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Old October 12th 16, 01:17 PM posted to sci.geo.satellite-nav
Terry Pinnell[_3_]
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Posts: 9
Default Why are a few timestamps missing?

Terje Mathisen wrote:

Terry Pinnell wrote:
I've been examining the GPS recordings of walks made on my iPhone. The
GPX files should of course contain one text line for each position
recorded, containing position, altitude and a timestamp. But
occasionally (less than 1%) the timestamp is obviously missing. The
4th line of this small extract shows an example:

trkpt lat="50.5573525932"

trkpt lat="50.5573393079" lon="-4.9283319153"ele22/ele/trkpt

trkpt lat="50.5573267350"

How does that flaw arise please? I can understand that for various
reasons, recorded trackpoints may be missing completely, or wildly
inaccurate. But how can some be without a timestamp?

Note that the two surrounding track points have just one second between
them: I'm guessing your logging app skips the timestamp when it is
identical to the previous one.

A very simple parser that ust picks up fields as it finds them will
remember the previosu timestamp and repeat it.


Thanks Terje, I'm sure that's the correct explanation. You may have
seen my recent posts...

Subject: Puzzling change in trackpoint rate
Date: Wed, 14 Sep 2016 20:35:43 +0100

....describing the unexplained huge increase in the trackpoint density
from my iPhone (using either of my two apps, Memory-Map or Pocket
Earth). Although my local Apple iStore says not, I conclude that there
must have been a radical change in the iPhone to cause this.

Here's a table illustrating that:

And here's a fragment of a track I recorded recently on the iPhone 6S+
(with the Pocket Earth app), which includes sections of walking,
standing and bus riding:

I'm still undecided as to my best approach. To restore a 'reasonable'
number of points, my only option seems to be to arbitrarily reduce
them. I may have missed it but I don't think GPSU's extensive list
under Tools can do that? But my desktop PC Memory-Map program has a
Reduce button that halves the number and I'll experiment with that. Or
use Excel, perhaps with a simple macro. Given that I have excessive
trackpoints, would it simply be best to delete any with no timestamps?

All extra unwanted work, whatever method used. Some may reckon short
intervals to be desirable, but so far I'm only seeing disadvantages.

Terry, East Grinstead, UK