This is a cache somewhat in the spirit of my 'Blue Moon' cache, but the number of intermediate points has been halved, to make it somewhat more approachable, based on the experience there.
In order to find this cache, you'll want to print out a copy of these clues and bring them; the coordinates alone are not enough to find the final geocache. yYou'll also want a pencil/pen to write down the intermediate clues as you go along.
The three intermediate locations you'll need to visit first are all points on a permanent compass training course located in the park. The total distance you'll need to cover is less than two miles, and most of it over fairly level ground on a good trail system.
You can either find them using their coordinates and a GPS; the nicely done trail map by the entrance will be a help in finding your way around, and perhaps in further explorations of the park.
At each one of these points, you will find a black metal sign with a red diamond. On each of the signs, there is a bearing (e.g, 90 degrees), and a number of paces. Write down the bearing and the paces in the space below.
Point 9: N41 12.887 W73 49.803 = ____ degrees (X1) ____ paces (Y1)
10: N41 12.888 W73 49.769 = ____ degrees (X2) ____ paces (Y2)
4: N41 12.831 W73 49.869 = ____ degrees (X3) ____ paces (Y3)
Then, plug these values into the two formulas below, which are used to determine the coordinates of the final geocache.
The coordinates are:
N 41 12.AAA W73 49.BBB
AAA = 3*X1 - 2*Y2 + 3*X3 - 78
BBB = -4*Y1 + 3*X2 - 4*Y3 + 190
This should be within 20 minutes walk from the other places you've been. Good luck!
(If you want to test out your compass skills, look for the lettered stake at that bearing from each of these signs, I believe with magnetic declination taken into account.)
Useful Notes: Admission and parking are free, and the trails are open from dawn until dusk.