› Forums › Geocaching in Wisconsin › Help › I have been wondering….
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wzbt03.
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03/07/2004 at 3:40 pm #1720093
Many times when we are out caching, my GPSr is routinely off by 20 feet. (Garmin GPS 72) I haven’t determined if it’s always 20 feet in one direction yet. Is this due to a difference between two different brands, such as me using my Garmin to find a cache placed by a Magellan user? Or could it be a calibration issue? I was just wondering if this is a common issue, or if it’s just my GPSr, (or user error) and if anyone else has had this problem as well.
I have had a few caches where the coordinates are dead on too. Those are mostly from one particular person’s caches though!
As I’ve gained more experience, the fact that it’s off doesn’t throw me quite as much, because I’m able to think it through as to where someone would place a cache. That’s fun when I can think it out and get it right. 😉 Maybe I would like my next career to be a detective.
~Mama Fishcacher
03/07/2004 at 5:08 pm #1739736Simply put, no GPS is perfectly accurate. Position accuracy will vary with GPSr configuration (receiver and antenna), location (geographic latitude, and surrounding objects possibly blocking reception), satellite constellation status, and ionosphere conditions.
Expensive survey-grade GPS receivers would generally be more accurate than the consumer-grade receivers that we use.With these things in mind, consider the likelyhood that the cache owners GPSr is several feet off when the waypoint was set. Your GPSr is also several feet off. The combination may get you right on target, or it may put you the maximum distance away that both measurements combined will lead you.
I don’t mind 20 feet, unless I’m looking for a matchbook-sized container. Then, I let the GPS get me close and put it away. The rest is trying to think like the cache owner.
[This message has been edited by Cathunter (edited 03-07-2004).]
03/08/2004 at 3:05 am #1739737I like as much accuracy as possible but the fact is that you can expect an error radius of around 60 feet. I’ve done many test where I’ve done waypoint averaging with WAAS showing an accuracy of 8 feet only to come back and find that the spot has moved more than that.
One thing that I have noticed at time, is if you are on a multicache, often the error is the same direction. So, if you find waypoint #1 30 feet to the north. #2 and so on often have the same error.
When you get about 100 feet from ground zero, stop and let the readings stabilize. You may find that the bearing and distance change. This is a good time to take a compass bearing if your GPSR is so equiped.
When money is available, I’d like to obtain a survey quality GPS. I was watching a fellow from the Waukesha water department walk around with one and take coordinates on where all the lateral valves were by the side walks. It fit in a backpack and had a nice tall antenna to keep it above his head.
As it is now, geocaching is like horse shoes, close counts. I’d like to get things even closer!
03/15/2004 at 3:53 pm #1739738quote:
When money is available, I’d like to obtain a survey quality GPS. I was watching a fellow from the Waukesha water department walk around with one and take coordinates on where all the lateral valves were by the side walks. It fit in a backpack and had a nice tall antenna to keep it above his head.
Those survey-grade GPS’s are nice, but the ones I’ve tested were a bit cumbersome – you’ve got that backpack, a boom antenna, and a nice hefty brick of a battery to power the thing. (We use them in conjunction with a Geographical Information Systems project where I work…)
Besides, what’s the fun of hunting if you can get within 3 inches of your target… You’ll wind up in the wrong spot almost all the time 🙂
03/15/2004 at 4:03 pm #1739739Survey GPS’s run about $70,000 each.
Accuracy is about 1 centimeter over 50 miles.
I think I will stick with my Garmin. -
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