The cache is NOT located at the listed coordinates.
The difficulty of this cache is based on the ground you have to cover in the adventure lab (which is still NOT excessive) and the amount of information you need to gather to get the final coordinates. Don't be afraid, though. It's not that hard; you just have to be willing to put in a bit of time to work for it. All of the locations in the Adventure Lab should be handicap accessible and should be available in Winter as well.
Unless we have an unusual amount of snow, this bonus cache should be available in winter.
In order to get the correct coordinates you will need to complete the 40 West ArtLine adventure lab. Each journal entry will provide you with a piece of information you can use to complete the puzzle below. If you start at Aviation Park and make your way through the lab in a "U" finishing at Walker Branch Park, you will be well positioned for this bonus.
Checksum for a through e = 53.
N 39° 44.[A-B-C][D-B-E][B/C]' W 105° 3.[D/E*2][B/C*2][ B ]'
Congratulations to LostinDenver for FTF (and my apologies for the typo in one of the formulas)!