What accuracy and precision can I expect from the iphone magnetometer? That is, how close is the reported result to the "true" bearing and what uncertainty is typically reported?
The iPhone 4S, 5 and 5C contain good quality components and the compass readings are generally accurate to +/-1 degree – better than can be achieved with a traditional hand-held Brunton or Silva compass. Unfortunately the magnetometer/gyroscope/accelerometer combination in the iPhone 5S is of lower quality and the compass is only accurate to +/-5 degrees.
This is of course assuming that the magnetometer has been calibrated, although the phone will typically prompt you to perform the calibration is the software determines the sensor input to be unreliable.
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I am trying to find out if the accuracy, plane detection and World Tracking of the ARKit will be better in iPhone 8 Plus and iPhone X comparing to an iPhone 7.
I googled it and I read thru this webpage.
There is no indication of dual cameras, no explanation of specs of the camera and if the processor power or better cameras in lates devices will make ARKit more accurate (read here).
I am working on an accuracy-related arkit app and I'd like to know more about this topic.
ARKit doesn't use the dual camera, so there's no functional difference between an iPhone (pick a number) Plus or iPhone X and other devices.
Apple's marketing claims that iPhone 8 / 8 Plus and iPhone X are factory calibrated for better / more precise AR, but makes no definition of baseline vs improved precision to measure by.
That's about all Apple's said or is publicly known.
Beyond that, it's probably safe to assume that even if there's no difference to the world tracking algorithms and camera / motion sensor inputs to those algorithms, the increased CPU / GPU performance of A11 either gives your app more overhead to spend its performance budget on spiffy visual effects or lets you do more with AR before running the battery down. So you can say "better" about newer vs older devices, in general, but not necessarily in a way that has anything to do with algorithmic accuracy.
There's a tiny bit of room for ARKit to be "better" on, say, iPhone X or 8 Plus vs iPhone 8, and especially on iPad vs iPhone — and again it's more about performance differences than functional differences. Among devices of the same processor generation (A10, A11, etc), those devices with larger batteries and larger physical enclosures are less thermally constrained — so the combination of ARKit plus your rendering engine and game/app code will get to spend more time pushing the silicon at full speed than it will on a smaller device.
Currently I have 3 transmitters and 1 receiver (4 devices are iPhones) setup allowing me to work on Trilateration.
However, I found that the signals fluctuate a lot (even when 4 devices are put steady on tables). Is there any strategy to stabilize the signals?
Should I calculate the average X and Y to assume its position, or can I get assistance with GPS, or Wi-Fi signals?
Given that Wi-Fi is turned ON, bluetooth is turned ON, but not all devices have SIM card installed.
The "accuracy" field distanxe estimate returned by iOS for a CLBeacon is already averaged over approximately 20 seconds, so further averaging is unlikely to help unless you are talking about very long periods.
You must be realistic about the precision you can get with beacon ranging in general and trilateration specifically. Radio noise, reflections and attenuation from obstacles limit the technology to rough positioning.
See here for more info: http://developer.radiusnetworks.com/2014/12/04/fundamentals-of-beacon-ranging.html
I captured raw magnetometer data on the iPhone 5 by accident, and I actually require the calibrated data. The problem is that I can't go and recapture the data I originally got. Does anyone know what the iPhone's hard bias (device bias) calibration values are and how I can apply them to my data to get a similar output to what the iPhone would have given me?
Alternatively what is the best approach to calibrate for the device bias? I don't care about soft bias in my measurements.
Thanks
The iPhone needs calibration data not to calibrate for the internal sensor (that is always accounted for, even in "raw" data, which actually isn't as raw as you might think). It is actually to calibrate for external factors that might disrupt or interfere with the Earth's natural magnetic field, like high voltage power lines, or steel beams overhead. The iPhone creates a 3D distortion map of the field (which is why the compass app asks you to make a figure eight) to offset these external influences.
Finally, even if you could recreate the exact distortions, Apple provides no way to peek into their black-box filtering, let alone apply your own distortion map to their data. So no, you cannot recompute the calibrated data after the fact.
For iPhone4+ devices gps resolution, how best can it be? 1 meter, 2 meter or?
I suppose that WiFi, cellular data usage and power are available.
You can specify your desired accuracy, and that will yield at best an accuracy of no smaller than 10 meters. This means that the device is at the coordinate, or somewhere in the circle around that coordinate with a radius of 10 meters.
Time-resolution is at best about one update per second.
Be mindful of the fact that the higher you desired accuracy, the more battery-power you consume.
I'm currently developing an iPhone App (on iPhone 5, iOS 7, Xcode 5) which requires a very accurate determination of the current attitude. The "attitude" of CMDeviceMotion does not fulfil these requirements because Apple's sensor fusion algorithm seems to rely too much on the gyroscope which drifts away rather fast (in my experience). That's why I decided to read out the bare sensor data and later I want to combine it within a sensor fusion algorithm by myself.
When asking for magnetometer data one has two possibilities:
via CMMagnetometerData in CMMotionManager
via CMCalibratedMagneticField in CMDeviceMotion about which Apple says
The CMCalibratedMagneticField returned by this property gives you the total magnetic field in the device’s vicinity without device bias. Unlike the magneticField property of the CMMagnetometer class, these values reflect the earth’s magnetic field plus surrounding fields, minus device bias.
In principle (2.) is exactly what I want.
There is a very simple test if magnetometer data is calibrated properly. For simplicity one can restrict oneself to two dimensions. When the device lies on it's back, the combination B_x^2 + B_y^2 must be constant, independent of the direction the device is pointing to. It must just equal the horizontal component of the Earth's magnetic field (assuming no other fields in the vicinity of the device). Thus, when performing a 360 degrees turn of the device which lies on it's back, the measured data B_y over B_x should display a circle. See here for details.
Now the point: the data of CMCalibratedMagneticField does NOT result in a circle!
Does anyone have an explanation for that? Or does anyone know, how the CMCalibratedMagneticField comes about? Is the magnetometer calibrated in the sense of the link from above when performing the "eight-shaped" movement of the device or what is the movement good for?
Btw. why the "eight-shaped" movement and not flipping the device around it's three axis, which would allow a calibration as described in the link from above?
I would be very glad for any clarification with this issue... Thanks!
There is a problem with the magnetometer in iOS 7, it has an error of +-7º. Try using the 7.1 beta version.
EDIT
The magnetometer has zero-drift over time, but is pretty inaccurate for sudden changes in position. The accelerometer and gyroscope on the other hand adjust quickly for sudden changes but, being inertial sensors, they lose accuracy over a period of time.
So when CMCalibratedMagneticField tries compensate for your rotational motion it uses data from the gyroscope and accelerometer. This is when the accelerometer and gyroscope's +-7º error creeps in and throws your circle off track. Check this answer and this wikipedia article for more info.
As regards to the figure of eight:
Both do the same thing, they orient the "North" of your device in each direction in hope of cancelling out magnetic interference. Flipping your device along all three axes will work better but it is harder to perform and not as easily understood by the user.
Hope this helps.