car mph versus tomtom mph, why different?

mcpitman

IanMSpencer wrote: »

ACPO guidelines will enforce only at 10% + 2mph (i.e. you need to do a real 57mph to get a ticket)
,QUOTE]

Not quite true, if this were I wouldn't have had to sit a speed awareness course for 34mph in a 30mph zone.

I questioned this on the course I attended and was told it is the word "guideline" that makes some people think you can get away with "10% +2mph".

There are also carying guidelines across police forces, so not all constabularies have the same rules/guidelines.

benjus

People seem to be assuming that GPS units calculate speed using distance travelled / time.

However, I was under the impression that they measured the doppler shift effect on the carrier signal to the satellites, which should in theory be very accurate ( http://www.aprs.net/vm/gps_cs.htm ).

Anyone know for sure?

Rover_Driver

mcpitman wrote: »

IanMSpencer wrote: »

ACPO guidelines will enforce only at 10% + 2mph (i.e. you need to do a real 57mph to get a ticket)
,QUOTE]

Not quite true, if this were I wouldn't have had to sit a speed awareness course for 34mph in a 30mph zone.

I questioned this on the course I attended and was told it is the word "guideline" that makes some people think you can get away with "10% +2mph".

There are also carying guidelines across police forces, so not all constabularies have the same rules/guidelines.

The ACPO guidelines for issuing a FPN start at 10% + 2mph, that is only a guideline. Anything above 30 in a 30 limit is an offence and it may have been considered that 34 was excessive at the time, and why you were offered a speed awareness course.

TrickyWicky

Strider590 wrote: »

Actually the speedometer only has to work, it's accuracy is not testable.

Yes it is, what do you think rolling roads are for?

Back in the late 80s / early 90s some guy was gaught doing around 90mph on the m25 in a reliant robin. The rozzers didn't believe it and ended up going to get their speedo tested on a rolling road to determine if the robin had really done that speed or if their speedo was at fault.

Rolling road confirmed that robin had taken lessons from batman

The RR and speedo are both of course dependant on the size of wheels / tyres fitted.

JustinR1979

TrickyWicky wrote: »

Yes it is, what do you think rolling roads are for?

For mapping/dyon runs.
Do you mean brake rollers, as in testing the brakes on an mot?
Which are for testing the brakes. and does about 2mph, and when not working/testing an lsd/4x4 equipped car they don't always use...

Car_54

Rover_Driver wrote: »

mcpitman wrote: »

The ACPO guidelines for issuing a FPN start at 10% + 2mph, that is only a guideline. Anything above 30 in a 30 limit is an offence and it may have been considered that 34 was excessive at the time, and why you were offered a speed awareness course.

There's also the attitude test. If you're stopped by an officer for exceeding the speed limit, he may decide to take no action if you recognise your mistake and listen to his advice. On the other hand ...

mcpitman

On the other hand... you may have been "caught" by a roadside speed monitoring exercise that don't physically stop anyone.

wongataa

benjus wrote: »

People seem to be assuming that GPS units calculate speed using distance travelled / time.

However, I was under the impression that they measured the doppler shift effect on the carrier signal to the satellites, which should in theory be very accurate ( http://www.aprs.net/vm/gps_cs.htm ).

Anyone know for sure?

For live speeds GPS devices do use the doppler shifts of the satellite signals.

mattyprice4004

Iceweasel wrote: »

I agree that a sat-nav won't be accurate on a hill - but how inaccurate?

Even on a road that climbed 3000ft in half a mile the error would be about 1%

I'm no maths professor so feel free to correct me.

I reckon that is still more accurate than any other instrument I could purchase.

I have never seen my sat-nav vary compared to my speedo - it's a constant error.

Sat-Nav says 70mph - speedo says 73mph - uphill and down.

Maths probably isn't a career for you
Check the post a few posts above yours.

Iceweasel

mattyprice4004 wrote: »

Maths probably isn't a career for you
Check the post a few posts above yours.

I take it that you mean the one with reference to Pythagoras?

I'll try to be a good student. LOL

Are we saying with 100% certainty that a Sat-Nav can only indicate the speed on a horizontal plane i.e parallel to the earth's surface?

How does the 'Doppler Effect' work?