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How accurate is the iPhone GPS? - Page 2

post #31 of 53
Quote:
Originally Posted by sibhusky View Post

The speed thing? For racers, maybe, then get a friend with a radar gun or whatever it is. And do it on a closed course.

 

Amen. Get just plain and simple angry when I see someone going by me at what is probably 45 mph (they'll recall it over dinner as 60) on an open recreational blue or green run (and it's always blue or green because only elite racers can handle the G forces of turning on steeps at those speeds, and your average I'm-so-fast idiot runs flat in a Jersey Tuck anyway). So if you think you're all that, either go race on a closed course or hit an honest steep black on an icy day and find out. 

post #32 of 53
Thread Starter 

I think blue trails are the most dangerous place to be.  I was taking a run with my daughter and  I had someone fly by me in a wedge, and so much in the back seat, that his tips were off the ground.  It ended up being a "yard sale" for that skier several hundred yards below us.

 

I feel safer on a black trail, where for the most part, I know what I can expect from the less numerous other skiers. 

 

But this is another thread!   wink.gif

post #33 of 53
Quote:
Originally Posted by Tog View Post

 

What's "SA"?

 

Speed reached according to the app: 75mph!!

That guy tucked with his hands held apart about torso width. (They weren't exactly racers)

Totally absurd. I'd give them 55 maybe, but after reading BootTech's info, maybe that's optimistic.

SA ==> Selective Availability

 

The US military designs & builds the GPS system and satellites.( No, Al Gore did not invent the Internet or GPS roflmao.gif)

 

 

SA is deliberate scrambling (small variable amounts depending on threat levels) of the GPS data so the enemy can't use it to

accurately guide aircraft or missiles.

 

Officially SA has been turned off since May 2000.  Unofficially who believes the govt!

If you're not sorry you asked, here's more on this and other GPS topics:  http://www.gps.gov/systems/gps/modernization/sa/

post #34 of 53
Quote:
Originally Posted by SkiMangoJazz View Post

You'd need to be connected to a minimum of 3 satellites to triangulate and probably would need 5 connections to get altitude accuracy not to mention speed accuracy with altitude changes. Maybe a high quality GPS unit could do that, but asking that of an iPhone is too much. I find it very bad just with altitude, forget about speed.  I've used my altimeter watch and Ski Tracks on the same day on a hill that I know the vertical feet of.  The watch is pretty accurate as to accumulated vertical feet descended.  The iPhone was so far off as to be completely useless.

 

Comparing iPhone linked with GNS5870 Blue Tooth GPS and just iPhone tested at Whistler, the tracking on both was very accurate via Ski Tracks.  Looking at the Google Earth view of the runs, they were exactly where we went.  iPhone has also compared well with SatSki's dedicated GPS device.  I gave up the SatSki due to the accuracy of the iPhone/SkiTracks.

 

One might see individual anomalies but overall GPS tracking on the iPhone is accurate.

post #35 of 53

The tracking of where you are is quite accurate, I'm talking about it's ability to track altitude, not latitude and longitude.  Ski Tracks shows vertical feet descended.  It tracked me as descending 600 feet when I'd descended 1,000 for example.

 

Satellites do great on where you are, just not very well on on your altitude.

 

Superimpose the lat/long on Google Earth and it can read the altitude, but getting that info from the satellites is the weakness.

 

imo.

post #36 of 53
Quote:
Originally Posted by SkiMangoJazz View Post

The tracking of where you are is quite accurate, I'm talking about it's ability to track altitude, not latitude and longitude.  Ski Tracks shows vertical feet descended.  It tracked me as descending 600 feet when I'd descended 1,000 for example.

 

Satellites do great on where you are, just not very well on on your altitude.

 

Superimpose the lat/long on Google Earth and it can read the altitude, but getting that info from the satellites is the weakness.

 

imo.


 The instantaneous altitude is accurate to within feet when we check, stopping at spots on the mountain with posted altitude.  The altitude graph on runs, top to bottom looks accurate matched against the known altitudes of the areas skied via the Google Earth map.

 

Granted I run mine with linked to the GNS GPS but when compared to those skiing with me using iPhones or Androids, the reading are very close and my simultaneous readings of iPhone and SatSki's dedicated GPS were also equal.

 

The GNS GPS was $80 if someone wants to be a real stickler for accuracy (why I have it) but the iPhone itself does a good job.

post #37 of 53

That's interesting.  I have only anecdotal evidence myself as I gave up on Ski Tracks quickly after the bad results I got from it.

