All of the Zumos that I know of from the 550, 660, 590, 595 and XT have recorded track logs, and you can turn them on and off. I have kept most of mine as a historical record of our longer tours. And they tell me where I was, what time and how fast I was going.
I do not know whether another copy is maintained in the system folder in a compacted format.
Speed is always calculated from positional fixes, and all sorts of stuff can affect the accuracy of the fix - trees, mountains, passing HGVs ...
If the position is subject to (say + or - 10m either way, then that is a difference of 20m for two consecutive readings for your actual position). 60kph is nearly 17m per second. So in just one second with, the satnav may have you covering anywhere between 3m or 37m in that 1 second- ie between 10.8km per hour or 133km per hour. (6mph to 83mph ish)
But - over a time period of say one minute, the positional error may still be as high as (say) plus or minus 10 metres. In a minute at 60kph you will travel one kiliometer. With the error, the satnav will calculate this (in the extremes) as either 980m or 1020m
980m in one minute is 58.8 km per hour
1020m in one minute is 61.2 kph.
I've used kilometres because the maths is easier to understand. But you can see that although a fix at a particular point in time is impossible to achieve, a distance / time calculation over a minute is much more convincing - even with the exaggerated + or minus 10m error that I described.
Especially if the speed limit was 40kph.
10m is a big positional error for a satnav, by the way - you may see it just turning it on, but once it has got a fix on a few satellites it drops pretty rapidly to 3-4 metres. I'm inside at the moment and mine is showing an accuracy of 3metres. It took about 20 seconds to get enough satellites to reach that level of accuracy.
Becasue of all of the above, the satnav never shows you the speed that you are currently travelling. Its shows you the speed that you have been travelling in the last few seconds. If this is in the last 5 seconds, it works out the distance travelled from where you are now to the point you were at 5 seconds ago, and displays that as a speed. It seems to update it every second and it settles to your current steady speed after around 5 seconds.
If you are travelling at a constant speed with no mountains around, a clear view of the sky, no buildings or passing trucks and the satnav speed is staying pretty steady and is showing a speed that is the same value different from you speedo - then it is a good indication of you true speed.
The speedometer will never show you travelling faster than your true speed - so if it shows you doing 60mph, you will definitley be doing 60mph or less. Never more.
You cannot rely on the satnav for that. It may show 60mph and you could be doing a good few mph faster than that. Not often, but it is possible.
And just for amusement. This is the speed profile drawn fromthe gpx file of my track log.
- Speed Profile.jpg (16.06 KiB) Viewed 3428 times
The red line is closer to the actual plot speed and I've just drawn that freehand. But if you take a look at the actual speed recordings in your GPS file, they can be quite alarming. Probably not as extreme as my 2nd XT satnav, but they certainly need smoothing out.
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