Informal GPS Logger Test: iPhone 4s GPS is Shockingly Good
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Tale of Three Tracklogs
on a three-hour stroll in Kyoto, Japan

In the comments of a recent post about GPS receivers, it was suggested that the GPS receiver in the iPhone was useful for keeping tracklogs. I had bad experiences with iPhone location services when I tested in 2009, but perhaps my test wasn't good, or perhaps the old iPhone 3 wasn't good, so I thought I'd give it another try.

So, the other day I took three GPS receivers with me while I did some errands. As I'm apt to do lately, I walked.

This post is a comparison of the resulting tracklogs.

Coming and Going
Garmin eTrex Legend HCx  ·  Garmin eTrex 20  ·  black is iPhone 4s

The three devices I took along on my unscientific test:

  1. Garmin eTrex Legend HCx that I bought in 2007. It supports GPS with WAAS for increased accuracy. It has a slate-blue outer shell, so its tracklog gets a blue line on the plot.

  2. Garmin eTrex 20 that I bought in Fall 2011 (product page at Garmin). Garmin has adjusted the hardware/software to make it even more troublesome to use than its predecessor, which is quite a feat, but on the plus side it not only contains a GPS/WAAS receiver, but also a GLONASS receiver, to tap in to Russia's own independent version of America's GPS. The theory is that more satellites in the sky means more satellites likely visible to the unit in any situation, and so better accuracy. I got the orange one, so its tracklog's plot is orange.

  3. iPhone 4s with the MotionX GPS app. This is a GPS (update: and GLONASS!) receiver, likely(?) without WAAS. My iPhone is black, so its tracklog gets a black line.

The results from the iPhone was much better than I expected, and the others were worse than I expected. In the short plot segment above, which shows both the start and end of my walk, the iPhone is the closest to where I actually walked, including crossing the street toward the top of the image.

Garmin eTrex 20 is best along here
I walked along the north side of the street

All-Around Fail

iPhone Least Bad of the Three

The place I'd planned to lunch was closed, so I wandered around until I found a place, at the center red dot in the next plot:


While there, I sat about two yards from a floor-ceiling window, and the two Garmin units reported accuracy as good as 10 meters, but the scatterplot above shows that they had no clue.

The more modern eTrex 20 perhaps had a better clue, because while in the restaurant it generally reported less accuracy than the older eTrex Legend HCx:

±16m     ±10m
both wrong, but the eTrex 20 less so

Again the Legend HCx is off


I spent a few minutes inside a building where I had poor-to-no reception, but instead of realizing the lack of reception and pausing the tracklog, the Legend HCx gave incorrect readings scattered across a span of 310 meters. The iPhone mostly realized that it had no signal, and its scatter was limited to 80 meters. The eTrex 20's scatter was about 60 meters.

For while the Legend HCx just drifted off, at one point more than 200m off, as illustrated by the outward bow in the blue line at the left of the overall plot, which I'll repeat here:

Legend HCx Going Crazy

I've never seen it show a suspected accuracy that bad, so it wouldn't surprise me if it was showing its normal accuracy of 3m-10m while it was actually off by 200m, but unfortunately I wasn't looking at it while walking, so I don't know what it thought its accuracy was.

Pedestrian Overpass

In the plot above, I crossed the street from north to south, then doubled back a bit on the pedestrian overpass (just to the left of center), to check out the view from above the street. Again, the iPhone seems the most accurate here, though the eTrex 20 at least makes it look like I used the crosswalk.

Not Too Bad Down This Stretch
but again, the iPhone is best

Cutting Corners
again the iPhone is best

Leaving Yodobashi Camera
it all looks crazy, but the iPhone plot is by far the best

More iPhone Bestness
I walked along the south side of the street

Heading Down to the River
eTrex 20 seems best

Walking Under a Bridge

The iPhone track looks best, but in this case it might be a side effect of laying down a plot point less often. The two Garmin units were set to log locations every second, but the app I used on the iPhone seems to have logged a location every six or seven seconds, which is just about the time it took to pass under the bridge.

Pretty Tight
the iPhone's less frequent updates smooths out the jaggies

Just Crazy
iPhone wins again


I don't know how far I actually walked, but I can tell you what each tracklog tells me about my three-hour stroll:

  • The iPhone 4s tracklog contains 1,163 data points, and the distances add up to 11.8km.

  • The eTrex 20 tracklog contains 9,973 data points, and the distances add up to 12km.

