Random Thoughts On Gps Accuracy

[+Team BAHumbug]

This has certainly been a learning experience... As a new cacher, I have scrambled over hill and dale and dodged the odd cyclist in the pursuit of those elusive containers... Naturally, the idea of placing my own became a strong desire too. Then the fun REALLY began!

 

The first instinct was to just "drop a box" and dump the co-ordinates on the web. Then after reading lots on the forums, I started to think "What is really fun?" - the most fun I have had has been on multi-caches. My desire swung in that direction.

 

Two weeks ago the planning began. Where would be a good location? Where would be good places to get the clues? while I thought about it, I put the first box of goodies together. Two weeks later, that box is still sitting in the basement.

 

The first day was spent eagerly running around getting co-ordinates of the three target sites, and scribbling the information from plaques contained at the first two sites to allow the deciphering of the third. That evening was spent "doing the math" and making sure things worked properly.

 

The next day was a dry run (paper only) to make sure the directions were clear without asking further questions. It became clear a re-write of those directions was in order. Test. Repeat. Test again.

 

Following that I went back using the gps to make sure everything would work smoothly. Oddly enough (and this will come of no surprise to anyone who has tried this before) - the co-ordinates were wrong! Up to 30 metres off in some cases! Back to the drawing board!

 

Fast forward to today. After re-visiting all of the sites 7 or 8 times, morning, noon, and night I have gotten the co-ordinates down to the point where they are only 6 - 8 metres off. One or two more dry runs and it should be worth registering the cache on the web site.

 

Things I have learned:

 

1) This is a lot harder than it looks!

2) GPS accuracy is an oxymoron.

3) GPSr's seem to work better at night than during the day.

4) Co-ordinates taken at night won't lead you to the same location in the day.

5) Two GPSrs held side by side can give different co-ordinates.

 

In closing, I just wanted to give a very heartfelt thank you to all those people that have done this before, so I could grab my handy-dandy gps unit, and wander about until I could shreik "I found it!" I really do appreciate your efforts, it's been a lot of fun.

 

Thanks again, and I hope that (when this is finally done) you can have some fun in return!

[+EraSeek]

1) This is a lot harder than it looks!

2) GPS accuracy is an oxymoron.

3) GPSr's seem to work better at night than during the day.

4) Co-ordinates taken at night won't lead you to the same location in the day.

5) Two GPSrs held side by side can give different co-ordinates.

 

1) true

2)to a point, yes.

3)Yes. The ionosphere is more active in the day which can cause signal delay and an inaccurate position. That where WAAS can be helpful

4)Depends, on ionospheric disturbance and geometry of the Satellites.

5) true. Different software, electronics, antennea, plus holding them close together itself can cause interference in the units.

 

Listen in most cases, unless you are under tree cover or next to walls (muti-pathing) you should be within about 10'- 15' of another reading.

[+NightPilot]

The biggest difference may be satellite geometry. You aren't seeing the same satellites in the same places at night as you are in the day, and I'll bet a dollar to a donut that you aren't doing this anywhere close to 12 hours apart. Different times, different satellites in different numbers = different coordinates. WAAS will take care of the ionospheric differences, that's why it was invented, for greater altitude accuracy for aircraft. It doesn't make as much difference for horizontal accuracy, but it does help. Of course, if you aren't in the US, WAAS isn't going to be of much help in any case.

 

The way to get a good set of coordinates is to go back many times, at different times of the day and night, and record the coordinates each time. Then take an average of the coordinates you recorded. You really only need to average the decimal portion, because everything else should be the same. This should give a good set of coordinates that everyone should be able to use to get close to the cache. But if they get within 10 meters, that's about as close as you can expect to do reliably, every time. If you're down to 6 or 8 meters, that's probably good enough.

[+Team BAHumbug]

I'll bet a dollar to a donut that you aren't doing this anywhere close to 12 hours apart.

