Find x.
1, 7, 11, x, 77, 111, ...
The cache is at: N 37 0N.NNN W 121 2W.WWW where:
N.NNN = (x * 5 + 3270) / 1000
W.WWW = (5197 - x * 16) / 1000
No checker is needed.
Congratulations to Geokashers and WERDAVE on co-FTF!
1/17/16: This puzzle was featured on teamajk's Geocaching Puzzle of the Day!