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  1. #16
    The saga continues ...

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    Quote Originally Posted by ferrarr View Post
    I believe that devices with only one connection (to GPS), will have better accuracy than one device using a second device, to get the GPS, then using the first device for displaying on yet a third device?
    Why? This is all digital information, not analog, so there is no loss of accuracy when transferring the information. I would think it would not matter.

    As it turns out there were some settings in the software for increasing accuracy in the gps information, something about including compass data and perhaps cell data as well, and changing those corrected the large speed error. When I tested with those changes made the digital speed display was much closer to the actual speed and thus the alarms were not a problem any more.

  2. #17
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    I think the increased accuracy actually comes from integrating the accelerometer data in the iPhone, not cell data. Compass data can show which direction the phone is facing, but that can be different from the direction it is traveling. Speed differences between the speedometer and GPS are common, with most of the error in the speedometer. Auto manufacturers have to meet certain standards for the odometer reading, so most of them choose to have the odometer be higher than the vehicle has actually travelled. There are penalties for under-reporting mileage, which they avoid by over-reporting. GPS then shows a speed generally slower than the speedometer. Also, as your tires wear out the speedometer speeds up. I'll put a paragraph on why that is after this but if you don't care, stop now.

    Tires have tread that wears off as the tire is used. The overall diameter of the tire thus gets smaller as the rubber wears away. So let's say you have pretty worn tires that have lost .25 inches/6.35mm of tread from when they were new. That means the diameter of the tire is .5 inches/12.7mm smaller. So now each revolution of the wheel goes a shorter distance than on new tires. The difference is equal to pi times the difference, or 1.57 inches / 40 mm. Not very much, but it adds up. Assuming a standard 15 inch rim, with a standard diameter of 24.1 inches tire new, that tire covers 75.71 inches each revolution. But the worn tire only covers 74.14 inches each revolution. So to cover the same distance in the same time the tire has to rotate faster, which means the speedometer will show a higher number for the same speed. In our example, the difference is about 2%. At 60 mph on old tires, although the speedometer may say 60 mph, you are, in fact only traveling 58.8 mph. But GPS doesn't care about tires, it calculates speed as the distance you covered between fixes and use the time between fixes to calculate your speed between fixes. So the GPS will show 58.8 mph. And that assumes the tires are the proper size for the vehicle. Change tire size or profile and the speedometer can be off even more.

    All that also assumes constant speed, which is nearly impossible to achieve in real life, so the actual speed will still vary on the GPS. The increased accuracy comes from using the accelerometers in the iPhone to calculate changes in speed during the time between fixes so that the GPS can forecast your next fix-based speed. I've seen at least one application on the iPhone that uses the accelerometer and compass to continue to provide tracking through tunnels where GPS is not available. Some cars also use the speedometer system to feed data to the navigation system inside tunnels, too.

    Finally, since we are here, the accuracy of the fix can affect the speed calculation from the GPS. Each fix is calculated independently and each has a certain probability of error. If the two adjacent fixes have errors in opposite direction the speed calculated between those fixes can be much higher or lower than your actual speed. GPS applications generally apply smoothing algorithms to limit the changes between fixes for speed, heading, etc. If in previous second you travelled this far in this direction, then this second you should be traveling about that same speed/direction. Older intervals get weighted less and less as they age, but are used to keep the speed/direction from jumping all over the place. Accelerate or decelerate really hard and those smoothing algorithms can be fooled. (I'm thinking of high speed drag racers and hitting an object slamming the vehicle to a stop kind of accel/decel.)

    When you think of it, SatNav/GPS is a pretty amazing technology. And we haven't even talked about how the system calculates a route from point A to point B for you!
    Jake

  3. #18
    The saga continues ...

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    Quote Originally Posted by MacInWin View Post
    I think the increased accuracy actually comes from integrating the accelerometer data in the iPhone, not cell data.
    Perhaps. I don't know. The nav software had some settings for increasing the GPS accuracy and I used those, but have no idea what iOS changes that made, if any. Whatever the case the mileage figure on the display was pretty much dead-on after the adjustment.

    Quote Originally Posted by MacInWin View Post
    I'll put a paragraph on why that is after this but if you don't care, stop now.
    Thanks. I did read what you wrote.

    The math is really pretty simple. The circumference of a circle is pi x d so it is directly proportional to the diameter. As the tread shrinks so does the circumference and since the speedometer is based on turns of the wheel the actual distance covered per turn decreases as well. A bit of simple math will give a percent figure as to how much actual speed loss there is between new tires and those ready to replace. And, of course, actual fuel economy suffers as well since less distance is actually being covered.

    Quote Originally Posted by MacInWin View Post
    Finally, since we are here, the accuracy of the fix can affect the speed calculation from the GPS.
    As with most of the stuff that goes on under the covers in technology, it is often far more complex than appears at first glance. The decisions that the software engineers have to make when implementing something like speeds and routing in nav software often have unexpected consequences and the whole thing rests on the integrity of the data, which I suspect is not as accurate as we think.

    I used to use a Rand McNally portable GPS unit. I liked it, and it was as good as any other navigation device I had used, but it often made some very strange decisions. I had it set for "fastest trip" but it always took me through the downtown of the city of Santa Fe even though there was a bypass with a higher speed limit and only 2 traffic lights. The bypass was at least 15 minutes faster, but it was 1 mile longer, and so the software always told me to go through the city. I can not tell you how many times it told me to drive through people's backyards or to turn into their driveways while navigating in rural areas, or how many times it told me to get off of the interstate only to tell me to get back on as soon as I got off.

    I spoke with tech support and they blamed bad data, and that may well be right. Like most other things, the technology is a mixed blessing.

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