Oxygen Deprivation Traditional Geocache
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Difficulty:
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Terrain:
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Size: 
(small)
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AKA: How much chest pain is normal?
This is not too far off of the Ball Hill trail. I've always liked this location, it is scenic and when I get here it's almost all downhill for the next couple of miles of trail. For us full figured guys, getting to this point on a mountain bike involves oxygen deprivation with the random orbiting gray spots, moderate chest pain and a sense of doom. From this point on however gravity is my friend.
Official Disclaimer: Virtually all of my Leominster State Forest caches are placed while on a mountain bike ride, I ride to the general area and walk into the woods and place the cache. If you're bushwhacking more than about 100 feet you're either lost or coming from the wrong direction.
A short note on GPS accuracy. Positional accuracy is usually defined by a number which is called the Circular error probability or CEP. The CEP is the number expressed in feet or meters that equals the radius of the circle that you are probably standing in 50% of the time based on the coordinates your GPS shows. The bad news is there is a 50% chance that you are outside of the circle. On a Garmin GPS the CEP is represented by the blue circle you see around your position icon.
When I place my caches I usually use an external, powered antenna that generally gets my CEP down under 4 meters +/- after I let the unit average for 300 fixes. 4 meters +/- means that both coordinates could be high or low by as much as 4 meters or 13 feet and because +/- implies a radius this works out to an 8 meter or 26 foot diameter circle. What this means is that there is a 50% chance that the location for any particular cache is somewhere inside of a 8 meter diameter circle because Garmin uses this 50% number for consumer grade GPS accuracy CEP. When a second GPS or even the same GPS at a the same GPS at a different time is used it will have a CEP or positional tolerance of maybe 8 or 9 meters +/- unless you put it down and let it settle for a few minutes. What this means is initially you can be any place inside of a 60 to 70 foot diameter circle for a given pair of coordinates with a 50% chance that you are outside of that circle. Tolerance stack-up is where this all really starts to become a problem, if the cache is actually on the eastern edge of my circular tolerance and your location is on the western edge of your circular tolerance it would appear that based on your numbers the cache is at least 75 feet or more away and the numbers are way off. Both of us believe that we are where we think we are but the probability is that we are both wrong.
I welcome suggestions as to the accuracy of my numbers but please set your unit down and let it average for at least 5 minutes and then log the actual numbers.
The website link shown below has a great explanation on GPS expected accuracy and his long term numbers suggest that plus/minus 8 or 9 meters is about the best we can actually expect.
(visit link)
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