  • The eTrex Legend HCx tracklog contains 10,428 data points, and the distances add up to 13.8km.

Remember, the older eTrex Legend HCx kept reporting a location even when it should have realized it had no idea where it was, jumping all over the place while I was in a building, and all that jumping added up to more “distance” and more data points. The other units were smarter in that they knew better when the accuracy was not good enough to count on.

Both Garmin units were powered on for the same amount of time and had the same 1-second logging frequency, so they should in theory contain the same number of data points, but the eTrex 20 recorded 455 fewer, representing seven and a half minutes where it choose to not record a location (because I was inside and it didn't have a good signal) while the Legend HCx continued recording its incorrect location.


Due to the nature of its design, the GPS system is not as accurate with altitude as with latitude and longitude, and on some Garmin units this is exacerbated with a worthless barometric barometer. I've learned to avoid those, so none of my units now have that, so the altitude in today's three plots is all from GPS (or, in the case of the eTrex 20 (and the iPhone!), both GPS and GLONASS).

It's almost comical how random it seems. Here are the elevation plots from each tracklog, along with an extra one for the elevation that Google Earth has on file:

Google Earth

Garmin eTrex 20 (GPS + GLONASS, 2011)

Garmin eTrex HCx (GPS, circa 2007)

iPhone 4s (GPS, circa 2011)
Google Earth
Garmin eTrex Legend HCx
Apple iPhone 4s
Garmin eTrex 20

The Google Earth data seems closest to reality, except for the odd hump in the center (Google thinks there's a hill just north of Kyoto Station). The elevation in Kyoto slopes down from the north, and you can see in the Google Earth plot that I'm mostly level as I walk east to west, then slopes down as I approach Kyoto Station, then at the end climbs steadily as I walk north along the river.

The other plots are just ridiculous. (According to the eTrex 20, at one point I was at 225m elevation, and according to the Legend HCx I was at one point walking 0.68 mach!)

In the end, I'm dismayed at how bad my old Legend HCx was, and how not-much-better the eTrex 20 is, especially for having twice satellites available to it.

But most of all I'm shocked at how good the iPhone 4s results are... even judging from this one simple test, it's clearly good enough in outdoor use for geoencoding photos.

All 21 comments so far, oldest first...

Very interesting. Let’s hope that police departments never use this data to decide if one was alcohol/drug impaired.

“Ossifer, I’m not drunk, it’s my internal GPS driffing…” —Jeffy

— comment by Grandma Friedl, Ohio, USA on January 15th, 2012 at 2:01am JST (5 years, 11 months ago) comment permalink

I use Motion X on my iPhone (just the 4 version but similarly good GPS) for this exact reason. Works better than my Garmin in my experience. Plus motion X allows you to predownload maps for free (their top maps are great for my sort of outings) that work in areas without cell coverage. I also use topo! maps (free iPhone app that pinpoints you on USGS topo maps) and national parks maps(shows you on the free national park maps) but those are obviously only useful in the US.

— comment by Jao on January 15th, 2012 at 4:00am JST (5 years, 11 months ago) comment permalink

Thanks a lot for this quite interesting comparison. Never thought that the iphone could be better than a Garmin. How did you carry the devices ?


Threw them into a sling pouch (“man purse” that looks like a mini version of a bicycle-messenger’s bag). —Jeffrey

— comment by Michael on January 15th, 2012 at 4:38am JST (5 years, 11 months ago) comment permalink

Jeffrey, this is fascinating!
I sort of “knew” about this location finding business of the smart phones from that great film “Enemy of the State”.
But in a safe and friendly world which we seem to be living in right now, where big brother does not want to kill us or lock either of us up its great isn’t it.
How long did it take you to plot your walking trek course data against the photos? Is it a simple app trick or do you have to be a bit of a geek to do the plotting?
It looks like you had a lot of fun!!

When I return from a photo trip, I copy the tracklog from the GPS unit to my computer (10 seconds) and use my geoencoding-support plugin for Lightroom to match the photos up to the tracklog… another 10 or 20 seconds. That’s all. The match up happens automatically by photo/datapoint time. —Jeffrey

— comment by Annie in London on January 15th, 2012 at 7:14am JST (5 years, 11 months ago) comment permalink

I had heard about using the iPhone as a GPS tracker before. It’s interesting to see just how well it did! I’ve heard that using it as a tracker tends to drain its battery fairly quickly – did you observe that as you were using it?

