Seems fairly quiet here but I'm hoping that one of the resident
experts can help me get to the bottom of some odd behaviour please.
I've tried several GPS, iPhone and iPad forums but still have no
satisfactory resolution.
I can't understand why my iPhone 6S+ recently started recording many
more trackpoints than previously: https://dl.dropboxusercontent.com/u/4019461/Trackpoints%20Comparison.jpg
Most of those have used the Memory-Map iOS app on my iPhone (plus a
few on my iPad) but I get the same averages with another app, Pocket
Earth. So it's app-independent, purely down to the iPhone. Here's a
track I recorded today (with Pocket Earth), which (unlike the examples
in my table, which were all walking) include sections of waiting and
bus riding too. https://dl.dropboxusercontent.com/u/4019461/GPS-TrkPts-Frequency.jpg
I'm still not sure if it's a good or bad thing! Unlike dedicated
devices, on this 128 GB iPhone I have no shortage of storage space. So
maybe I should be celebrating the unexpected availability of vast
numbers of trackpoints, instead of fretting about it!
But ... why would the rate have *changed* so greatly around early
July?
On 2016-09-14 15:35, Terry Pinnell wrote:
Seems fairly quiet here but I'm hoping that one of the resident
experts can help me get to the bottom of some odd behaviour please.
I've tried several GPS, iPhone and iPad forums but still have no
satisfactory resolution.
I can't understand why my iPhone 6S+ recently started recording many
more trackpoints than previously:
https://dl.dropboxusercontent.com/u/4019461/Trackpoints%20Comparison.jpg
Most of those have used the Memory-Map iOS app on my iPhone (plus a
few on my iPad) but I get the same averages with another app, Pocket
Earth. So it's app-independent, purely down to the iPhone. Here's a
track I recorded today (with Pocket Earth), which (unlike the examples
in my table, which were all walking) include sections of waiting and
bus riding too.
https://dl.dropboxusercontent.com/u/4019461/GPS-TrkPts-Frequency.jpg
I'm still not sure if it's a good or bad thing! Unlike dedicated
devices, on this 128 GB iPhone I have no shortage of storage space. So
maybe I should be celebrating the unexpected availability of vast
numbers of trackpoints, instead of fretting about it!
But ... why would the rate have *changed* so greatly around early
July?
Perhaps it updates less often when there is poor quality in the position >solution. Where do you have the phone when you're recording such
tracks? (pocket?). When the points were more numerous did you have it
in a better position? Was it hilly or urban canyons? (The later should
have good cell aiding, however).
Even sat geometry could have an impact at the time that you're recording
on the position quality.
Just a guess.
Many thanks Alan, much appreciate that comprehensive explanation.
I'm fairly satisfied with my distance measurements (after editing out
any serious errors by visual inspection). It's those gross ascent
descent estimates that vex me!
Walks (including holidays with my wife) are sometimes led, sometimes self-led. The former often include a briefing beforehand from the
leader along the lines of "I'll be leading the higher of the three
walks tomorrow, which will be 10 miles, 1500 ft of gross ascent and
1750 ft of gross descent." Leaving aside the obvious question as to
the source of his/her estimates, they do at least give us a rough idea
of expected difficulty based on past experience. And for our own
planned self-led walks I like to prepare my own estimates.
BTW, my walks never get repeated exactly. When they're close I do try
to make some comparisons.
Do you think that estimates based on digital elevation models, like
the one that Google Earth uses, or the UK's LIDAR model with 2 m
accuracy
https://data.gov.uk/data/search?q=lidar+2m
are more likely to give a more realistic GA/GD than recorded GPS data?
(BTW, no altimeter on the iPhone.)
I like the hat idea, although I suspect I'd quickly lose my iPhone!
Used to have my ancient Garmin strapped to my rucksack, so maybe I'll
try that again.
On 2016-09-26 13:47, Terry Pinnell wrote:
<S>
Many thanks Alan, much appreciate that comprehensive explanation.
I'm fairly satisfied with my distance measurements (after editing out
any serious errors by visual inspection). It's those gross ascent
descent estimates that vex me!
Walks (including holidays with my wife) are sometimes led, sometimes
self-led. The former often include a briefing beforehand from the
leader along the lines of "I'll be leading the higher of the three
walks tomorrow, which will be 10 miles, 1500 ft of gross ascent and
1750 ft of gross descent." Leaving aside the obvious question as to
Hmm - aren't you arriving where you departed?
the source of his/her estimates, they do at least give us a rough idea
of expected difficulty based on past experience. And for our own
planned self-led walks I like to prepare my own estimates.
BTW, my walks never get repeated exactly. When they're close I do try
to make some comparisons.
Do you think that estimates based on digital elevation models, like
the one that Google Earth uses, or the UK's LIDAR model with 2 m
accuracy
https://data.gov.uk/data/search?q=lidar+2m
are more likely to give a more realistic GA/GD than recorded GPS data?
Sure - but not sure how you'll integrate all that - though a coarse
estimate of longest ascending/descending paths should be close.
