Hi, following up on Richard's comment, and recalling a similar thread a
few months ago...
- a traditional compass is built around a permanent magnet that aligns
to the earth magnetic field without any need for calibration. the body
of a traditional compass is built with non-magnetic material and does
not influence the reading (don't measure close to your hammer or phone!).
- a smartphone is equipped with a three-axis magnetometer which must be
calibrated from time to time, and (1) it is difficult to say if the
calibration is successful, (2) many android devices do not have a
calibration app (mine - a sony - has got a hidden calibration utility
that can be accessed with a strange hacker code).
- a smartphone is composed of a lot of magnetic parts: battery,
electronic components and radio antennas! this means that the magnetic
field around your three-axis magnetometer is given by the earth magnetic
field PLUS the phone magnetic field. the latter varies according to the
phone activity (e.g. sending a call). in a high-end electronic compass
most of these problems are solved because non-magnetic material is used
where possible, and the rest is very stable and calibrated (e.g. there
are no antennas around).
So, as said a few months ago:
- test your phone against a traditional compass.
- turn off antennas, using "airplane mode".
- DO A SIMPLE TEST: place your phone on a flat surface, one side aligned
with a reference line, and read the bearing = B. turn the phone around
the vertical axis at 90°, 180°, 270°, aligning the other sides with the
reference line. the bearing should be exactly B+90, B+180, B+270.
however on most phones there is a systematic error of 10-20° in some of
these positions. this error is reduced but not totally eliminated in
airplane mode... this gives you a rough estimate of the errors you can
experience even when the phone is correctly calibrated.
Finally my personal opinion is that using a smartphone as a compass
might be (still) questionable (I totally agree with Richard), and I
always warn students on this, but on the other hand apps like FieldClino
etc. are great because they allow to PLOT and ANALYSE data in the field.
so it would be great to be able to collect the data with a traditional
compass and quickly transfer the data on the phone... a suggestion for
Roddy and other software developers: would it be possible to "dictate"
measurements to the phone using the speech recognition function that is
already built in in most operating systems? this would be very useful
also when measuring in nasty places...
Best,
andrea
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