3 ms·

Oh, and BTW, it's not mentioned, but it would be quite interesting, and much more of a burn, if Elon is actually making some of the charges, like driving in loo

by cpg 14y ago

Oh, and BTW, it's not mentioned, but it would be quite interesting, and much more of a burn, if Elon is actually making some of the charges, like driving in loops, with a proof via GPS logs of the test drive.

lutusp 14y ago
Yes, and after a little research, I find that the Tesla log contains GPS positions, so this could be done in principle. But for a short distance like within a small parking lot (as in the account of driving in circles), the positions might not produce a useful graph (typical GPS accuracies are +- 10 meters or 32.8 feet).
- Robin_Message 14y ago
  GPS is only accurate to a certain amount (maybe 1.5 metres, maybe 10 metres) in an absolute sense, but in a relative sense they are much more accurate and would easily show circles being driven in a parking lot (it's just that the circle might be 10 metres away from the road). This is also used in surverying to do something called differential GPS with two receivers: one GPS receiver fixed in a known location, and another to measure GPS locations relative to the first GPS in the known location - absolute error is completely removed by this process (http://en.wikipedia.org/wiki/Differential_GPS http://en.wikipedia.org/wiki/Differential_GPS claims 10cm accuracy). If you think about it, two receivers separated by a little space is more or less equivalent to two receivers separated by a little time, so you'd expect a similar relative accuracy of 10cm.
  - lutusp 14y ago
    Yes, all well understood. The problem comes up in cluttered terrain and an ordinary GPS unit that's in motion (no differential GPS option). In such a case, if the receiver encounters obstacles in its satellite line of sight, it may switch to more favorable satellites and recompute its position. As a result, the apparent position will jump around within the normal error bound (see below). I know, I've had the experience any number of times. > GPS is only accurate to a certain amount (maybe 1.5 metres, maybe 10 metres) Statistical civilian GPS accuracy is better understood than this. It's 7.8 meters (25.6 feet) for two standard deviations (i.e. 95% confidence): http://www.gps.gov/systems/gps/performance/accuracy/ http://www.gps.gov/systems/gps/performance/accuracy/
    - Robin_Message 14y ago
      Good point about changing satelites. I guess an open parking lot would be fine, but who knows. I gave up looking for an accuracy figure on wikipedia, but knew I had seen 1.5 metres in the past, and that 10 metres is too inaccurate. Thanks for the link.
      - lutusp 14y ago
        > I gave up looking for an accuracy figure on wikipedia, but knew I had seen 1.5 metres in the past, and that 10 metres is too inaccurate. It all has to do with the statistical error bound. For a specified accuracy, one must also state the deviation for which that error is true. My point is that GPS doesn't have a specific accuracy, always true, with a probability cliff on each side. For a given accuracy specification, one must always include the probability for that accuracy.
  - bigiain 14y ago
    I suspect the probem is not the gps positional accuracy, but rather the data loggers recording frequency - for 99.9% of what you'd want from a gps log on a vehicle, recording the position and speed every 10 or 20 seconds would be more than enough. To generate useful plots of a car driving in circles round a small parking lot would need a much higher recording rate. Even slow old-school gps chipsets output 1Hz position data, but logging all that would needlessly produce 10 or 20 ties more data than youd need for anything except showing up lying journalists. I'd put good money on Musk getting 1Hz or better gps logs of the _next_ bunch of test drivers...
- bfe 14y ago
  I use Google's "My Tracks" app every day to record my running, with a GPS tracked path imposed on a Google map, and it easily shows tiny shifts in position such as which side of the street I ran on or whether I ran on or slightly off of a dirt path. There's no doubt something equivalent in a Model S would be able to accurately record laps in a parking lot.
markisonfire 14y ago
I don't think GPS would show anything substantial if his claim of a "100-space parking lot" where Broder drove in circles is true. I'd be impressed if the GPS was that accurate. Regardless, I don't think that's a huge sticking point to this story. Broder was at the charging station - it's possible he wanted to see just how long it would take until the battery shut down.
smackfu 14y ago
It makes me wonder why Musk didn't provide that proof then.

by cpg 14y ago

lutusp 14y ago

Yes, and after a little research, I find that the Tesla log contains GPS positions, so this could be done in principle. But for a short distance like within a small parking lot (as in the account of driving in circles), the positions might not produce a useful graph (typical GPS accuracies are +- 10 meters or 32.8 feet).

