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:
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.
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:
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 (New York Cupcakes), coffee (Starbucks) and a bagel shop (Blazing Bagels).
When I created the puzzle, I worked out the solution using three separate methods. (Update: I’ve worked it five different methods. For fun.) I also allowed for some round off error, accepting a solution that’s within 30m of what I measured. 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! [Update: Now 57 solvers.]
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!
Nearly eight years afterwards, many of the tools have gone away: Garmin Basecamp is EOL, GSAK is now freeware (as its founder has had medical issues and is deservedly retiring), etc. I’ve not found a tool that works for three waypoints. For example, this one: http://geo.javawa.nl/coordcalc/index_en.html
will produce a result about 600′ away. However its two circle solution works fine. One can pick any two points and produce two candidate spots. One spot will be obvious.