Stupid question? When is a mile not a mile?

MikeP

MikeP

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BMW Nav II. As it a is Garmin with a few bells and whistles, are the miles US or Imperial?

Two things have struck me, one is that the options are for Metric or Statute distances. As far as I am aware Statute is US terminology.

The second is that on the bike and in two cars the variation between the SatNav speedo and the vehicle speedos are about the same. The vehicles all under-read compared to the SatNav.

Am I right in thinking that the US mile is a bit shorter than an Imperial mile? This would explain the variation.

I was using the SatNav speedo thinking it might be more accurate but with Gatsos set at 10% + this could be a bit risky!

:confused:
 
Oh well it's back to the old fashioned way I suppose!

I'll try a few runs along the local measured mile with a stopwatch against the speedo and the Nav II to see which is the most accurate.
 
... The vehicles all under-read compared to the SatNav.:confused: [/B]
Click to expand...


Are you sure? My vehicle over reads the GPS by about 10% - 80mph bike = 72 GPS / 135mph bike - 122mph GPS... or thereabouts.

If I'm a tad over on the bike speedo I'm not worried anymore as it's obviously optimistic... even more so than myself... surely not officer :D

Adam
 
A measured mile may not be a measured mile, unless it's in a dead straight line.

A close friend quite some time ago developed a measuring system for race tracks. Developing the actual system was easy, measuring the tracks was not.

At the time he was developing his system the distance of a racetrack was measured by running a rolling measuring device around the track in the centre of the track.

Because of variances one got using a racing line and/or a preferred line, the actual distance of a track could be measured quite differently. Having been involved in the frustrating job of measuring a race track I would take any time you get as a ± arrangement at best.

Another thing to consider is that I believe the signal from the GPS satellites are only sent every second, this, on face value, could give you a possible 2 second variation. For short distances it's not good but for longer distances the variables of time should be evened out.

Then one has the elevation factor of a GPS as you rise and fall. When you go up or down I don't think a GPS system measures, or takes into account, the real distance travelled as it measures from point a to b in a straight line not allowing for an incline which means that you could travel 1 mile uphill but the GPS only registers .89 of a mile, or something like that.

One of the reasons your speed cameras give you a 10% ± variation is more than likely the impossibility of proving in a court of law the absolute accuracy of their readings, so they give you leeway and know that if you're nabbed, then you really are over the posted limit.

Mick.
 
But a mile is a mile, 1760 yards, 5280 feet, a standard. There are not two different miles as Mike O said. Odometers and other measurements are just estimates of a standard. Every stock BMW speedometer that "I personally" am aware of, reads high on MPH. Changing the speedo gear on my R1150GS brought the odo very close to matching the GPS. When the speedo reads high you are putting phantom miles on your bike but it may avoid tickets.
 
Mick Fagan said:
Don't forget Nautical Miles!

MIck.
Click to expand...

And nautical miles vary in length around the globe as it is defined as being the length arc msubtended by one minute of latitude. Thus, there are always 60 nautical miles per degre of latitude, but because the Earth is not a perfect sphere the length of the arc varies. Thus the nautical mile varies.

This means that for navigational purposes (at sea and in the air) the unit of speed is the knot (a knot being one nautical mile per hour). It also means that for accurate navigation you need to aggregrate short distance measures to accurately arrive at longer distances.

It doesn't matter when using Statute miles (NOT an American term but very much a British one). Or kilometres, either, of course - nasty French invention defined by the length of a brass rod in Paris. (Well until SI units came about when measurement and standards sudenly became very obscure).

But if you're travelling north/south at sea, it can make quite huge differences to your calculations.
 
I have a TT IMO fitted which is dead accurate as calibrated against a measured mile (the only error being tyre wear).
I recently did a 1200 mile run across to Germany and back and when I compared the mileage with the trip reading on the SPIII it was within 3 miles.....prettty good I thought.
 
With regard to Mick Fagans "variation with the ups & downs" thing. I would say that, if you've got an SP3 or similar which is "3D Navigation" capable, the GPSR would calculate the distance travelled accordingly. It can, after all, calculate the altitude by triangulating the satelite directions.
But don't quote me on that. I'm only guessing.
 
But the GPS only measures distance from its reference or sample points. Hence in twisties it can read low.

Generally I use the SP to ensure reasonable compliance with the speed limits but have noticed it reading low on fast short bends where trhe straight line is short...

A statute mile is referenced because it was legislated and therefore became a legal statute. I am sure that the US would not appreciate an Imperial measure related to the British Empire though the US Empire may be acceptable though it is perhaps significant that they got short changed on a gallon.....
 
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