Hi again,
A big 'thank you' to everyone who replied to my inquiry about phone
compasses and the setting in place of two posts. Responses are
summarized below.
- A couple of people suggested that I should use specialist equipment
such as theodolites or astrocompasses. Unfortunately, I neither own one
of those nor know anyone locally who does, and the daily rental fee for
a theodolite is well beyond my budget (~ $100 / day).
- A couple of other people suggested using a hiking compass. Since
posting my original inquiry, I have borrowed one and, after a bit of
fiddling around, I got it calibrated for the local magnetic declination.
In practice, I found it difficult to use the compass for judging whether
my two posts were lined up as required. Once I had both posts installed
however, I got a reassuring compass reading indicating they were
aligned as required.
- The third approach is Kevin Karney's suggestion to use Google Earth to
measure the position of the sundial and of a reference point, so that I
could get a reference bearing. This, coincidentally, is a technique that
I had referred to in a talk at last month's NASS conference; it's also
the basis of the method I ended up using to put my second post in place
today (but I used Google Maps). I was able to estimate the position of
my dial very precisely and, fortuitously, Google Maps showed a
convenient landmark exactly north of the dial location at about 30m
distance. By standing at that landmark and using a pair of binoculars,
it was fairly easy to line up my second post on the line to my first
post. I had prepared a large hole in about the right place so I just had
to call to my assistant to move the post by a centimeter or two at a
time, until we were happy with the placement. A light tap on a nail then
fixed the post in place against a temporary support, and I have now set
it in place with concrete.
With respect to the specific question of the directional accuracy
achievable with a cell phone:
- Responses were contradictory. Richard Langley reported that he
estimated his phone's compass to show bearings good to within about 1
degree; I used the borrowed hiking compass to make the same comparison,
and also found that my phone matches to within about a degree. On the
other hand, a number of people reported having found discrepancies of up
to 20 degrees between magnetic compass and cell phone. Clearly, the
calibration state of the phone effects the accuracy achieved; I suspect
the make and model does too.
- I wanted to double check the accuracy value that I had found for my
phone. Now, it so happens that according to Google Maps a section of
road that runs near my home is exactly North-South. My phone runs iOS;
there is an iOS app called Theodolite that overlays the phone's camera
with orientation information. So, I positioned myself over the edge of
the sidewalk and used the app to sight the same edge at about 100m
distant. It gave a bearing of 359 degrees (in one degree steps) so
assuming the sidewalk is actually true NS, then his method also suggests
my phone is good to about a degree.
- I have looked on the web for manufacturer's specifications, but have
not found the required details.
- I found an academic study which suggests that a sample of 2013-era
iPhones were found to match compasses to within about 4 degrees. The
study mentions work by a different group who had investigated Andriod
devices and found greater discrepancies. Note, the study relates to
phones in use in 2013, and I don't know how relevant it would be to
today's phones. ["Structural data collection with mobile devices" by
Richard W. Allmendinger, Chris R. Siron, and Chelsea P. Scott / Journal
of Structural Geology v. 102, pp. 98-112]
Cheers,
Steve
**
*From: *Steve Lelievre <mailto:steve.lelievre.can...@gmail.com>
*Sent: *September 11, 2018 12:39 PM
*To: *Sundial List <mailto:sundial@uni-koeln.de>
*Subject: *How good is a cell phone compass
How good is a cell phone compass? I mean, if I have no metal nearby and
I have the phone set to show True North, what kind of accuracy can I
expect if I lay my phone flat and use the compass app?
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