06 Geocaching.com-specific puzzles

What could be more obvious than using Geocaching.com to create a mystery puzzle? There are a few possibilities here. For example, you could refer to caches, profiles, log entries or images uploaded to geocaching.com, which can be used to obtain or piece together the coordinates.

Listing/GC code

The first thing to mention for cache listings would probably be the GC code. That is, the 6-7 character code beginning with GC, which is used to uniquely name each cache.

It is worth knowing that Groundspeak has numbered its caches since the beginning and converts this consecutive number into the GC code. At the very beginning this was only the hexadecimal value of the sequence number, preceded by a GC. In 2003, this range of values was no longer sufficient and from then on a separate number system was used. In principle, a place value system with a base of 31, except that the alphabet was not used up to the 21st letter, but the complete alphabet with the exception of individual letters. I, L, O, S and U are missing, supposedly to avoid swear words being possible as GC code. Until 2006, this sufficed for six digits, since then the GC code has been seven digits.

Conversion to and from this Groundspeak-owned base-31 can be done, for example, online at fizzymagic or via the mobile phone app GCC.

My cache with the GC code GC5DWQB has the sequence number 4620439.

For example, if you call up a cache listing on geocaching.com with only the GC code via the URL shortener http://coord.info and now look at the URL to which the browser has been redirected, there is much more there than just the GC code.

My cache http://coord.info/GC5DWQB is redirected to http://www.geocaching.com/geocache/GC5DWQB_duzzels-nutzlose-suche?guid=11f1e948-cfa1-48a5-bf12-44aac0177f7b .

The listing is apparently called “GC code plus cache name” on the Groundspeak pages (which is also much more search engine friendly) and has a Guid attached to the back. Guid means “Globally Unique Identifier” and is a globally (more or less) unique number. Every listing at Groundspeak gets such a guide, just like every log entry, every uploaded picture and every user profile. As a mystery owner, you cannot influence these guides, but you can of course somehow muddle up the characters used in them and have the puzzler create a coordinate from it.

An ID like this (more precisely, the one used by Groundspeak is a UUID random 4 http://de.wikipedia.org/wiki/Universally_Unique_Identifier ) is always built from the same pattern and consists of five groups of hexadecimal values in the format XXXXXXXX-XXXXXX-XXXXXX-XXXXXXXXXX , where each X stands for a character from the hexadecimal system ( digits 0-9 and letters a to f).

Converting the above hexadecimal values from the Duzzel listing-guid, you get a slew of decimal numbers, at least the middle ones of which could almost pass for coordinates:

11f1e948 = 301066568
cfa1 = 53153
48a5 = 18597
bf12 = 48914
44aac0177f7b = 75500157894523

Tricky owners can, of course, include some kind of arithmetic operations in their puzzles


Just like the listing and its GC code, user profiles at geocaching.com are also given a sequential number and are converted into a kind of profile code (analogous to the GC code) via the “GC-Base-31”. Nicely, the URL shortener http://coord.info would link them to the correct profile with a PR in front. Unfortunately, it is not so easy to find out which code or which serial number a user profile has.

For example, if I click on my user profile, I get the following link with a Guid:


Possibly there is even another Guid hanging back here; depending on where I got to the profile from. However, this unique ID does not reveal the sequence number or the profile code. However, if you now look at the “See the Forum Posts for This User” link on the main page of any user profile, it has the running registration number at the very end!

My account was obviously the 2,892,759 geocaching.com account (I feel like I started this hobby a few years too late!). If I now convert this sequential number to GC-base-31 via the links above and add PR to the front instead of GC, I have the profile ID PR3hwyq, which I can actually use via http://coord.info/PR3hwyq and refer to my profile.

Travel Bugs

Travel bugs have always been popular objects for “vemystification”. They also have a sequence number, e.g.: 1880999, which can be converted into a TR code via “GC-Base-31”, and via the URL shortener as http://coord.info/TB2EY4A as

http://www.geocaching.com/track/details.aspx?id=1880999 can be called.

When searching for the mystery solution, it is often worth looking for the owner’s TBs or the TBs logged in first in the listing anyway

Logo entries, images, waypoints and attributes

Since each log entry has its own guid (e.g.: http://www.geocaching.com/seek/log.aspx?LUID=6b1d5e5f-7c4b-4bd8-a93d-da8c13707534 ), you can of course bury it somewhere in the puzzle and use it to nudge the puzzler towards a log entry that contains information necessary for solving the puzzle. The same applies to pictures that you can upload to geocaching.com (e.g. http://imgcdn.geocaching.com/cache/large/e6395a95- f788-4572-bf60-089835130a05.jpg). Groundspeak meanwhile outsources these to cloudfront.net (https://d1u1p2xjjiahg3.cloudfront.net/e6395a95-f788-4572-bf60-089835130a05_l.jpg).

Waypoints also have their own guid, e.g.: http://www.geocaching.com/seek/wpt.aspx?WID=17625587-748c-4673-bfee-f94a3a686f71 .

And finally a note on the attributes, which can also be used excellently for a mystery. The owner can choose them himself. For example, if he takes 10 of them and uses the terms of these attributes in the listing, numbers can be generated.

In addition, the attributes also have IDs. You can see them, for example, in the GPX file of a cache that has included them. Or, if you have already published your own cache, go to the area where the attributes can be edited and have a look at the source code. For example, 17 are the snakes, 32 are the bicycle, 1 is the dog, 3 is the climbing rope. And last but not least, there are the smilies that you can add to a log and whose order (or the letter word value of the labels or the digits that are underneath the special characters from which you generate the smiley code) are often used for a smiley cache.

Much more detailed than I did, “West468” dealt with the secrets around the GC site on his blog: GC: Searching Smart for Geocaching Sites.