS unit - not only does it know where you currently are, but it can also work out how to get you to where you want to be.

Travel Planning and Assistance
Road Warrior resources
Noise Reducing Headphones
International Cell Phone Service
GSM cell phone unlocking FAQs
Portable MP3 Players
GPS series of articles
Should you choose an iPhone or Android series
Apple iPhone review series
iPhone 3G/3GS Battery replacement
Third Rail iPhone 4/4S External Battery
Apple iPad review series
iPad/Tablet Buying Guide
Google Nexus 7 review
Netflix Streaming Video
Sharing Internet Access series
Microsoft OneNote review
T-mobile/Google G1 phone review series
Blackberry review and user tips
Palm Tungsten T3
Motorola V3 Razr cell phone review
Motorola V600 cell phone review
Nokia 3650 cell phone review
SIM Saver GSM Phone Backup and Copy Device
Clipper Gear Micro Light
Amazon's Wand review
Amazon's new (Sep '11) Kindles and Fire review
Review of the Kindle Fire
Amazon Kindle eBook reader review
Amazon Kindle 2 preview
Sony PRS-500 eBook reader review
Audible Digital Talking Books review
Home Security Video Monitoring
Quik Pod review
Joby Gorillapod review
Satellite Radio Service
Satellite Phone Service
All About Speech Recognition Software
2005 Best Travel Technology Awards
How to connect to the Internet when away from home/office
Bluetooth wireless networking
How to Choose a Bluetooth Headset
Logitech Squeezebox Duet
Packet 8 VoIP phone service
Sugarsynch software review
iTwin remote access device
Barracuda Spam Firewall review
Cell Phone Emergency Power Recharger series
First Class Sleeper
Roboform Password Manager review
Securikey USB Computer Protection Key review
Steripen UV Water Purifiers
ScanGaugeII OBDII review
SafeDriver review
Expandable Bags for Traveling Convenience
USB Flash Drive
Vonage VoIP phone service
Laptop Screen Privacy Filter
AViiQ Laptop Stands
Aviator Laptop Computer Stand
No Luggage Worries
Pack-a-Cone roadside safety flashing cone
Emergency Self charging Radio
Evac-U8 Emergency Escape Smoke Hood
MyTag Luggage Tags
Beware of Checked Baggage Xray Machines
SearchAlert TSA approved lock
Boostaroo Portable Amplifier and splitter
Dry Pak protective pouch
Boom Noise Canceling Headset
Ety-Com Noise Canceling Headset
Nectar Blueclip BT headset holders
Skullcandy Link Headset Mixer
Lingo Pacifica 10 language talking translator
Nexcell NiMH rechargeable battery kit
Jet Lag Causes and Cures
SuddenStop License Frame
CoolIT USB Beverage cooler
Travel ID and Document Pouches
Protect Yourself Against Document Loss
Personal Radio Service
PicoPad Wallet Notes
Times Electronic Crossword Puzzles
Slim Cam 300 micro digital camera review
Stopping Spam
BottleWise Bottle Carrier review
The End of the Internet as We Know it?
How to Book and Buy Travel
Scary, Silly and Stupid Security Stories
Airline Reviews
Airline (Mis)!Management
Miscellaneous Features
Reference Materials
About the Travel Insider
Looking for something else? Search over two million words of free information on our site.
Custom Search
Free Newsletter

In addition to our feature articles, we offer you a free weekly newsletter with a mix of news and opinions on travel related topics.


 View Sample
Privacy Policy

Help this Site
Thank you for your interest in helping this site to continue to develop. Some of the information we give you here can save you thousands of dollars the next time you're arranging travel, or will substantially help the quality of your travel experiences in other, non-cash ways. Click for more information
Reader's Replies

If you'd like to add your own commentary, send me a note.



A Beginner's Guide to Using GPS Part 2

All about Maps, Routing, and ETAs

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 2 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!

Routing Logic

When you ask the GPS unit to navigate you to a specific destination, the unit goes through an amazing number of 'what-if' type calculations to calculate what it believes to be the 'best' route for you to follow.

Now, you know that any time you are choosing to drive anywhere, you have choices of route, choices such as do you take the surface streets or the freeway?  Do you take the main roads or the back roads?  And so on.  Many times you're never quite sure which is the best, and you might have many different ways to get some where.  You make your ultimate decision based on all sorts of factors - maybe even the weather and which route you want to have a break from driving all the time.

How does a GPS handle these choices?  It makes a decision based on a number of assumptions.  Some of these assumptions are built in to the unit, and some you can program in yourself.

