GPS or Speedo, What do you believe???????

[GS]

We are all going to get booked for speeding considering we are actually travelling at somewhere below 1000 miles per hour within the earth's rotation.  I wonder if they take that into account when booking you?  Is it really an accurate reading you've got there officer, I put it a few 100 miles per hour short ... 

"The circumference of the Earth at the equator is 25,000 miles. The Earth rotates in about 24 hours. Therefore, if you were to hang above the surface of the Earth at the equator without moving, you would see 25,000 miles pass by in 24 hours, at a speed of 25000/24 or just over 1000 miles per hour.
Multiply by cosine of your latitude to see how fast the Earth is rotating where you are.
Earth is also moving around the Sun at about 67,000 miles per hour."
From:  http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970401c.html

Kit_e

So speeding 10k over the limit is insignificant in the big picture!!

GS

[oldblade]

So speeding 10k over the limit is insignificant in the big picture!!

GS

Was that 10kph or 10000kph

[Patr80l]

Rules changed a few years ago.  I think it was 2006.  Speedos on vehicles built since then are not allowed to give you a reading where you can be travelling faster than the speed given which is why you usually find speedos are now on the low side of your actual speed.

 
Dave

No, you mean the speedo reads higher than the actual speed.

While the GPS position may be out by a several meters, they will be very accurate for interval distance, and the time measurement has the accuracy of an atomic clock.   Distance/Time has to be more accurate than any speed measurement that involves rubber do-nuts.

[Patr80l]

Was that 10kph or 10000kph

Neither: it's km/hr

[oldblade]

Neither: it's km/hr

technicality

[cruisindub]

Having a speedo that is out, mine is reading about 8km/h faster than the sat nav at 80km/h, (the old VW doesnt go much faster than that!!) an about 9km/h if I can get it to 90km/h downhill,
How does this affect the following:
-Fuel consumption calculations, L/100km.
-Odometer reading over a long period of time, say if it curently reads 200,000 kms.

-on a lighter note,.....does this make distances just that much further away?

Cruisindub

[savanahkelpy]

Waiting for the end of the year so i can get a new calender for my old Landy,,,,

[jasonw]

a speedo gives you ground speed, a gps gives you your location on the earth. what a gps cant tell is if you are going up or down a hill. compare your speedo to your gps on a flat road and then set your speed at 100 then as you drive up a hill maintain your speed by the speedo and watch the gps speed drop slightly. i use my gps to check my speedo and i havnt had a ticket yet .

[Redback]

If you want to eliminate all this, get a speedo correction unit if your speedo is electronic, I have a Truspeed in mine.

Get it here, there are other places to get one, this is just the first one I found in Google.
http://www.superlift.com/accessories/truspeed.asp

Baz.

[Patr80l]

a speedo gives you ground speed, a gps gives you your location on the earth. what a gps cant tell is if you are going up or down a hill. compare your speedo to your gps on a flat road and then set your speed at 100 then as you drive up a hill maintain your speed by the speedo and watch the gps speed drop slightly. i use my gps to check my speedo and i havnt had a ticket yet .

How steep are these hills???  An incline of 10 degrees is quite a steep hill yet will only make a 1.5% difference! (cosine of 10 deg = 0.985)

[rescue1]

a speedo gives you ground speed, a gps gives you your location on the earth. what a gps cant tell is if you are going up or down a hill. compare your speedo to your gps on a flat road and then set your speed at 100 then as you drive up a hill maintain your speed by the speedo and watch the gps speed drop slightly. i use my gps to check my speedo and i havnt had a ticket yet .

Actually my GPS gives me elevation as well as do most.....

[WilSurf]

what a gps cant tell is if you are going up or down a hill.

Yes they do.
They are 3-dimensional: longtitude, lattitude and height as long as you have more then 3 satelites.

[jasonw]

happy to put my hand up to say im wrong  but im sure i read somewhere that altitude in a gps isnt that accurate as its based on a mathmatical average sea level and assumes the world is perfectly smooth not hilly. 

[Heiny]

happy to put my hand up to say im wrong  but im sure i read somewhere that altitude in a gps isnt that accurate as its based on a mathmatical average sea level and assumes the world is perfectly smooth not hilly. 

I would not expect the indicated elevation (altitude is the distance above ground level) on in car GPS units to be very accurate because over the whole of Australia there are millions of local datum values for reletively small areas, one suburb can have several local datum values ( im unsure of the size of these local datum areas outside of developed areas). When we set up a project at work that will use the GPS systems on our graders and other equipment to auto control horizontal position and elevation we need to calibrate (site cal) the GPS base station to the local area datum value and a minimum of three surveyed control points on site, the base station will then correct the satellite signals before they are sent via radio frequency to the machines, only then we can we obtain the required accuracy which can still vary + - 40mm of the design level throughout the day and day to day.
 
So as mentioned above I would assume that an average of local datum values (which are in relation to sea level) are used with these in car units to give an indication of elevation and they do not "assume that the world is perfectly flat". but as for how close these units are to being accurate or how accurate you want them to be

I might compare one up against the Trimble system one day for interest sake.

[Patr80l]

I would not expect the indicated elevation (altitude is the distance above ground level) on in car GPS units to be very accurate because over the whole of Australia there are millions of local datum values for reletively small areas, one suburb can have several local datum values ( im unsure of the size of these local datum areas outside of developed areas). When we set up a project at work that will use the GPS systems on our graders and other equipment to auto control horizontal position and elevation we need to calibrate (site cal) the GPS base station to the local area datum value and a minimum of three surveyed control points on site, the base station will then correct the satellite signals before they are sent via radio frequency to the machines, only then we can we obtain the required accuracy which can still vary + - 40mm of the design level throughout the day and day to day.
 
So as mentioned above I would assume that an average of local datum values (which are in relation to sea level) are used with these in car units to give an indication of elevation and they do not "assume that the world is perfectly flat". but as for how close these units are to being accurate or how accurate you want them to be

I might compare one up against the Trimble system one day for interest sake.

You're describing differential GPS which demonstrates that if you give it an accurately known position (the surveyed point), it is very accurate in determining distances from that reference point.   Using the same logic, it is very accurate in determining speed.
To determine speed, you don't need an accurate position, you just need to know how far you've moved in a certain time.

[outback jack]

Let's lighten up, who really cares 

[griz066]

Thanks for the reply's you confirmed what i thought but as usual it is way off topic. Time to lock this thread please someone.

[Manjimike]

I found some of the techo stuff quite interesting - thanks

 
Mike