the reading on my gps seems to be quite consistent with my speedometer when i'm maintaining my speed. It only seems to lag when accelerating/decelerating.
Most speedos read several kph higher than actual, and have for decades, if not since the day they were invented. Tends to make the car seem a better performer (and saves you from the odd ticket) which is why manufacturers do it.
You are really talking about two different things GPS cordinates and mapping software, some units only do the former most anymore do both. There is as I believe we all realize a major difference in saying where am I on this map or chart within + - 25 meters or from the corner of Bay and King Streets how far and in what direction is the corner of Wall and Broad Streets in New York, and what is the quickest way to drive from the Toronto Zoo to Syracuse University.gps told a woman to turn into a lake, how accurate could it really be.
Car speedometers can be + or - depending upon where there are speedwise. I had a vehicle that showed I was going slower than I was under 40K then faster than I was actually doing at 110K. As for GPS, in theory they should be more accurate, however civilian GPS has some constraints built into it by demand of the US military and US government. They are not nearly as accurate as military use GPS.
Most of those military constraints have been removed since they were ordered to remove Selective Availability - see my earlier thread on the subject-. However as you pointed out, they remain still less accurate; the main reason is that manufacturers, in order to meet the low cost demands of the mass market, are using a less accurate receiver clock. An extremely accurate -and expensive- clock is required for the GPS receiver to work with extreme accuracy. An error of one microsecond (0.000 001 second) corresponds to an error of 300 metres).
Even with very precise clocks there can be some diffenences between satellites clocks. These differences are corrected to within a few nanoseconds by regular updates originating from ground stations, The field of inertial navigation is very complex, and beyond the scope of this discussion.
But the situation is that I was driving at a steady speed measured at 100kph on the speedo and at 93kph on the GPS. level straight roads. I kept up the constant speed for several minutes. neither of the indicators showed any fluctuations. just the steady difference. so it can't be anything to do with nanosecond errors.
i am just gonna assume you are kidding and go about my day.
The situation which you describe is encountered more often than one would think. Analog devices readouts are the results of many variables such as tires diameters, tires inflation, temperature, speed sensors and various interconnecting devices. Each of them introduces a potential marging of errors. Cars odometers are well known for their inaccuracies.
Of the two I will favor the GPS reading. Try several different GPS and I am sure they all will read the same, or within a km.
I have one car which gives me an 8km error. (108 on the speedo, 100 on the GPS. My second car gives me 103 versus 100.
As someone pointed out earlier car manufacturers stay on the safe side, which I agree is good. How much of a difference should there be is a question which should be addressed to the automotive controlling body. Maybe drivers should start complaining about it.
It depends on the year and make of the vehicle. There was a scandal about one famous car maker who got fined heavily for 'speeding up' the spreedometer and odometer registration, which worked favourably for them with the warrantees expiring soonner than they should have. And no, it wasn't one of the Big Tree (or the former Big Tree).Is it the same on your car, that the GPS speed reading is significantly lower than the speedo reading? I checked it on my car, and at a steady 100kph on the speedo, the GPS indicated I was actually doing 93.
I just changed to winter tires, and it was exactly the same, so it's probably not due to the tires being slightly out of spec.
Do car manufacturers deliberately make our speedos read way above the speed we are actually doing?
I take it that the GPS is the more accurate reading. Is it possible the GPS speed could be wrong?
If the margin of error for a GPS is 20 m that can introduce a big error into speed calculations. Depending on the distance and time over which the calculation is done. So until there was more information on the actual mechanics of the calculations I don't think one should automatically assume it is more accurate.
A speedometer can be very accurate if you are using the same tire sizes, etc. that it was designed for. Whether or not they are calibrated to be lower or higher than they should be...I don't know. But a direct mechanical measurment from the road to your instrument panel would seem to have the potential to be more accurate than a triangulation process from satellites hundreds or thousands of miles away.
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Having said that, the licensed used car sales moron who sold me my last car installed a replacement set of tires two sizes too big put and it threw the speedometer and odometer reading off by 10 to 15 %. So, it can happen. The GPS, if it works according to principles, should give you a more accutrate reading, as it triangulates with at least two or three reference satellite positions. In the GTA, I think that I was reading off of about 7 to 10 satellites.
I certainly hope he was. I could be wrong, but that would mean that every speeding ticket could be challenged and grind traffic court to a stand still. Odometers maybe, but not speedometers.
It is not an assumption. Engineering studies have been made which describe the greater precision of GPS or any electronic navigation system. The fact that a satellite is positioned thousands of miles away does not affects its performance. How do you think NASA is able to precisely dock a vehicle in space. If one were to follow your argument, calculations made with paper and pencil would be more accurate than those obtained through use of modern electronic computers; hardly the case.
That is not my argument at all. I'm only going by what someone else here said...how can something with a 20 m margin of error (positioning) translate into high accuracy in measuring speed. Speed is distance over time. If you are calculating distance several times per second and basing speed on that then you could have a significant margin of error. However, if you calculate the distance less frequently then the distance measure becomes less important, and "average" speed over that distance more accurate. So if the GPS tells me my average speed over the last kilometer then it should be relatively accurate (20-40/1000 being a rough margin of error). But if it calculates that every second then the margin of error could be very much higher - depending on sampling and smoothing algorithms.
The reason that satellite triangulation has a margin of error is precisely because they are so far away. If you triangulate with field mounted PS transmitters you can get accuracy down to fractions of a centimeter.
Docking a space vehicle is done manually and by local control systems once they are there. Finding it with a margin of 20 m accuracy would not be a problem. You could also find it with you eyes. I bet you they don't use GPS to do the actual docking. Not when they have a far more accurate alternative.
i am just gonna assume you are kidding and go about my day.