The types of assumptions that it considers might include :

  • How fast can you drive on different types of roads (so as to work out the fastest route)

  • Do you prefer to drive on freeways or surface streets

  • Do you want to drive on or avoid driving on toll roads

  • What type of vehicle are you driving (if you are on a motorcycle you'll be happy on different routes than if you're in a big truck)

  • Would you rather go a longer route that takes less time to drive, or a shorter route that takes more time to drive

  • Which route is least congested with traffic (some units get information about traffic on some roads, other units don't)

Most units allow you to choose the answer to some of these questions yourself, and a few units allow you to answer all the questions.

When the unit has decided on a route for you, some units will then present you with some route choices, while others just show you the one best route.

It is important to understand that the route you are presented with is not necessarily truly the best route, because it reflects the assumptions made by the unit, and is also based on what the unit thinks it knows about different roads and traffic conditions, etc, and what the unit assumes your preferences might be.

Sometimes the unit will come up with very strange routes, and - even harder to understand - sometimes the unit will give different routes to the same place on different occasions.  That is a bit like asking a calculator to add 2 + 2, and sometimes getting an answer other than 4!

For example, my Garmin 680 sometimes tries to save me a few feet of traveling distance by having me take a freeway off ramp, then immediately go back onto the freeway on the matching onramp.  This is a crazy suggestion - usually there is a traffic light or stop sign at the end of the offramp, making it much slower to go off and on the freeway.

What to do if you know there is a better route

If you know a better route than the one suggested, just follow the route you know.  Almost every unit these days will automatically recalculate its route based on where you are, and when you drive off its route, it will be forced to recalculate based on your current direction of travel, and sooner or later, it will probably end up selecting the same route you are wanting to use.

Sometimes my Garmin gets really fixated on a specific route, and keeps trying to get me to return to it, even though it ends up being a longer distance and longer traveling time.  Then, when it finally gives up and accepts the route I'm clearly proceeding on, suddenly the travel time and distance massively reduces.  Even more strangely, the point at which it gives up sometimes varies from day to day.

I've no idea why the unit is so 'temperamental', but it is, and after using it extensively, I've started to get a feel for how it 'thinks'.  When you first get a new unit, that is not possible, and the key concept here is just to realize that if you think you know a better route than the one being suggested, you very well might be correct.

Illogical preferences

An extreme example of this is in my Landrover.  Although, in theory, the Landrover is the ultimate 'go anywhere' vehicle, the built-in GPS, which Landrover is very proud of, refuses to route me on minor roads, even when it shows the minor roads on the map.

It will detour me many miles on major roads rather than take a shortcut on a perfectly normal, two-lane, fully-sealed minor road, and if I do end up driving on minor roads, it refuses to navigate at all until I go back onto major roads.

My Garmin unit is perfectly happy on minor roads, but the Landrover GPS is not.

This is a good example of an illogical routing preference, and there is no way I can override it.

A different example is when I'm driving between two points that are almost exactly opposite each other on a circle.  To get from one place (Leavenworth, WA) to the other place (Redmond, WA) I can either go clockwise (130 miles with over half on freeways) or counter-clockwise (110 miles with no freeway driving).

The 130 mile route is faster, easier, and more predictable; the 110 mile route is more of a hassle, slower, and if you get stuck behind a slow moving vehicle or two, can be very slow indeed.  The Landrover GPS correctly defaults to the 130 mile route but offers the 110 mile route as an option, the Garmin GPS really likes the 110 mile route and never offers the 130 mile route.  I have to drive about 10 miles in the opposite direction to the way it insists I go before it relents and allows me to proceed in that direction.

Estimated Time of Arrival

Many units will show an estimated time of arrival, and some units, if they offer you different route choices, will show you how long they expect each route choice will take to drive.

Units will often give you a choice of showing your ETA either as the time at which you will arrive, or the number of minutes driving time remaining.  Some people prefer to see their ETA expressed one way, others prefer the other way.  There's no right or best way to display this data.

This information is based on information the unit has about the road types you'll be driving on, and the typical average speeds you'll achieve on these road types.  Some units will also adapt their ETA based on the actual speeds you are driving at - for example, if it had assumed the average freeway speed would be 65mph and you have been averaging 75mph, it will change its assumption for your present route to reflect that experience.

On the other hand, bad units don't differentiate between different types of roads at all, and just estimate your arrival time based on miles to go and your recent average speed - a totally useless calculation.  If, perhaps, you have 10 miles of surface streets (average speed 20 mph) and then 40 miles of freeway (average speed 60 mph) the correct ETA would be 70 minutes, but if the unit took the 20mph average from your first surface street driving, it would estimate a 150 minute driving time - more than twice as long as it will actually take you.  Alas, some units are this unhelpful.

ETA information is notoriously unreliable and should be considered only as a very approximate guide.  I've generally found that units are typically very optimistic about surface street speeds, and more realistic about highway speeds.  That is, if the unit says it will take 20 minutes to drive a route on surface streets, it is likely to take at least 20 minutes and maybe 30 or even 40 minutes.  But if it says it will take an hour of freeway driving to go somewhere, it is usually more accurate about that.

