[OSM-talk] logging with precission/amount of data set to maxi mum

[Robert Hart]

> 
> Example Speed:
> If you have the speed driving along a road you can get a pretty good guess
> which highway= tag this part of the street would be appropriate to tag.
> Some of you might think; that is no problem, we have the timestamps and
> the
> position, so we can calculate the speed later too. But the timestamps in
> your
> gpx log have a precision of 1 second. So the speed might vary by almost
> factor 2 (between 1 sec and 1.9sec) which for example at 90Km/h could also
> be
> 180Km/h which is not really precise. 

This is a bogus argument. If you have track points at ~1sec spacings, then
it's fairly easy to use a moving average over 5 or 6 points to give a more
accurate speed. If the points are at >>1sec spacing then the aliasing of the
time value isn't important.

As an aside, I've no idea how the compression algorithm actually works, but
if it were me writing it I'd do something like:

do {
	if ( current location deserves saving ) then save location to log
	wait 1 second
}

Which would mean that all points stored would be at exactly 1 second
intervals.

> If you get the speed directly from
> the
> GPS device its evaluated by measuring the Doppler shift of your signal
> which
> is pretty precise (less than 1Km/s error).
> 

Hmm. I'm always suspicious of the max speed my garmin etrex reports. I went
jogging with it the other day, and apparently peaked at 30km/h.

Rob


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