I have a fairly cheap GPS logger that I’ve been using for the past year, an AMOD AGL-3080. It gets the job done well enough, but my tracks tend to look like the one from your eTrex loggers. Even with a clear signal, if I’m standing still for any period of time (which tends to happen frequently, as I engage in bird photography), the tracks tend to go all over the place.

So, question for you: I’ve heard of software that makes for easy editing of GPS logs. In particular, I’ve heard that some software automates the editing process by cutting out lines if they don’t meet certain parameters (such as if the distance covered per unit time is much higher than you’d be capable of by foot – as occurs if the signal goes wild). Do you use any such software? Any recommendations?

I use my tracklogs verbatim, but perhaps I should. I believe that GPS Babel can tidy up tracklogs. As for battery, the phone was down to about halfish after the three hours, which was better than I expected. —Jeffrey

— comment by David K. on January 15th, 2012 at 7:58am JST (5 years, 11 months ago) comment permalink

How much of this is as a result of the urban environment? I don’t know much about Kyoto but could the buildings be affecting the signal? I’d be interested to see similar tests for open country side and maybe the bamboo area you have posted images from.

The long walk up the river toward the end is pretty wide open, though the bank does rise up 5m on the right, so maybe not the best test afterall. There are few really tall buildings in Kyoto, so I wasn’t in any major “valleys”, though some mild ones near Kyoto Station. —Jeffrey

— comment by Chris on January 15th, 2012 at 8:46am JST (5 years, 11 months ago) comment permalink

Another interesting post here.
Does the battery of eTrex 20 really last 25 hours as it is stated in the spec? wondering 4s might be the least durable… so eTrex could be the main recorder or at least the backup.

I don’t measure the time it’s on, but it seems to last pretty well. I keep a spare set of batteries in my camera bag, and have to dip in every so often, but I can go quite a while in between, so 20-25 hours doesn’t seem unreasonable. —Jeffrey

— comment by nodust on January 15th, 2012 at 10:13am JST (5 years, 11 months ago) comment permalink

The iPhone 4S also has GLONASS support.

Holy cow, you’re right! I just checked the specs at Apple Japan, and there it is. Wow. So why can’t my dedicated GLOSNASS-enabled Garmin unit do as well!? —Jeffrey

— comment by Brad Mohr on January 15th, 2012 at 11:36am JST (5 years, 11 months ago) comment permalink

For the Android users amongst the audience, the My Tracks app works well. It’s designed for exercises (joggers, walkers and I guess cyclists) to find out their average speed, etc. but it creates tracks that can be exported in various formats as well. On a recent visit to Japan, I used it when my on-camera GPS unit failed and it worked well even without assistance from the phone towers (as the Japanese phone companies were charging nearly the value of the phone to rent a SIM for two weeks!). The tracks followed the roads I walked down very accurately; that was on a Samsung Galaxy S2.

— comment by David on January 15th, 2012 at 7:23pm JST (5 years, 11 months ago) comment permalink

Your Garmin tracks are typical to what I’ve gotten in urban environments (and some not-so-urban) with 2-4 story buildings. I used to have multiple GPS Babel batch files on my desktop that I could drag/drop my tracks onto which would simplify and remove the high error hdop and vdop points when I used my AMOD AGL-3080 (same as David K) which recorded the raw NMEA signals from the satellites. Very useful for removing those spikes in the track like the blue point to the right in the center of “Pause” above. That’s likely one of the points that caused your .6 mach, jumping from one side of the street to the other and back in one second. Those batch files unfortunately got dumped with a poorly timed hard-drive crash. Not that it matters anymore, as I’ve mentioned in the LR4 Geoencoding post, my AGL-3080 got mistakenly left in the UK and I’m now in Korea.

It’s also unfortunate that the Garmin records in GPX and removes all that extra info so you’re left with what you see. Smoothing is with GPS Babel is possible at the cost of some of the 1-second interval points, so finding the right “fuzziness” factor in your plugin can take some trial and error. I was going to ask if you geoencode your photos from your tracks as-is or if you go back through and tweak to get them as accurate as possible (via your Google Earth target) as I do, but you’ve answered before I could ask!

The few times I’ve used my Samsung Galaxy S2 phone (6-months old) for tracks, it’s been reasonably accurate for a phone, but the battery drain is probably double what you’ve experienced with the 4S. I think I’d get about 3 hours total use before the phone shut down… if not from the loss of power then from overheating. GPS on warms up the battery fast!