 

From what you say altitude accuracy is not a big problem. What do you think about speed?

post #38 of 53

But you don't know if it is taking readings every 1 second, every 2 seconds, every tenth of a second, etc., do you??  If you do, let us know how often it is taking readings.  And given that time period and the possible lateral error rate, how accurate could the speed possibly be?  That is what the original question was.  If it takes it every 10 seconds, it will be assuming you are running in a straightline, and measure distance A to B, time A to B, then come up with a speed.  The distance error % may be less of an issue, but the deviation from a straight route will be bigger.  If it is taking it every 1/100th of a second, the need for a straightline route is reduced, but the possible % error due to actual location (six inches southwest of some latitude and longitude or whatever) is going to be huge.  Is it only taking ONE reading to come up with "max" or is it averaging 25 readings?  If you think it is accurate, please explain how you know this.  Finding you STANDING STILL is one thing, and I have yet to see a GPS that will even SHOW YOU STAYING PUT if you zoom in and watch for a while.  My tablet, our SPOT device, our DeLorme GPS and our Garmin GPS all have drift.  Now factor in movement in a zigzap pattern.  

 

There are augmentation systems in use that help the vanilla GPS (http://www.gps.gov/systems/gps/performance/accuracy/).  But I don't pretend to know what is in the iPhone in this regard.  I say, put six of them in your vest, station someone with a radar gun, run multiple tests during various weather and terrain situations and report back to us.  

post #39 of 53

There are discussions on the Apple forum about the accuracy of the GPS in the iPhone 5.  Didn't read the whole thing, but they talk about lack of WAAS, time to identify current location, etc.  https://discussions.apple.com/thread/4288839?start=15&tstart=0  Searching various other discussions shows about 5 METER accuracy.  Fine if you are standing still or going in a straight line, pretty useless if you are descending steep terrain and making turns.  

post #40 of 53
Quote:
Originally Posted by SkiMangoJazz View Post

That's interesting.  I have only anecdotal evidence myself as I gave up on Ski Tracks quickly after the bad results I got from it.

 

From what you say altitude accuracy is not a big problem. What do you think about speed?

 

Speed seems to get a rare anomaly.  Out of my 29 Ski Tracks records this year, I have one day showing a peak of 59.3 mph.  The rest more reasonable 35-50 mph so maybe 1 out of 29 you might see a reading that seems off.  Then again that was Park City day skiing with PCST alumni who go fast so maybe it was right.

 

Most consistent test was iPhone/Ski Tracks vs. the SatSki dedicated GPS for a season.  Occasional and minor differences.

post #41 of 53
An additional issue that nobody brought up is you are in the mountains. The signals can reflect off the rocks which can lead to bad data much more than flatland. Especially if you are calculating speed, where the reflected signal can lock on for a period of time rather then getting averaged out or thrown out by software.

Radar gun, or do out the math for the run using stopwatch and distance youve plotted.
post #42 of 53
Quote:
Originally Posted by raytseng View Post

An additional issue that nobody brought up is you are in the mountains. The signals can reflect off the rocks which can lead to bad data much more than flatland. Especially if you are calculating speed, where the reflected signal can lock on for a period of time rather then getting averaged out or thrown out by software.

 

Doesn't seem to affect GPS, SatSki's, GNS or iPhone.  The SatSki always had a fix on five or six satellites whenever I looked.   Might be a problem in a deep, narrow canyon but skiing areas are likely too open with access to many different satellite angles.

post #43 of 53
Quote:
Originally Posted by Eagles Pdx View Post

 

Doesn't seem to affect GPS, SatSki's, GNS or iPhone.  The SatSki always had a fix on five or six satellites whenever I looked.   Might be a problem in a deep, narrow canyon but skiing areas are likely too open with access to many different satellite angles.

 

It does, do remember that the number of satellites does not necessarily matter, it is moreso the location of the satellites. You can have 5 fixed, but if they are all viewing the user from the same exposure, it doesn't matter because each one will be getting a similar reading. This is usually the case when you are on a mountain, as roughly 30-40% of the sky is blocked by the mountain itself.

post #44 of 53
Quote:
Originally Posted by sibhusky View Post

But you don't know if it is taking readings every 1 second, every 2 seconds, every tenth of a second, etc., do you?? 

 

Skitracks on android shows me skiing in diagonal zigzags down the hill. I can only conclude it's taking readings every second or so. 

post #45 of 53

I have a Garmin GPS/HRM on my bike.  One day I averaged over 40 mph on my lunch time ride and my familiar route was suddenly several miles longer.  When I looked at the route later there was a point where it showed I went several miles off to the left and instantly returned.  Personally I think all GPS units are pretty suspect.  I've had times when my phone was seeing more than 10 satellites but not using any and only rebooting the phone, sometimes more than once, would make it start actually using the satellites.

post #46 of 53

The standard IOS API ("LocationManager") does not provide the developer direct access to the raw data from the GPS chip. Rather it returns location information based on GPS, Cell triangulation, WiFi and (possibly) other sensors (like the accelerometer).

 

The LocationManager can also return the device's "speed", which "Represents the speed at which the device is moving in meters per second".  (emphasis mine) While I'm not certain of the underlying details, it's quite conceivable that an instantaneous speed reading of the device could differ significantly from the top speed as measured by subsequent data points on a GPS plot.  Because LocationManager is using multiple sensors it may use (but not sure) the device's accelerometer as part of the "speed" reading. In which case you would expect fairly wild swings from a device in a sleeve pocket in a skier's jacket (as opposed to a device mounted in a moving car).