 

16 hours, the last time... This has been done over a couple of weeks. Mostly during daylight hours.

 

Of course, if you aren't in the US, WAAS isn't going to be of much help in any case.

 

Nope, not in the US, and to top it off, the final point is on the north side of a hill... (And just to make life difficult, buildings on either side and tree cover! Woo-hoo!)

 

If you're down to 6 or 8 meters, that's probably good enough.

 

Thanks, I'm pretty much there, I guess. Now just for the polishing of the documentation.

[+GeoCyclist]

Sounds like you're doing a good job with your first cache. I had my wife follow my clues after I finished to make sure there weren't errors and that everything made sense.

[virgo91967]

Then on top of any initial iinaccuracies of a caches coords, you have to also add in the fact that the cache hunter's GPSr may also have the same amount of (in)accuracy. If your coords are good to witn ,say, 5 meters and the cacher's is off by 5 meters as well, then it is probable that that cacher can be up to 10 meterrs ( abt 15-20 feet) off.

 

I have learned to be very very retentive about my coords... I shoot 20 different readings and average them out . I also like to use UTM coordinats then convert them over . Partially becaue my ancient Magellan 310 only has 2 decimal accuracy using Lat Long... and because I know it will be precise to within 1 meter/yard.

 

my advice is to take different readings ( at different days if you like) average them out then go back and check them at leat once more.

 

Yep... is aggraviting to go looking for a vcahce for well over an hour ( and a 2/2 at that) to discover that the original coords were 20 to 30 feet offf to begin with

 

also a good idea to use a service such as topozone.com to double check the location with nearby landmarks ( streets, roads, streams...)

 

You have bitten of a pretty big bite with making your first cache a multi team bah Humbug. best of luck

[+Alan2]

Save the waypoint when the accuracy is 20 feet or under and you will be good to go for the cache coordinates. SHooting 3 and averaging with the same under 20 feet works better but you'l find they variance wil onyl be a couple of thousandths..

[+Alan2]

That's more accurate then my speelling.

[+EraSeek]

Your accuracy reading is just a programmed guestimate. But it is a good relative tool. Yes, mark your postion during your lowest accuracy reading. Take maybe four readings, and compare for consistancy. Do a search for nearest waypoint on your GPS and see which ones settle in the closest. Test with the "goto" function. Walk away, goto, search for your own cache, see how it reacts on your approach.

[+Thot]

If your coords are good to witn ,say, 5 meters and the cacher's is off by 5 meters as well, then it is probable that that cacher can be up to 10 meterrs ( abt 15-20 feet) off. 

I hate it when I nitpick, but 10 meters is really 33 feet.

[+EraSeek]

If your marked position is messed up, there is a good chance the finders coords will be messed up too, therefore at some point they will come across the very same coords as you. Don't sweat it. Do your best.

[+Team BAHumbug]

[

I had my wife follow my clues after I finished to make sure there weren't errors and that everything made sense.

 

Mine too - She's much better at the "get the message across to EVERYBODY" than I am...

 

You have bitten of a pretty big bite with making your first cache a multi

 

(Findahappyplace! Findahappyplace! Findahappyplace!) It's another lesson in patience I am slowly learning.

 

I shoot 20 different readings and average them out .

 

I'm up to nine on some of them... When I start getting four repeats of the same number I start to feel a little more confident. Naturally, the last point where co-ordinates are the most important is the hardest to get consistant readings from - go figure!

 

Thanks all for your advice and words of encouragement.

[+TotemLake]

And that will happen. I have a park near me where the coordinates can wander by as much as 30 meters. It sounds like youv'e taken this more seriously than most newbies of the same experience. Get the best coordinates you can, include the fact of bouncing coordinates in your description, provide good hints in case all else fails, and don't fret too much. When most people realize they will have to deal with widely wandering coordinates, they're more apt to forgive the inaccuracies that can come up. Be sure to up the difficulty a by 1/2 - 1 star to reflect this too.