Squonkpups Posted December 8, 2003 Share Posted December 8, 2003 I hid my first cashe and would like to register it for all to find but when I marked the position and drove back to it after a successful hunt the coordinates were way off. I thought the government shut that system down. How accurate are GPS's? I even had the position pinning option activated. The original decimal latitude was .917 and while sitting there while sipping some cold ones it varied from a high of .917 to a low of .883 and on longitude from a high of .129 to a low of .104. That's too much varience. That's 300 feet so theoretically the cashe is on the North goalpost but the GPS says it's on the South goalpost. The GPS didn't giggle, the satellites don't jiggle and the planet doesn't jiggle so how can this happen? I want accuracy within 3 ft. like the government. Quote Link to comment
+Sparky-Watts Posted December 8, 2003 Share Posted December 8, 2003 First of all, there are all kinds of variables at work here. What type of GPS are you using, are you in an open area (nearby trees, hills, canyon walls, etc.), what is the temperature, is it clear or cloudy? Also, most GPS units are not accurate down to 3 feet, and 99.99% of them won't hold an accurate reading when stationary, you must be moving for accuracy. To average out your coords on your cache, mark it and write the coords down. Clear the GPS and walk 50 to 100 feet away and come back, mark it, write the coords, continue this from 4 or 5 directions, add the coords and divide by the number of times you measured. This will give you an average coord for the cache, and be more accurate than just one reading. Some cachers even come back on different days to confirm their coords and average again. Quote Link to comment
+DavidMac Posted December 8, 2003 Share Posted December 8, 2003 Some useful instructions on getting coords can be found here under Step 3- Placing Your Cache. If your GPS has a field that reports accuracy (some Garmin units with updated firmware), I find it best to wait until this number is as low as possible (under heavy tree cover or near buildings, 16-21 feet is acceptable) before taking wypoints. If your unit has WAAS, be sure it's enabled, as this can theoretically get your accuracy to within 6-8 feet. This is only a best-case scenario though, and I wouldn't expect to be able to get below about 16 feet without a completely unobstructed view of the sky. Also, be sure you have fresh batteries installed. Quote Link to comment
+Prime Suspect Posted December 8, 2003 Share Posted December 8, 2003 Don't bother averaging. It's a pointless exercise. The reason for averaging (Selective Availability) went away years ago. Now, you're just as likely to make your initial reading worse, rather than better. See this thread, especially the final post. If you have a unit where you can't turn off the averaging (like many some Magellans), you may want to turn the unit off, go to the site, set the unit down and turn it on. Leave it until it gets a number of good locks, and store the waypoint. Quote Link to comment
+Sparky-Watts Posted December 8, 2003 Share Posted December 8, 2003 Don't bother averaging. It's a pointless exercise. The reason for averaging (Selective Availability) went away years ago. Now, you're just as likely to make your initial reading worse, rather than better. See this thread, especially the final post. Yes, I know Selective Availability is a thing of the past, but I've played with my Legend enough to know that I can still get more accurate waypoint markings by averaging the way I described above. However, I also know that most people don't do this, and still have their caches found quite regularly. I think the best thing to point out is this: Once you're close, you still gotta do a whole lot of thinking and looking. In the end, Prime Suspect is right, you don't really need to do this, so long as you have a fairly good number of sats locked before you take your initial reading. Quote Link to comment
tlg Posted December 8, 2003 Share Posted December 8, 2003 When I placed my one and only cache I thought of grabbing one of the GeoExplorers from work and using it to log the coordinates of the cache but in the end I decided I didn't really need it. No one looking for the cache is going to have a unit that brings them closer then 20-30 feet. Quote Link to comment
Peregrine007 Posted December 8, 2003 Share Posted December 8, 2003 Garmin's eTrex models don't feature 'averaging'. The position will change slightly as you stand there, but the GPS will just think you're moving around. Many Magellans will start averaging as soon as you've stopped moving, and contrary to some posts above, the position accuracy WILL get better the longer you stay there. Try this out at a known benchmark and you'll see what I'm talking about. Some units, like the Garmin GPSMap76S, will allow you to start averaging when you like, not when you tell it to start. So if you get there and only have a few satellites, you can wait to get more sats before you start averaging. It will also place more relevance on positions it records when your EPE is best - that is, if you position for 2 minutes and the first minute it says your EPE is 15 metres but the 2nd minute you're at 3m EPE, it'll place a lot more weight on the time you're at the lower EPE. Thus, a more accurately averaged final position. Users who have experience with some of these more complex units will know that, given a good enough GPS accuracy (EPE) and averaged over a long enough time, it's quite possible to record a position that is within less than 1 metre (3 feet) of a known position (usually a benchmark of some kind). Pretty darn good for a hand-held unit. Quote Link to comment
+xenophon10k Posted December 8, 2003 Share Posted December 8, 2003 I want accuracy within 3 ft. like the government. Yeah. The government spent a lot more money for their units, and they make use of more technology than can currently be crammed into your handheld at present. At least for the prices we currently pay. The best you can hope for is 10 ft, and that's under ideal conditions. Quite frankly, the fact that something that fits in your hand can be that accurate is no small miracle. You'll find a number of factors degrade GPS performance. Very heavy cloud cover, heavy precipitation, heavy tree cover, and tall buildings or rock walls can all cause either signal degradation or signal bounce. Both have an effect on accuracy. The best thing you can do is check the number of signals you are receiving and the signal strength. Let your unit auto average for a few seconds before you take the waypoint. If the signal strength is low, or you are only receiving a few signals, then you might want to take several readings and average the results. Quote Link to comment
Kerry. Posted December 9, 2003 Share Posted December 9, 2003 .... I want accuracy within 3 ft. like the government. Bit of a misconception and whoever told you that one forgot the rest of the infomation. 300 feet well that's possible as well. As for "Position Pinning" that rather different to averaging as it basically stops/locks the position until one has moved a short distance or moving at a slow speed. This was just to prevent the wandering affect of SA and gave this disallusion that the position was steady, which of course it wasn't. While Position Pinning is ON the unit is basically not navigating and simply displaying the last available position before the unit stopped. Much more important than averaging, which can be rather deceiving is being in the required location at the best possible time and take advantage when the best conditions exist. Cheers, Kerry. Quote Link to comment
Kerry. Posted December 9, 2003 Share Posted December 9, 2003 .... and contrary to some posts above, the position accuracy WILL get better the longer you stay there. That really should be "the position accuracy CAN/MIGHT/COULD get better the longer you stay there." THe other half of the time averaging makes absolutely no improvment or even decreased accuracy. Just depends how long one is prepared to stay and these days without SA most aren't prepared to spend the amount of time that is really required for averaging to be usefull have have some integrity. Cheers, Kerry. Quote Link to comment
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