# How accurate is accurate enough?

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I am a fairly new geocacher and I had a Question about the accuracy of coordinates when placing a cache. I've placed two multis already, both with positive results,but I was wondering how to get more accurate coordinates? The technique I use to is to place my cache, set the GPSr on top of it( with a good track on satelites of course) and use the average feature. I let the gps click away and at times have let it take 2000 to 2500 readings with an estimated accuracy only up to 14 feet. Considering the size of the planet earth, 14 feet is pretty good but I think I can do better. When averaging the position, how many readings do you need to get the most accurate results? Obviously, the longer you let the Gps average, the more accurate your results will be.I sat and waited for close to two hours on one cache and it still wasn't as accurate as I wanted it to be. Do you wait 3 hours, 4 hours 5 hours? What's the limit and where's the threshold, the point where it doesn't matter and its not any more accurate?

Geocaching.com's hide a cache suggestion is this:

"Some GPS units have the ability to do averaging, but if yours can't, the best suggestion is to take a waypoint, walk away from the location, then return and take another waypoint. Do this around 7-10 times, then pick the best waypoint (I've done this with a Garmin eTrex on a cache),"

More specifically, I walk away from the location in all ordinal and cardinal compass directions (N, NE, E, SE, S, SW, W, NW), and walk out a distance of at least 75 feet in theses directions, then return to the location and wait 1 to 3 minutes (depending on satellite acquisition--more time for less acquisition). Repeat this until eight directions have been noted (bring a pencil and paper to jot it down. Do the math for averaging and....voila! Time involved: 10 to 30 minutes.

David Berne

If your gps Averages, let it average for 10 or 20 min. If you get with 30 feet you are doing good. Remember the handheld GPS receivers are only so accurate. Even the \$12k units will only get within a meter on the best days. Don't let the worry spoil your fun. I don't even average because my unit does not average. I just take 3 or 4 readings and let it go. No one has complained yet.

ENJOY THE OUTDOORS

If it says accuracy 20 feet, thats good enough. Then write some clues as to the location of the Cache. On two of my Caches readings were taken with a map program prior to planting the cache. Went to the mark, and then wrote the clues.

If I have to look for a place to hide it your going to have to look also.

Tahosa - Dweller of the Mountain Tops.

Don't sweat it *too* much. Remember that although the GPS meter may say "accurate to 20 feet," the civilian GPS system at best suggests an accuracy within 15 meters, or just under 50 feet. (In fact, as part of my book research, I'll be talking to a number of experts to find out just how much the accuracy readings on the GPS units mean.)

Now, that's the accuracy of *one* GPS unit. But, theoretically, yours can be off by as much as 15 meters in one direction, while the GPS unit of the person seeking your cache could be off by as much as 15 meters in the other direction. That means, you can have 30 meters (or almost 99 feet) variation from the "true" coordinates. So, yes, let it average, take a number of readings, but don't get too worked up over it, because you cannot create more accuracy than the system allows.

It's all about satellite geometry. The better the positions of the satellites, the better your "true" accuracy ought to be. Look for birds spread out in a big circle, not all bunched up together. If they are all bunched together and my accuracy is higher than 20 feet, I won't use those coordinates here on the website.

I try whenever possible to average coordinates not only using the averaging feature, but also by returning to the cache location at least two times before posting the cache to the website. Go at different times of the day when the satellites are arrayed differently. It takes around a half hour, according to the tech experts here, for satellite positions to change materially. So, averaging for 20 minutes can just mean 20 minutes of inaccurate readings.

I have had a grand total of one complaint about my coordinates. I went back and checked, and navigated to a zero point within 15 feet of my cache. That's close enough.

x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x

Next time, instead of getting married, I think I'll just find a woman I don't like and buy her a house.

Generally averaging in the time available is a waste of time even to the point of being less accurate. One generally spends time averaging without knowing if the result will be better or worse.

Basically a 24 hour average has some meaning but is not practical as it traps 95% of the possible errors. 99.99% of all errors is a 30 day average issue. None of these options is practical.

Of more importance is the state/strength of the constellation at the time and that is basically more important than averaging, being at a certain location at the best possible time.

