# The Naked Scientists Forum

### Author Topic: Which Method of Distance Calculation Does My Sat Nav Use ?  (Read 3754 times)

#### neilep

• Withdrawnmist
• Naked Science Forum GOD!
• Posts: 20602
• Thanked: 8 times
##### Which Method of Distance Calculation Does My Sat Nav Use ?
« on: 13/02/2008 23:57:16 »
" In one hundred yards turn left.....turn right at the roundabout, take the 4th exit .......You have arrived at your destination, the route guidance will now finish.............would you like a B.... Job ?"

I'm In Love With My Car sang Roger Taylor on Queens quintessential album A Night At The Opera in 1975 !.........well...if he had my Sat Nav back then..... he'd be singing about 'Sat Nav Babe'.....she's gorgeous !!..........but........she's a bit ditzzy !!......

I notice that the distance for a whole journey can differ quite dramatically between what my Sat Nav originally calculates and the final distance actually indicated by the odometer !

So, does SNB (Sat Nav Babe) calculate distance as the crow flies or by the actual road length ?

You see....I really do not know.........do you ?

Thanks

neil
xxxxxxxxxx

« Last Edit: 13/02/2008 23:59:02 by neilep »

#### another_someone

• Guest
##### Which Method of Distance Calculation Does My Sat Nav Use ?
« Reply #1 on: 14/02/2008 00:51:38 »
Given what has already been said about the accuracy of speedometers, you can easily expect a 10% error in your odometer, but anything beyond that would imply another cause.

#### neilep

• Withdrawnmist
• Naked Science Forum GOD!
• Posts: 20602
• Thanked: 8 times
##### Which Method of Distance Calculation Does My Sat Nav Use ?
« Reply #2 on: 14/02/2008 00:59:38 »
Given what has already been said about the accuracy of speedometers, you can easily expect a 10% error in your odometer, but anything beyond that would imply another cause.

THANK YOU George.

But do you know if Sat Navs do calculate distance because they know the lengths of the roads (with all their bends and twists) or by triangulation ? (is that the right phrase ?)

#### another_someone

• Guest
##### Which Method of Distance Calculation Does My Sat Nav Use ?
« Reply #3 on: 14/02/2008 02:00:53 »
THANK YOU George.

But do you know if Sat Navs do calculate distance because they know the lengths of the roads (with all their bends and twists) or by triangulation ? (is that the right phrase ?)

Not having a satnav myself, I have not observed it in action to ba able to comment.

Even if calculated the route not just the endpoint, it would still have to make approximations, depending on how often it took readings, and how many gaps there were in the readings.

I suppose the best way you could answer your question would to to take two very different routes between the same endpoints, and see if it judged that you had travelled the same distance.

Actually, thinking more about this - simply do a 10 mile circular journey, and see if the SatNav tells you that you have driven 10 miles or zero miles.
« Last Edit: 14/02/2008 10:47:47 by another_someone »

#### JimBob

• Global Moderator
• Neilep Level Member
• Posts: 6564
• Thanked: 7 times
• Moderator
##### Which Method of Distance Calculation Does My Sat Nav Use ?
« Reply #4 on: 14/02/2008 22:35:33 »
It is also quite possible which base map projection the satnav uses.

If the map used is an orthographic projection the map would look like this county in Texas:

If the British Ordinance Survey projection of 1937 was used, your map would look like this, only much more squashed. The higher the latitude, the more squashed the map becomes. This map is at about 31° north. The UK is above 50° north

The first image is almost a true distance map. It is what the county would look like from above at a land distance from the earth. The second is obviously skewed. It is the county as viewed from the equator. The distances measure become almost useless especially if they are not also corrected on the fly by mathematical programs to adjust for the skewedness running in the satnav and constantly updating the position and adjusting for the map projection used. The map may appear as orthographic on the screen but the map in the satnav may not be.

See http://en.wikipedia.org/wiki/Orthographic_projection_%28cartography%29 for orthographic and http://www.mapref.org/MapProjectionsGB.html#Zweig818 for systems in the UK. The ordinance survey is the basis of all of them.

#### The Naked Scientists Forum

##### Which Method of Distance Calculation Does My Sat Nav Use ?
« Reply #4 on: 14/02/2008 22:35:33 »