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sci.geo.satellite-nav (Global Satellite Navigation) (sci.geo.satellite-nav) Discussion of global navigation satellite systems (GNSS). Topics include the technical aspects of GNSS operation, user experiences in the use of GNSS, information regarding GNSS products and discussion of GNSS policy (such as GPS selective availability).

Why are a few timestamps missing?



 
 
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  #1  
Old October 12th 16, 07:31 AM posted to sci.geo.satellite-nav
Terry Pinnell[_3_]
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Posts: 9
Default Why are a few timestamps missing?

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="50.5573593826"
lon="-4.9283146486"ele22/eletime2016-08-03T09:57:59Z/time/trkpt

trkpt lat="50.5573615199"
lon="-4.9283183366"ele22/eletime2016-08-03T09:58:06Z/time/trkpt

trkpt lat="50.5573525932"
lon="-4.9283242878"ele22/eletime2016-08-03T09:58:07Z/time/trkpt

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

trkpt lat="50.5573267350"
lon="-4.9283414707"ele22/eletime2016-08-03T09:58:08Z/time/trkpt

trkpt lat="50.5573160900"
lon="-4.9283459131"ele21/eletime2016-08-03T09:58:10Z/time/trkpt

trkpt lat="50.5573066185"
lon="-4.9283490144"ele22/eletime2016-08-03T09:58:11Z/time/trkpt

trkpt lat="50.5573010445"
lon="-4.9283564743"ele22/eletime2016-08-03T09:58:12Z/time/trkpt

trkpt lat="50.5572247692"
lon="-4.9284306541"ele21/eletime2016-08-03T09:58:20Z/time/trkpt

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?

--
Terry, East Grinstead, UK
  #2  
Old October 12th 16, 12:14 PM posted to sci.geo.satellite-nav
Terje Mathisen[_3_]
external usenet poster
 
Posts: 25
Default Why are a few timestamps missing?

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"
lon="-4.9283242878"ele22/eletime2016-08-03T09:58:07Z/time/trkpt

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

trkpt lat="50.5573267350"
lon="-4.9283414707"ele22/eletime2016-08-03T09:58:08Z/time/trkpt

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.

Terje
--
- Terje.Mathisen at tmsw.no
"almost all programming can be viewed as an exercise in caching"
  #3  
Old October 12th 16, 02:17 PM posted to sci.geo.satellite-nav
Terry Pinnell[_3_]
external usenet poster
 
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"
lon="-4.9283242878"ele22/eletime2016-08-03T09:58:07Z/time/trkpt

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

trkpt lat="50.5573267350"
lon="-4.9283414707"ele22/eletime2016-08-03T09:58:08Z/time/trkpt

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.

Terje


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:
https://dl.dropboxusercontent.com/u/...Comparison.jpg

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:
https://dl.dropboxusercontent.com/u/...-Frequency.jpg

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
  #4  
Old October 13th 16, 01:14 AM posted to sci.geo.satellite-nav
Alan Browne
external usenet poster
 
Posts: 1,339
Default Why are a few timestamps missing?

On 2016-10-12 02:31, 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="50.5573593826"
lon="-4.9283146486"ele22/eletime2016-08-03T09:57:59Z/time/trkpt

trkpt lat="50.5573615199"
lon="-4.9283183366"ele22/eletime2016-08-03T09:58:06Z/time/trkpt

trkpt lat="50.5573525932"
lon="-4.9283242878"ele22/eletime2016-08-03T09:58:07Z/time/trkpt

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

trkpt lat="50.5573267350"
lon="-4.9283414707"ele22/eletime2016-08-03T09:58:08Z/time/trkpt

trkpt lat="50.5573160900"
lon="-4.9283459131"ele21/eletime2016-08-03T09:58:10Z/time/trkpt

trkpt lat="50.5573066185"
lon="-4.9283490144"ele22/eletime2016-08-03T09:58:11Z/time/trkpt

trkpt lat="50.5573010445"
lon="-4.9283564743"ele22/eletime2016-08-03T09:58:12Z/time/trkpt

trkpt lat="50.5572247692"
lon="-4.9284306541"ele21/eletime2016-08-03T09:58:20Z/time/trkpt

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?


You could ask the app provider. Other than that use previous,
interpolate, tolerate or reject.

Terje points out the time difference issue, and that is probably the
best explanation. In non-critical processing that is sufficient unto
the day as long as the positions remain close.

--
She hummed to herself because she was an unrivaled botcher of lyrics.
-Nick (Gone Girl), Gillian Flynn.
 




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