First, you need to go to the given co-ordinates and follow the fence along (East), until you can line up the dog bin, and the gap in the middle of the ridge in the park. Look for a sign with three lines of writing (It's Behind You!).
Using the usual technique of A=1, B=2, C=3 etc, the next stage is at:
N(3rd letter, top line)(8th letter, top line) (3rd letter, bottom line - 1st letter, middle line)(7th letter, middle line).(3rd letter, top line + 11th letter, bottom line)(2nd letter, top line)(last letter, middle line)
W00(5th letter, middle line) (10th letter, middle line)(16th letter, bottom line).(5th letter, top line)(3rd letter middle line)(16th letter, bottom line x 5th letter, middle line)
When you reach the second and final stage of this cache, you must align the mast in the distance with a telegraph pole (not saying which one!) and ... do something ....