speedometer v gps - Page 2 - Motorhome Forums, Motorhome Discussion, Motorhome Chat

User Tag List

 6Likes
Reply
 
LinkBack Thread Tools
post #11 of 17 (permalink) Old 03-07-2015, 00:43
Senior Member
 
Blizzard's Avatar
 
Join Date: Sep 2009
Location: County Durham
Posts: 1,201
Thanks: 31
Thanked 38 Times in 36 Posts
Quote:
Originally Posted by bigtwin View Post
Think of a right angle triangle.

The car is traveling downhill along the hypotenuse.

The GPS measurements (as observed by the satellites) only detect the horizontal movement (the base of the triangle).

Since the hypotenuse is longer than the base of the triangle the car has travelled a greater distance than that detected by the GPS (over the same time period). Thus, in this case the GPS will under estimate the true speed of the vehicle. In practice, this error is likely only to close the delta between the over estimating speedo and that indicated by the GPS.

Ian
I'm liking the simple maths, which is spot on in a situation where only one satellite is being tracked by the satnav and where the car travels downhill/uphill in a perfectly straight line, directly towards or away from the satellite.

A modern satnav or GPS will gather its information by tracking several satellites, in various positions and elevations, the combined effect of which will minimise the effect of undulations on the Earth's surface, be they horizontal or vertical and the end result should remain a fairly accurate speed, no matter the terrain.
Blizzard is offline  
Sponsored Links
Advertisement
 
post #12 of 17 (permalink) Old 03-07-2015, 07:16
Senior Member
 
Kev_n_Liz's Avatar
 
Join Date: Sep 2008
Location: Leeds, Gods County.
My Motorhome: 2002 Autocruise Wentworth
Posts: 31,222
Thanks: 1,065
Thanked 617 Times in 582 Posts
Personal Aire
Send a message via AIM to Kev_n_Liz Send a message via Yahoo to Kev_n_Liz
Quote:
Originally Posted by bigtwin View Post
Think of a right angle triangle.

The car is traveling downhill along the hypotenuse.

The GPS measurements (as observed by the satellites) only detect the horizontal movement (the base of the triangle).

Since the hypotenuse is longer than the base of the triangle the car has travelled a greater distance than that detected by the GPS (over the same time period). Thus, in this case the GPS will under estimate the true speed of the vehicle. In practice, this error is likely only to close the delta between the over estimating speedo and that indicated by the GPS.

Ian
I knew that, but not for the reason you give, I surmised it was an altitude gain and loss, which in effect I suppose it is, but thank you for a very good & accurate explanation, I am a wiser person for it, but don't ask me to reiterate

Cheers Kev

Semper ubi, Sub ubi

Just because you're PARANOID it doesn't mean they're NOT out to get you

link to my album
Link to my build thread on MHF
Kev_n_Liz is online now  
post #13 of 17 (permalink) Old 03-07-2015, 09:09
Senior Member
 
BillCreer's Avatar
 
Join Date: Jan 2010
Location: Chorlton-Cum-Hardy
My Motorhome: Hymer B584
Posts: 3,265
Thanks: 44
Thanked 77 Times in 69 Posts
I'm not so sure that it is an accurate description. GPS is also measuring your hight above sea level so it is tracking you in three dimensions.
If it knows you are going up and down hills as well as in any horizontal direction I don't see why it can't work out an accurate speed at all times. It's only maths.

Cheers
Bill
BillCreer is online now  
 
post #14 of 17 (permalink) Old 03-07-2015, 09:29
Senior Member
 
Join Date: Jun 2006
Location: The Berwyn mountains in the paradise of Powys
My Motorhome: Hymer S740 on Sprinter 616 (2004)
Posts: 11,516
Thanks: 105
Thanked 171 Times in 152 Posts
If we are venturing into the esoteric. Is it not velocity measured by the sat nav if it records through 3 dimensions.

Dick
Glandwr is online now  
post #15 of 17 (permalink) Old 03-07-2015, 09:36
Senior Member
 
BillCreer's Avatar
 
Join Date: Jan 2010
Location: Chorlton-Cum-Hardy
My Motorhome: Hymer B584
Posts: 3,265
Thanks: 44
Thanked 77 Times in 69 Posts
Quote:
Originally Posted by Glandwr View Post
If we are venturing into the esoteric. Is it not velocity measured by the sat nav if it records through 3 dimensions.

Dick
I thought that was what I said Dick
Glandwr likes this.

Cheers
Bill
BillCreer is online now  
post #16 of 17 (permalink) Old 03-07-2015, 15:31
Senior Member
 
Join Date: Jun 2005
Location: Wirral
My Motorhome: Hymer 504 B Class
Posts: 1,042
Thanks: 3
Thanked 12 Times in 12 Posts
The difference being seen between speedo and satnav is the same as I had on my 2005 Fiat 2.8. I seem to recall that before I sold it in 2013, it had a recall which was something to do with the binnacle [I might be wrong about the cause of the recall]. After that, the difference was 70mph on satnav, 63/64mph on speedo.

On a flat motorway, my 2013 Fiat 130 speedo shows 60mph and the satnav 56mph.

I'd say that the satnav is more accurate but not necessarily consistently accurate whereas the speedo is never accurate. I rely on the speedo to keep me within speed limits.
Brock is offline  
post #17 of 17 (permalink) Old 03-07-2015, 16:40
Senior Member
 
Kev_n_Liz's Avatar
 
Join Date: Sep 2008
Location: Leeds, Gods County.
My Motorhome: 2002 Autocruise Wentworth
Posts: 31,222
Thanks: 1,065
Thanked 617 Times in 582 Posts
Personal Aire
Send a message via AIM to Kev_n_Liz Send a message via Yahoo to Kev_n_Liz
Kev_n_Liz is online now  
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Motorhome Forums, Motorhome Discussion, Motorhome Chat forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in









Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.



Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome