At any point on the Earth the angle between magnetic north (the direction the north end of a compass points) and true north is called declination. We GPS users don’t worry much about declination since our GPS units are typically set to true north which corresponds to how most maps are drawn, but in the pre-GPS days declination was a big deal. Within the last couple hundred years the declination in the St. Louis area has varied over a range of about 10 degrees, but the change is relatively slow and predictable. The line where the declination is zero (magnetic north equals true north) is called an agonic line. The current agonic line extends from the tip of Florida, through Missouri and across Lake Michigan and Superior up to the Artic Ocean. This line is the only place in North America where magnetic north is the same as true north. In 2006 the agonic line was located in the area of this cache. The following are good resources discussing declination: Wikipedia which has a neat time lapse of declination values since about 1600 as well as a US historic declination calculator which is recommended for a less traditional solution.
Wikipedia Page
National Geophysical Data Center (NGDC) has answers to FAQs and a declination calculator here
If you choose the more traditional route, data to find WP2 and WP3 is provided at the WP1 coordinates provided on this page. Now that the educational part is complete … let us apply it. The basic cache layout consists of a vertex at the starting point WP1. From this point WP2 is found by projecting at the declination from 1961 a distance of 5217 feet. ALSO from WP1, find WP3 by projecting at the anticipated declination in 2034 (assuming the rate of change remains the same as it was between 2000 and 2010 a distance of 5296 feet. Connect WP2 and WP3 with an imaginary line. The final is located where this imaginary line crosses the 2006 agonic line. Now that you know what you need to - go find it! There are many ways to solve this cache. Please indicate in your log if you completed via the armchair or traditional method, or just got the final coordinates from a caching friend!
Although mathematical solutions may vary a little bit they should get you close enough to find the final ... if done correctly. I found the GPS only solution to provide the most accurate coordinates for the final since conversions and rounding were not required.
Armchair solutions can be checked: here
All WP containers are small camouflaged containers and are located close to trails with little bushwacking required.
FTF honors go to: Doc Funk - traditional method Strider - armchair method. They both found it at the same time?!?! Thanks to Dustin Hills with Mo DNR for continued permitting of this cache.