 

So if the app takes advantage of this reading in the core location services of IOS (and does not compute it based on subsequent data points) I absolutely would not rely on the max speed reading from the app itself.  The data is very likely precise ... but not likely accurate :)

 

Bottom line ... unless you know more about how the app is coded, I wouldn't rely on some of the readings .. particularly max speed.

 

To be fair, I've tested the iPhone against a dedicated Garmin GPS for hiking/trail running and found the iPhone's plots to match up very well against the Garmin.  There are some issues with dropped track points when in dense woods or partially obstructed sky views as the Garmin's receiver is much more sensitive than the iPhone's, but overall a GPS app on the iPhone is pretty accurate in terms of track, distance and elevation over a longer course.

post #47 of 53
Quote:
Originally Posted by Rise To The Top View Post

 

It does, do remember that the number of satellites does not necessarily matter, it is moreso the location of the satellites. You can have 5 fixed, but if they are all viewing the user from the same exposure, it doesn't matter because each one will be getting a similar reading. This is usually the case when you are on a mountain, as roughly 30-40% of the sky is blocked by the mountain itself.


Not according to the dedicated SatSki GPS.  I think it is fair to assume the GSN GPS would have similar good performance. The iPhone does not show the satellites but the metrics it delivers are the same as the GPS.  The resulting Google Earth maps very accurately show where the person skied.

 

Actual performance on the mountain says iPhone and GPS work fine.

post #48 of 53
Yeah, that's why we've got so many people telling us they clocked 50 to 70 mph.. Please.
post #49 of 53
Quote:
Originally Posted by sibhusky View Post

Yeah, that's why we've got so many people telling us they clocked 50 to 70 mph.. Please.

It's why you have so many peope reporting a speed anaomaly so infrequently, one out of thirty times.
post #50 of 53

I have a bunch of experience using various GPS units for offshore fishing but only recently became acquainted with SkiTracks on my iPhone. Without going into the discussion of speed accuracy or lack thereof (I can understand why there are lots of possible causes for anomalies), I have to say I was pretty impressed by the accuracy of the altitude readings. Multiple times during trip to CO last week I checked the instantaneous read-out vs published altitudes at various points on various mountains. Most impressive was the reading at top of Imperial Express lift at Breck. There's a sign there advertising 12,840 elevation, SkiTracks reported 12,838 - that's a mighty small percentage difference from a basically free GPS!

 

btw, I'm a Philly native - go Birds!

post #51 of 53
The Ski Tracks user manual states: "Our current tests show that our accuracy is about: Speed +/- 5% Distance +/- 5% Altitude generally +/- 50 meters Slope Angle +/- 4% Obviously different condition and particularly GPS in general has inherent inaccuracies and although we have strived to improve this using filtering and other techniques accuracies can vary depending on the quality of locations."

5% seems about right in my experience skiing with it and testing it in the car a couple times. No consumer GPS device will be perfect, but Ski Tracks is close enough for my needs. When I play back a run on Google Earth, I can easily see places where I crossed over from the left side to the right side of a run, or where I dropped into a bowl.
Altitude is tricky, and their claim of 50m (150 ft) accuracy seems close. My friends and all all get slightly different readings at the top of the lift, but within about 100 feet of each other, maybe as much as 200 feet difference -- all pretty close to the official altitude posted at the top of the lift. It helps if you calibrate your phone's GPS before use. On the iPhone you do this by making repetitive figure eights while holding the phone in landscape mode.
Another thing to consider: the faster you're moving, the less accurate the speed measurement will be simply due to the sampling intervals. To save power, the app only wakes up the GPS every so often -- maybe less than once per second (they don't say). So the faster you go, the more distance you travel horizontally & vertically between samples, and that will make the speed data noisier.
post #52 of 53
Quote:
Originally Posted by AZskidogg View Post

The Ski Tracks user manual states: "Our current tests show that our accuracy is about: Speed +/- 5% Distance +/- 5% Altitude generally +/- 50 meters Slope Angle +/- 4% Obviously different condition and particularly GPS in general has inherent inaccuracies and although we have strived to improve this using filtering and other techniques accuracies can vary depending on the quality of locations."

 No consumer GPS device will be perfect, but Ski Tracks is close enough for my needs. When I play back a run on Google Earth, I can easily see places where I crossed over from the left side to the right side of a run, or where I dropped into a bowl.

 

Agree.  I add in a full bore GPS Bluetoothed to the iPhone to improve it further.  I'm sure it does but when comparing same day with someone just using iPhone, the iPhone results were almost identical.

post #53 of 53

This is the second season that I have used skitracks and the first time I clocked over 90mph (147Km) on the first run NOT IN YOUR LIFE POSSIBLE,  All other runs hit the approximate correct speeds strangely enough.

 

Thinking back at the run, it occurred at a little drop of point. where the horizontal speed changed to a rapid vertical speed and the software must have miscalculated the speed here because of the sample rate.

 

Other wise a neat app.

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