(BTW, no altimeter on the iPhone.)
Yes, there definitely is an altimeter on the iPhone 6S+. And various
apps read it. And the Health App should be able to estimate your climbing.
I like the hat idea, although I suspect I'd quickly lose my iPhone!
Used to have my ancient Garmin strapped to my rucksack, so maybe I'll
try that again.
As high up as possible.
Alan Browne <alan.browne@freelunchvideotron.ca> wrote:(BTW, no altimeter on the iPhone.)
Yes, there definitely is an altimeter on the iPhone 6S+. And various
apps read it. And the Health App should be able to estimate your climbing.
You're right, of course, what I meant to say was no altimeter used by
the iPhone GPS apps I use, Memory Map and Pocket Earth.
I like the hat idea, although I suspect I'd quickly lose my iPhone!
Used to have my ancient Garmin strapped to my rucksack, so maybe I'll
try that again.
As high up as possible.
FWIW, here's a screenshot of the GPS record of a short walk yesterday,
opened in GE. My iPhone was in a shoulder pocket. I'm guessing that
the great variation in the density of the trackpoints depends on how
open or wooded it is. There were no fewer than 2,166 points recorded
in this 4.3 mile walk.
On 2016-09-28 04:40, Terry Pinnell wrote:
Alan Browne <alan.browne@freelunchvideotron.ca> wrote:(BTW, no altimeter on the iPhone.)
You're right, of course, what I meant to say was no altimeter used by
Yes, there definitely is an altimeter on the iPhone 6S+. And various
apps read it. And the Health App should be able to estimate your climbing. >>
the iPhone GPS apps I use, Memory Map and Pocket Earth.
You can run several apps at once. As long as they work in BG, there's >probably something that can record the altitude profile. I use a
separate GPS (photologger) and it has altitude (GPS) as well as position
and so on.
I like the hat idea, although I suspect I'd quickly lose my iPhone!
Used to have my ancient Garmin strapped to my rucksack, so maybe I'll
try that again.
As high up as possible.
FWIW, here's a screenshot of the GPS record of a short walk yesterday,
No link.
opened in GE. My iPhone was in a shoulder pocket. I'm guessing that
the great variation in the density of the trackpoints depends on how
open or wooded it is. There were no fewer than 2,166 points recorded
in this 4.3 mile walk.
That's a point for every 3 metres of travel or about every 2 seconds at
a good walking speed.
I'm heavily committed to Mem-Map and my iPhone but anyway I don't know
of an iOS GPS app that records altimeter-based altitudes as well as
x,y positions. Nor one that reports gross asc/desc for a track, which
would be a useful supplementary estimate.
On 2016-09-25 03:16, Terry Pinnell wrote:
Alan Browne <alan.browne@freelunchvideotron.ca> wrote:
Perhaps it updates less often when there is poor quality in the position >>> solution. Where do you have the phone when you're recording such
tracks? (pocket?). When the points were more numerous did you have it
in a better position? Was it hilly or urban canyons? (The later should >>> have good cell aiding, however).
Even sat geometry could have an impact at the time that you're recording >>> on the position quality.
Just a guess.
Thanks Alan, and apologies for not acknowledging sooner.
There has been no change of those kinds at my end. Still carrying the
iPhone either in shirt pocket or (thin) trouser pocket. Same types of
terrain (mainly English countryside).
How about the satellite end? Could there have been some change there?
Not enough to matter. IAC the position solution on your phone is
composed of:
GPS (and SBAS P-R and corrections if in view)
GLONASS (and SBAS corrections if in view (not sure about P-R))
Cell Tower ranging
WiFi reception (if any - probably not out in the fields...)
So the various changes to the satellites (in/out of service, etc.) won't
show in user land unless you're into plate tectonics - and even then...)
--------------------
More important (as the very high rate seems here to stay) am I right
that this significantly increases estimates of gross ascent and
descent? Elevation profiles chart now look very much noisier. Should I
just increase the smoothing factor until they 'sort of look right'?!
Depends on how your ascent estimator is designed. If it's simply based
on delta-alt from the GPS and/or the altimeter in the iPhone it should
be pretty good. Smoothing 'til it looks right should be okay - 10
second time constant, I'd guess.
There is an app you can download called SensorLog which will record all >(most?) of the sensors in the iPhone to a file. (or o/p to an IP
address via WiFi - not practical for you...). Makes a CSV file that can
be easily processed or loaded into a spreadsheet for plotting and
processing.
On my tracks I don't care much about elevation (though maybe I should)
and when plotting a trail I usually have 6 or more recordings of it,
walking in each direction. In the woods, in winter, the variance is
easily 5 - 10 m. (No SBAS received either as at 45° N or so, in the
woods, the WAAS sats are too low; the 1 EGNOS sat I get is only when on
an open southeast facing hill)
Opinions vary widely on this topic, including the 'fractal' view that
you can regard just about *any* estimate as 'accurate'. (Distance as
"Fractal" - "Estimate" - "Accurate".