After a while, you'll get a feeling for how the ETA calculation is done in your unit and will know whether to rely on it or not, and what type of adjustment factors to apply to the unit.  But until then, be aware that this is one of the most error prone parts of your GPS.

One last cautionary note - if your ETA shows you are 'falling behind schedule' don't feel that means you are driving too slow and need to speed up.  As always, drive at a safe comfortable speed, no matter what happens to your ETA.

The Map used by the GPS Unit

The saying about computers - 'garbage in, garbage out' is even more true when it comes to GPS units.  A GPS unit will only direct you to your destination via roads that it knows about.

While modern day map data is vastly improved over that of even just five years ago, it still remains unavoidably imperfect.  You will typically find several different types of potential problems in any given set of map data - some roads may be missing entirely, other roads may be present in the map data but no longer in the real world, and some roads may be in the wrong place or with wrong connecting data.

In any given twelve months, there are literally hundreds of thousands of changes to the US map database.  Most of these of course won't concern you, and changes in your local area you'll automatically adjust for, but you need to realize that the data in your GPS starts off as being imperfect to start with, and then proceeds to get worse and more out of date with every passing day.

Unfortunately, the map data in your GPS doesn't automatically update itself (this is under development but not yet commonly available).  The only way to get more up to date map information is to buy a newer copy of the data - this typically costs about $100 or so, and may be published about once a year.  Remember also that when you purchased your unit to start with, the map data you received with it was already of an uncertain vintage.

Any time you're traveling somewhere that is experiencing rapid growth and development (eg Florida) you might want to consider updating your map data before you travel, so as to be sure you've the latest information in all the new roads, freeways, exits on existing freeways, etc, that is being added.  A GPS becomes totally useless when it doesn't know about the place you're trying to go to, or the road you're driving along!

The important thing to keep in mind is that while the map in your GPS seems extremely detailed and exact, it may not be perfectly correct and may not be perfectly complete.  Trust the real world around you and what you see on the street more than what you see on your screen.

There have been some classic situations where people have blindly followed the directions on their GPS screen, causing them to drive down roads that have been clearly marked as closed, or even to turn onto railroad tracks instead of a road.  Don't end up as another such joke/story!

An important tool - an old fashioned map

If you're in an unfamiliar part of the country or world, you probably should travel with a regular map as well as your GPS.

Hopefully you'll never need to refer to the paper map, but there may be times when it is very helpful.  Sometimes I've found it a real life saver, particularly in situations where the GPS map is wrong - most commonly caused by a road closure on the route it is suggesting.

Sometimes the 'detour' feature on the GPS will work to route you around a closed road, but sometimes it won't, and in such cases, a regular paper map is very much quicker and more convenient to help you see the broader picture of all the roads in an area and how to route yourself around the obstruction.

Because GPS units have small screens, they can't display as much information as a regular map can.  Here's a startling comparison - a large 4.3" GPS screen has 130,560 pixels of information on it.  By comparison, a typical Rand McNally type map on one page of a map book has about 12.5 million pixels of information on it - it can display up to 100 times as much information.  Of course, it doesn't show you 100 times more information, but it does show you a huge amount more than the GPS can, and you don't have to fuss about with zooming in and out and being frustrated by, as you zoom, having roads appear and disappear on the GPS screen.

A map can also help you understand the good sense (or lack of sense) in the route the GPS is suggesting, and can help you understand what other alternatives you might prefer.

So, as counter-intuitive as it seems, it is prudent to supplement your GPS with a regular map, especially if you're in an unfamiliar region.

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 2 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

Related Articles, etc

If so, please donate to keep the website free and fund the addition of more articles like this. Any help is most appreciated - simply click below to securely send a contribution through a credit card and Paypal.


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.


GPS Related Articles

About GPS
Beginner's Guide to GPS part 1
Beginner's Guide to GPS part 2
Beginner's Guide to GPS part 3
How to Choose a GPS Navigation Receiver
GPS Receiver Reviews Menu
Enhanced GPS Data services
GPS Based Realtime Gas Prices
Using a GPS internationally
Dash Express review
Garmin Nuvi 660 review
Garmin Nuvi 680 review
Garmin StreetPilot 7200 review
GlobalSat GV-370 review
Hertz NeverLost Portable GPS review
Landrover LR3 built-in GPS review
Plenio VXA-3000 review
TomTom Go 930 GPS review
Trafficgauge Mobile Traffic Map review
Via Michelin X-930 review





Your Feedback

How Would You Rate this Article


Was the Article Length and Coverage

Too short/simplistic
About right 
Too long/complex

Would You Like More Articles on this Subject


Back to Top