For David K –
You can always switch the firmware on your AMOD logger to the Static Navigation ON version from AMOD’s website:
It’s very good at removing the spaghetti-like mess from those tracks, especially when out and about in the field. The downside is that it’s not very good when walking as it puts a 1-2 meter per second threshold on the logger. As long as you’re moving faster than that, it records. Slower and it assumes you are standing still and doesn’t add more points to the track. So, a slow walk will produce a very jagged track as it thinks you’re standing still and then it suddenly records what it considers an accurate point many meters away and jumps to that point. Using it when running or in a car, I got decent tracks straight from the logger without having to do any GPS Babel conditioning to it. In your case, being out in the field at a blind for an extended period of time, it would only record a point every few minutes maybe. Any photos on the move would be reasonably accurate as long as you were keeping up a decent speed… which isn’t necessarily good for photographs!

— comment by JasonP on January 15th, 2012 at 8:30pm JST (5 years, 11 months ago) comment permalink

JasonP: I managed a whole day (something like 8am to 6pm) with the Galaxy S2 in the Iya Valley in Shikoku, and don’t remember battery drain as being a problem. I did, however, switch off the tracking when stationary, eg when having a meal. Also, I have no idea whether not having a data or phone connection at the time helped significantly with battery life. And perhaps it being late autumn/early winter helped with the battery temperature issue – not that I’ve ever noticed that as being an issue. It isn’t one here in Australia in summer :-).

I should have said in my earlier post that I have no connection with My Tracks; just a satisfied user. It’s a free app anyway.

— comment by David on January 15th, 2012 at 9:21pm JST (5 years, 11 months ago) comment permalink

Jeffrey – GPS Babel, that was it! I think it was that or some software called Hadoupi (?) that people suggested. I looked just now and it seems that GPS Babel is free, so I’ll check it out and see how well it works.

JasonP – my AMOD came with the static navigation ON firmware pre-loaded, and I actually flashed it with the static nav OFF for the reason that you stated – that it isn’t so accurate when on foot. I looked at some of my GPS logs taken with static navigation on, and was surprised to see that there were still little spaghetti-like messes at the points where I stopped for long periods of time. They seem to be a bit more confined and less messy than with static navigation off, though (as would be expected). The way I see it, if I’ll still have to deal with the spaghetti trails (or edit them out), having the GPS occasionally skip massively distant waypoints because it perceives itself as standing still isn’t worth the trade-off for me.

But I do want to thank you for pointing it out and linking to their firmware page, because I see that they released a firmware update a few days ago! I wish that they posted a changelog… I’ll go ahead and update anyway.

— comment by David K. on January 16th, 2012 at 12:44am JST (5 years, 11 months ago) comment permalink

It occurs to me the iPhone probably has significantly more processor power available to it than either of the Garmins. It seems likely the iPhone may be doing some smoothing on the fly, or perhaps depreciating the data from the individual satellites based on anything from how off they seem based on the plot provided by the others to the angle of the bird and how that generally effects the accuracy of the raw data.

Sort of like Olympic voting. First you throw out the highest and lowest scores…

Mind you, this is only my speculation. It’s possible the Garmins already do this, though it does seem likely they’d skip a lot of it if their processor was already taxed to near capacity.

— comment by Sean on January 17th, 2012 at 3:36am JST (5 years, 11 months ago) comment permalink

Hi Jeffrey,
I use “GPX Master” on my 4S and it looks promising (did not have the chance to use it for real yet). You can set a lot of parameters to adjust precision and battery consumption.
They even thought about the file transfer and it can use your Dropbox account to do it automagically.

— comment by Marc Gibeault on January 18th, 2012 at 5:51am JST (5 years, 11 months ago) comment permalink

If look into the appstore, you can find some apps called GPS info or GPS status, or GPS data, there are several of them. These apps can tell the real time accuracy of the postion data from the chipset, for both horizional and vertical directions. Will anybody instal one of them on a iPhone 4S, do a outdoor test and tell us the accuracy readings? On my iPhone 4, the horizional accuracy in a good ourdoor condition usually reads 5 or 10 meters.
I am spending hours on internet to look for the accuracy data of GPS on 4S, but until now did not find any figures.

