#19 Update: Should you trust your digital compass?


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In the wake of my last post on digital compass clinometer functions, I was left feeling rather dissatisfied that I had found out the whole truth about the accuracy of digital compasses found in most modern smartphones and tablets. I set aside half an hour over the weekend to gather some data to see for myself what was the case.

Here I present a very rough experiment using:
1 x iPhone (built-in compass app)
2 x Analogue compass clinometers (1 x Suunto MC-2, 1 x Silva type-15)

Data Gathering
Here we are testing the ability to measure magnetic north, so any readings must reflect that. The analogue compasses used are “working” compasses so have had the baseplate correction set to zero. The iPhone compass was also set to measure magnetic north.

To ensure meaningful data, both compasses must always be measuring the same direction of travel, and must not be too close as to magnetically interfere with each other. To achieve this I used a 130 mm wide piece of plastic with parallel sides (an empty DVD dox) to separate the two devices, whilst ensuring they are pointed in the same direction. I also used the right and side of the iPhone as this side has no buttons, and thus the straight side of the device should ensure the “top” of the phone is in the correct line.

I also needed to make sure that there were no significant external magnetic interferences. Since it was pissing it down with rain outside, I opted to do it in the centre of a large room. There were no metal/electronic objects within 2m in a horizontal radius. Assuming there is no large neodymium magnet under my floorboards (?) then I also assumed there would be no interference from above or below. I was not wearing a watch, necklace, bangle/bracelet, earrings or any other piercings, or chastity belt on my person that could also interfere.

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On a side note, I am very confident that compass 1 (an by association, compass 2) is indeed accurate. This is because only a matter of weeks ago I used the compass on fieldwork and on a following hiking holiday where it was used countless times for triangulation. Most of the time I was showing students how to triangulate, locating ourselves on a known location on a map. These positions were also verified by a high-accuracy mobile GPS. In short, if my compass was inaccurate or damaged, I would be well aware of the fact. Interestingly, whilst on that fieldwork a colleague of mine discovered she had fallen victim to the phenomenon whereby the magnetic polarity of your compass becomes reversed due to prolonged proximity to a smartphone.

Data was gathered in two job-lots. Using firstly compass 1 (C1, Sunnto) and then compass 2 (C2, Sylva), I sequentially adjusted the baseplate in increments of 10 degrees from 0 to 180. I had the digital and analogue compass on my lap, separated by the plastic rectangle. Sat on my swivel chair, I then shwiffled (shuffled and swivelled) round until the magnetic needle of my compass overlay the north arrow on the baseplate as well as I could get it. My lap provided much needed stability. I then recorded the reading on the iPhone as a counterpart measurement.

The Results
I must say that I was alarmed at the apparent error in the iPhone. The mean difference in reading was 4.8 degrees (s.d. 9.2) and 3.2 degree (s.d. 6.8) for compass 1 and compass 2 respectively. Interestingly, however, there seems to be a pattern in the data. Both compasses seem to show a shift in positive to negative/less positive error right around 90-110 degrees. While this pattern is more symmetrical for compass 1, both seem to have a similar negative excursion over approximately 70-80 degrees.

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Theoretically, any static disturbance in the room should cause a consistent error in the measurement as neither the needle, point in the room of measurement, nor the site of any potential disturbance actually move. Even if there was something in the room causing the interference then one would expect that the analogue compass would have been affected too, and thus there would be a homogenous error in the two measurements.

So what do these errors indicate? Is this error acceptable? Of course the answer is no. Sure, the internal magnetometer in the iPhone or any other device which uses a similar piece of hardware is probably good enough for giving you a bearing on a city street, but any remotely consequential use of that information would surely lead to undesirable results. Yet, although the vast majority of people out on mountainsides around the world are unlikely to be sorts of individuals who would use an iPhone for navigation, my concern surrounds the use of “smart” devices as geological tools. Indeed in my last post I was very positive about the app GeoId. That’s not to say that the app itself is not useful. With the prospect that the app’s data-gathering capabilities is hampered by hardware accuracy, it seems like the app GeoId is probably more useful and reliable if you were to collect data with an analogue compass clinometer and then input it into the program. What a laborious and time-consuming task that would be. A task that negates the entire convenience and quirkiness of the app. If you were going to sit there for a couple of hours and input that data into something, why wouldn’t you rather input it into a piece of software such as those discussed in this blog post on structuralgeology.org.

I encourage you to gather some data yourself and let me know what you find. I’m a big advocate of using technology wherever possible, especially in geology, so I’m really interested in knowing more about the limitations of such technology.

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3 thoughts on “#19 Update: Should you trust your digital compass?

  1. I’ve tried to do a little digging to see if other phones have more precise magnetometers. This is the spec sheet for the chip inside the iPhone 5 http://www.akm.com/akm/en/product/datasheet1/?partno=AK8963 but I can’t find the information on any other high end smartphones.

    It would be interesting to see how the iPhone stacks up against other smartphones. I imagine that the construction of the phone could have a fairly significant effect on the accuracy of the digital compass as well.

  2. I just started a ‘mapping project’ with the Lambert app on my Iphone 4. I was very excited at first because I was setting up 1×1 meter grids to focus lineation points on based on plant structures in a 45deg dipping bed. The app seemed awesome at first because it would give you the trend/plunge, dip, strike, and even long/lat up to 6 decimal places and would plot the data points in several different stereo net diagrams. I kept switching between my bearings in the Lambert app and comparing it to the actual Compass utility in the Iphone and they would vary by about 10 degrees (On a good day). Unfortunately, I need to have higher resolution than + or – 10 degrees (min) on my map since the structures are very dense and concentrated.

    I’ve also noticed that the lat/long value would vary by about the last 3 digits after the decimal place. I tried converting this to meters and checking the distance between two points in the lat/long coords, which were actually only about a meter apart and got a distance of about 20 meters. Not only that, but when I reviewed it even more, I noticed that the app put had a point I measured twice at 20 meters apart as well. I’m pretty sad as I am a broke-unpaid intern who’s still in school, so somehow I’ll have to find a way to get my hands on a brunton since I can’t destroy this mapping project.

    Regards

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