Robin_Message 14y ago
GPS is only accurate to a certain amount (maybe 1.5 metres, maybe 10 metres) in an absolute sense, but in a relative sense they are much more accurate and would easily show circles being driven in a parking lot (it's just that the circle might be 10 metres away from the road). This is also used in surverying to do something called differential GPS with two receivers: one GPS receiver fixed in a known location, and another to measure GPS locations relative to the first GPS in the known location - absolute error is completely removed by this process (http://en.wikipedia.org/wiki/Differential_GPS http://en.wikipedia.org/wiki/Differential_GPS claims 10cm accuracy). If you think about it, two receivers separated by a little space is more or less equivalent to two receivers separated by a little time, so you'd expect a similar relative accuracy of 10cm.
- lutusp 14y ago
  Yes, all well understood. The problem comes up in cluttered terrain and an ordinary GPS unit that's in motion (no differential GPS option). In such a case, if the receiver encounters obstacles in its satellite line of sight, it may switch to more favorable satellites and recompute its position. As a result, the apparent position will jump around within the normal error bound (see below). I know, I've had the experience any number of times. > GPS is only accurate to a certain amount (maybe 1.5 metres, maybe 10 metres) Statistical civilian GPS accuracy is better understood than this. It's 7.8 meters (25.6 feet) for two standard deviations (i.e. 95% confidence): http://www.gps.gov/systems/gps/performance/accuracy/ http://www.gps.gov/systems/gps/performance/accuracy/
  - Robin_Message 14y ago
    Good point about changing satelites. I guess an open parking lot would be fine, but who knows. I gave up looking for an accuracy figure on wikipedia, but knew I had seen 1.5 metres in the past, and that 10 metres is too inaccurate. Thanks for the link.
    - lutusp 14y ago
      > I gave up looking for an accuracy figure on wikipedia, but knew I had seen 1.5 metres in the past, and that 10 metres is too inaccurate. It all has to do with the statistical error bound. For a specified accuracy, one must also state the deviation for which that error is true. My point is that GPS doesn't have a specific accuracy, always true, with a probability cliff on each side. For a given accuracy specification, one must always include the probability for that accuracy.
- bigiain 14y ago
  I suspect the probem is not the gps positional accuracy, but rather the data loggers recording frequency - for 99.9% of what you'd want from a gps log on a vehicle, recording the position and speed every 10 or 20 seconds would be more than enough. To generate useful plots of a car driving in circles round a small parking lot would need a much higher recording rate. Even slow old-school gps chipsets output 1Hz position data, but logging all that would needlessly produce 10 or 20 ties more data than youd need for anything except showing up lying journalists. I'd put good money on Musk getting 1Hz or better gps logs of the _next_ bunch of test drivers...
bfe 14y ago
I use Google's "My Tracks" app every day to record my running, with a GPS tracked path imposed on a Google map, and it easily shows tiny shifts in position such as which side of the street I ran on or whether I ran on or slightly off of a dirt path. There's no doubt something equivalent in a Model S would be able to accurately record laps in a parking lot.

markisonfire 14y ago

I don't think GPS would show anything substantial if his claim of a "100-space parking lot" where Broder drove in circles is true. I'd be impressed if the GPS was that accurate. Regardless, I don't think that's a huge sticking point to this story. Broder was at the charging station - it's possible he wanted to see just how long it would take until the battery shut down.

smackfu 14y ago

It makes me wonder why Musk didn't provide that proof then.