If you're looking for 5 m accuracy and the estimates have 10 metre >uncertainty then the fractal nature of the data (if so) doesn't matter a >whit, does it?
More useful to get a feel for the error by making repeated recordings on
a given trail or route. You won't get the truth, but you'll get a range.
With many recordings you can plot the middle (eyeball it or write some
code - the later is much harder to do than say) and get a feel for the
errors in different situations. Open field, good antenna position,
receiving SBAS ... expect a couple m most of the time; in dense woods,
north side of the mountain (northern hemi), expect 20 - 50 m error.
well as cumulative ascent/descent.) But I haven't given up on trying
to come up with reasonably realistic estimates, so that helpful
comparisons can be made, both about walks done and planned.
Any position estimate has errors. When I plot trails, I do it in the
winter when the leaves are down and I put the receiver under my tuque so
that there is minimal blocking of the satellites. This tightens up the
error considerably - esp. if SBAS is received.
If you have your iPhone in your shirt or trouser pocket, you are denying
the receivers a huge part of the sky as the signal will not go through
your body - the errors will be larger than if you can get that iPhone up >above your head where it can see as much sky as possible, all of the
time. I'd bet that would calm down your elevation plot a bit too.
Note that smartphone sat receivers are excessively sensitive. When you
put the receiver in a pocket, it still gets the signal at a very
suppressed level. Noisy - therefore less accurate. Further, it would
be very prone to multipath signals bouncing off rocks and perhaps trees.
That of course makes for more error.
So, if accuracy is important, get a hat, put the iPhone in a ziplock bag
and stuff it in the hat and wear it.
Locus Map does all this, albeit only on Android. It can even
fill in the altitudes after the fact, using the SRTM (Shuttle
Radar Topography Mission, http://jpl.nasa.gov/srtm/ ) data, if
you put that on the phone. I have done exactly this to try it.
It works.
On Thu, 29 Sep 2016 13:25:11 +0200, Hans-Georg Michna wrote:
Locus Map does all this, albeit only on Android. It can even
fill in the altitudes after the fact, using the SRTM (Shuttle
Radar Topography Mission, http://jpl.nasa.gov/srtm/ ) data, if
you put that on the phone. I have done exactly this to try it.
It works.
Correction: It does this even if you do not put the SRTM data
onto the phone first. It auto-loads the relevant data segment
automagically.
If you like your track to be enhanced with altitude values, send
it to me, I will do it on my phone and send the result back.
You will have to do a small and obvious correction to my email
address, by removing the NoEmailPlease.
Hans-Georg
Terry Pinnell wrote:
Thanks Hans, that's very kind of you. But I have so many such tracks
that it would be impractical.
Have you considered Gogle Earth (Pro is free now)?
I know that it by default will import GPX tracks and lock them to local >ground, so if you can import a bunch of tracks you should get altitude
data for all of them at once.
What I don't know is if the resulting KML/KMZ files will store that
altitude data or not, if they do then gpsbabel would be able to convert
them back to GPX.
Track point rates are typically controlled by an algorithm which tries
And any thoughts on the cause of the subject under discussion?
to minimize the number of points needed to generate a given
approximation to the raw (once per second or more) GPS measurements.
I.e. you store a track point each time the direction and/or speed changes.
I know that this is the way Garmin have handled it for many years, but
when I collect tracks of my orienteering races or when surveying new
paths, I always configure the GPS to a fixed 1 Hz logging rate.
A change on the same device without any reconfiguration on your part is >probably due to an update from the vendor which have tweaked the
filtering algorithm.
Alan Browne <alan.browne@freelunchvideotron.ca> wrote:
Perhaps it updates less often when there is poor quality in the position
solution. Where do you have the phone when you're recording such
tracks? (pocket?). When the points were more numerous did you have it
in a better position? Was it hilly or urban canyons? (The later should
have good cell aiding, however).
Even sat geometry could have an impact at the time that you're recording
on the position quality.
Just a guess.
Thanks Alan, and apologies for not acknowledging sooner.
There has been no change of those kinds at my end. Still carrying the
iPhone either in shirt pocket or (thin) trouser pocket. Same types of
terrain (mainly English countryside).
How about the satellite end? Could there have been some change there?
--------------------
More important (as the very high rate seems here to stay) am I right
that this significantly increases estimates of gross ascent and
descent? Elevation profiles chart now look very much noisier. Should I
just increase the smoothing factor until they 'sort of look right'?!
Opinions vary widely on this topic, including the 'fractal' view that
you can regard just about *any* estimate as 'accurate'. (Distance as
well as cumulative ascent/descent.) But I haven't given up on trying
to come up with reasonably realistic estimates, so that helpful
comparisons can be made, both about walks done and planned.
I can't understand why my iPhone 6S+ recently started recording many
more trackpoints than previously:
Thanks Hans, that's very kind of you. But I have so many such tracks
that it would be impractical.
And any thoughts on the cause of the subject under discussion?
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