WangGang from Jakarta, Indonesia

— comment by WangGang on February 13th, 2012 at 12:44am JST (5 years, 10 months ago) comment permalink

Thanks for a fascinating discussion.
I have recently bought an eTrex 10 and have used it with GPS/EGNOS and Glonass turned on but have begun to suspect that using both may increase battery drain?
I would make a couple of points re phones v eTrexs –
a) I live in a rainy country (Ireland) and hike in the hills so the waterproofness of the eTrex is essential.
b) the eTrex uses AA batteries and I carry two spare sets (for various reasons I may not get the claimed 25 hours use out of a set). I’m not sure if all phone batteries are replaceable and I certainly wouldn’t like to try it in rain.
c) Maps – In my opinion the screen size on the eTrex makes maps almost useless which is why I have an eTrex 10. Within limits bigger screen size is better though I find using topographic maps on the slightly bigger but still small screen of a Nokia 5230 still gives a very limited view. I’m in the process of buying a Samsung Tab 7″ in the hopes that it will be pocketable and show a useful amount of map. In many ways even better than any of the foregoing is an A4 printed map of the area in a plastic pocket. It’s at least water resistant, I can roll it up, put it in my back pocket and it remains perfectly legible!

— comment by FSheehan on February 21st, 2012 at 12:43am JST (5 years, 10 months ago) comment permalink

@Sean the iPhone definitely can apply error correction to the GPS position. It uses the magnetometer (digital compass), gyroscope and accelerometer to smooth out the GPS position. I think some Garmin devices might do this too, but perhaps the iPhone does a better job.

This process does significantly increase battery drain, so not all apps will activate it. Apple’s documentation recommends it should only be activated when plugged into a USB car charger, but they don’t enforce it.

@WangGang the reported accuracy of a phone or GPS device is not accurate. The only way you can really know is by walking around and comparing your track to satellite photos. Beware satellite photos don’t always line up perfectly either.

— comment by ackar on April 9th, 2012 at 2:48pm JST (5 years, 8 months ago) comment permalink

Hi Jeffrey.

I was looking for a way to turn an iPhone 4 into a GPS receiver but I stumbled upon your site.

Can you tell which software you used for the elevation plots?


Google Earth, IIRC. —Jeffrey

— comment by fluxflax on May 18th, 2012 at 6:53pm JST (5 years, 7 months ago) comment permalink

I would be interested to see the results with a similar track log recording interval on all three devices and in a rural environment.
I have stopped buying Garmin hand held devices because they are so poor these days, but my devices (Garmin GPSmap60CSx and Magellan eXplorist 710) both show pinpoint accuracy on track logs in a rural environment.

I think this followup suffices… it was pretty rural, deep in the mountains. —Jeffrey

— comment by Pete on January 21st, 2013 at 6:51pm JST (4 years, 11 months ago) comment permalink

The author said he threw the devices into a bag. However, even if these devices have high sensivity receivers, and probably will work on most positions, the best way to compare the three units is to hold them correctly all the time the test lasts. Just by holding the devices incorrectly will cause higher errors in positioning.
In this case, with patch antennas, the correct position is with the antenna facing up and as far from the body as possible. Placing the eTrex units with the screens facing up and level would be necessary for an accuracy comparison.
Also, the tracklog set to record at 1 second interval is quite excessive, and thus it does not allow for the device to smooth the tracklog. Setting it to Auto, Most often would result in a much more correct tracklog.
Could you test the units again with these changes? I believe this would do a lot of difference!
Thank you very much for your article!
Cheers from Portugal.

— comment by Pedro on June 8th, 2013 at 7:15am JST (4 years, 6 months ago) comment permalink

I’ve been evaluating several GPS loggers to use on my Android device (Nexus 6P) during an upcoming 2-week wilderness trip, and like you, have been very impressed at the accuracy and ease-of-use compared to older Garmin devices I have. The app I’ve decided to use (Ultra GPS Logger) creates NMEA, KML, GPX and CSV files that can be “published” by email or uploaded to cloud storage. The logging trigger time can be adjusted to save on battery usage: for a long horse trek, an interval of even a few minutes should conserve the battery, yet still give me a track log that will be sufficiently accurate for geotagging my images.

When I ran several apps at the same time while also carrying a handheld GPS device, I was very surprised to see how accurate all were compared to my Garmin GPSmap 76CS. All logs included minor (but slightly different) errors when I was close to cliffs or in dense woods, but the apps recovered more rapidly than the Garmin device. Repeated logging while horseback riding during my testing gave me log tracks that seldom deviate much more than the width of the forestry roads.

Best of all? The resulting GPX track files work just fine for geotagging using a certain well-designed LR plug-in!

— comment by Eric Fletcher on June 7th, 2016 at 11:32pm JST (1 year, 6 months ago) comment permalink
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