Cheers, Kerry.

I never get lost everybody keeps telling me where to go

Thank you for your help guys and gals . I appreciate the feedback that I get in these forums.

One could go so far as to take a laptop and use available programs to do the averaging for you with greater precision than the display of any recreational GPSr allows (such as SA Watch).

The practicality of it dims when you consider that anyone looking for those coordinates will not be sticking around for several hours averaging to find out if they are close enough. For searchers it's a walk up, find it, walk out proposition. So at a minimum you end up with a probable 10-40+ ft error anyway, depending on WAAS, geometry, canopy, etc., even if the hider used survey equipment to locate the cache to the centimeter.

You would get satisfaction of knowing that you have taken steps to ensure the coordinates are the best you can measure, but it can be overdone.

Cheers,

Don

I used to worry about averaging, but now I just make sure I get a good sat lock and take I one reading. I get as many complaints and compliments about the accuracy of my coordinates as I did when I averaged them.

"You can't make a man by standing a sheep on its hind legs, but by standing a flock of sheep in that position, you can make a crowd of men" -Max Beerbohm

I used to average the heck out of my caches. Now what I do instead is to wait until the numbers stop moving. I quickly move the gps back and forth to stop averaging and let it settle again. After I do this a couple of times the numbers stay the same and I know my coordinates are good.

But make sure you have a good sattelite reading first. If the signals aren't strong no one will find your cache. I've made that mistake when I first started a few times.

I hear voices.....and they don't like you!

quote:
Look for birds spread out in a big circle, not all bunched up together. If they are all bunched together and my accuracy is higher than 20 feet, I won't use those coordinates here on the website.

Now I need to look for birds when setting a cache?? This sport gets more complex every day...

You could always set your cache close to a trig point!

On a more serious note, if your accuracy is not ideal check to see whether something is obscuring your view of satellites. If so take a fix elsewhere and extrapolate, if not record the poor accuracy. If baggers are unaware of poor accuracy it can lead to ambiguity as to the possible search area. This is not to suggest that you should not set a cache where the accuracy is less than perfect.

[This message was edited by M&Ms on December 10, 2002 at 08:38 AM.]

quote:
Originally posted by enfanta:

quote:
Look for birds spread out in a big circle, not all bunched up together. If they are all bunched together and my accuracy is higher than 20 feet, I won't use those coordinates here on the website.

Now I need to look for birds when setting a cache?? This sport gets more complex every day...

I think the term "bird" should not be taken too literally. I would use the term "harbinger of doom" instead. If I see a large flock of evil-looking things hovering over my cache site, I assume that it won't be there too long after I leave. I always try to place my caches in areas with singing chipmunks and dancing rabbits.

You could go to school for 4 years, work for a surveyor for 4 years, become licensed with you state, buy a \$30,000 total station theodolite, a few thou in tripods and optical targets, traverse loops from survey monuments or shoot a reading off a star, crunch numbers in a good survey software package for a few thou more, and then get down to sub-inch accuracy level.

Well, maybe that's overkill. You could hire a surveyor and they might charge a few hundred perh cache.

well... if you can get sub 6" groups from 25 yards i would say you are accurate enough. i have a hard time doing that without really concentrating with a rest. it blows my mind that some people can do better offhand. i guess im just a lousy shot. maybe if i wouldn't have drank that six pack before hand. oh, wait... i thought this was the Glocktalk forum!

quote:
... i guess im just a lousy shot.

Well, that's why! Look at yer picture. Yer holdin' it all wrong!

quote:
Originally posted by Cachetrotters:

quote:
... i guess im just a lousy shot.

Well, that's why! Look at yer picture. Yer holdin' it all wrong!

Yep, thats right...yer right, his grip is off. No good aim game with a grip like that.

------> BUGOUT to avoid the FALLOUT ------>

I've about abandoned averaging. Instead, I will let the receiver lay for a while until I see little change in coordinates or accuracy and punch the coordinate in. I'll then check it by approaching the point from a few different directions. If I feel that it is off, I might add or subtract a thousandth or two until I'm happy.

Steve Bukosky N9BGH

Waukesha Wisconsin

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