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--- Begin Message ---A remark must be done: Some covers for iPhones or tablets have magnets inside that greatly disturbs thr accuracy of the positional devices as well as compasses. When using the device for position or orientation purposes, the device must be used without the cover and its cover must be kept as far as possible from the device. If a read must be taken close to metallic structures, is not advisable to use any magnetic compass either an in-tablet or a conventional magnetic compass. For this purpose a sun compass is the right instrument to use. No need of an Astrocompass or a Cole or Abrams compass, they are too expensive. Actually a sun compass similar to the British Asrmy MK III Howard Sun Compas is very easy to construct, a good explanation on how to construct a home made sun compass can be found here: http://www.jgiesen.de/SundialBox/index.html The formulae to calculate the azimuth of the sun at every latitude and at every day of the year can be obtained from this link: https://www.nrel.gov/docs/fy08osti/34302.pdf The calculated data can be obtained from this link: https://www.esrl.noaa.gov/gmd/grad/solcalc/ And complete sun path diagrams can be obtained from this link: http://solardat.uoregon.edu/SunChartProgram.html Sincerely yours. Stanislav [email protected] > On 13 Sep 2018, at 06:09, Steve Lelievre <[email protected]> > wrote: > > 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 >> Sent: September 11, 2018 12:39 PM >> To: Sundial List >> 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? >> > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial >
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