As GPS-enabled phones become more popular, there have been a lot of geocaches placed by people using phones.   Many of these will have serious “adjustments” to their posted coordinates because the person placing it just took a single reading, using whatever their phone was reporting and called it good.  Usually these adjustments are anywhere from 50-500 feet, but that’s a lot when you consider the cache may be the size of a pinky and located in an area with a lot of hiding places.  Like a forest, perhaps!I think one problem is people aren’t letting their GPS receivers settle down a bit.  When I first flip on the geocaching application, it often displays a wildly imprecise value.  This is the most egregious example I’ve been able to capture on the screen:
Geocaching.com application showing an error radius of 39.6 miles.

No, we are not lost, I'm not stopping to ask for directions.

What this is telling you is there’s a 95% chance that you are within a circle of radius 39.6 miles from the coordinates displayed.   Put another way: Finding a thumb-sized container “somewhere in the greater Seattle metropolitan area may take a while.
How we think GPSes work

C'mon, GPS, I'm trying to find the Tupperware in the woods (Thanks, SMBC)

More typically, the phone is off by a lesser amount before it starts to settle down to 16-128′ error (flopping around a lot). It’s not just geocaching.com’s application. Geosphere showed me this a few weeks ago:
1.4 miles to target, plus or minus a mile

The existential equivalent of: you are here, but where is here?

This observation, combined with my newly-acquired knowledge on how to do a screen capture on my iPhone led to a puzzle cache titled “My first iPhone hide.”  This will no doubt cause groans in the puzzle community.The “puzzle” part is to realize that I’m providing three points of reference from which you would then perform a 2D-trilateration calculations.  In other words, plot out a circle originating from each point and radius, then find where they intersect.  And, perhaps a symptom of my being hungry when I created this – all three points have a food-based theme: cupcakes, coffee and a bagel shop that makes a bacon-topped breakfast staple.
Projecting the circles around each radius

Projecting the circles around each radius: click to enbiggenate.

Showing the intersection

At the intersection is the prize.

When I created the puzzle, I worked out the solution using three separate methods. I also allowed for some round off error, accepting a solution that’s within 30m of what I measured (Update: I reduced it to 10m as everyone who’s understood the approach has fallen within the tighter boundary) . Still, after it was published, I kept checking the audit log, hoping I didn’t make it too obscure or make a colossal mistake in the math. Finally, late morning, someone posted a find. Since then, three other people have solved it. whew!
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2 Responses to My first iPhone hide

  1. Kiri says:

    As usual, you prove yourself to be too cool for words (and worthy of admiring gazes from the rest of us), plus possessing an awesome sense of humor. Go Jim!

  2. [...] are limitations of the phone.  First, the GPS takes a while to settle down, as I alluded to in my puzzle cache.  It’s not a huge problem if the person placing it chose a reasonable area and measured [...]

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