Does The Gps Just Get You To The Right Part Of Tow

[+Thot]

I’d never heard of geocaching until I stumbled onto it 2 nights ago, and it sounds like fun. I’d never seen a GPS receiver before this afternoon. A friend had a castoff Magellan GPS 315 he lent me to give this game a try. But this thing can only accept coordinates to the nearest hundredth minute. By my estimate that’s nearly 60 feet. This seems to mean it will have one standing in a 120 foot diameter circle of uncertainty trying to locate a hidden object. It seems like that would be quite a challenge if it’s anything more cluttered than an empty Walmart parking lot. The caches near me are in a semi-urban area, so it seems like a detailed street map would get you that close or closer.

 

Am I missing something here?

[+Renegade Knight]

Yes you are.

 

60' is your maximum error ignoring anything but the missing last digit. On average your error will be 30' since that missing last digit will on average only be 0.005 off.

 

Given that a GPS is 20' accurate give or take a few feet you are not doing too bad and should not hesitate to go geocaching.

 

Besides unless you are hunting a microcache or standing in a huge rock pile there are only so many places a cache can be.

[+Thot]

60' is your maximum error ignoring anything but the missing last digit.  On average your error will be 30' since that missing last digit will on average only be 0.005 off.

 

Given that a GPS is 20' accurate give or take a few feet . . .

Well, maybe, but the gadget says it's only accurate to about 50’ (not 20), so one could think of it that this is an 80' radius – entry error of 30' plus location finding error of 50'. The person who provided the original coordinates probably wasn’t accurate to better than 10’. So, it seems the ambiguous circle could be as large as 180’ in diameter - 2 X (80 + 10) = 180.

 

If the cache placer was using my GPS his error would be 50' not 10 so the total would be 2 x 130' or 260'.

 

Now I think my original estimate of 120' was too small.

Edited June 25, 2004 by catch_22

[+Nappy10]

When placing a cache...the hider generally takes several readings at different times. The error is most likely not going to be in the same direction every time. Therefore, if you average all the readings, a cache hider can be pretty dead accurate.

 

Also see if you are standing under any cover (trees, etc.). This may cause a larger error range.

 

Try this method: When you are in the general area of the cache, stand in one spot a look at the GPS reading. Draw an invisible straight line between you and the direction the GPS is indicating. Make a note of it. Move to a different location and do the same thing. Whereever those two lines intersect, thats a good indication where the cache is near.

 

 

 

 

Source: Markwell's Update to FAQ

Edited June 25, 2004 by Nappy10

[+Renegade Knight]

You are looking at the worst case possible.

 

The 50' isn't reality. It's probably closer to 20 if you update your firmware. Do a search on the Magellan 315 in the GPS and Software section . You will be suprised at how many people have a lot of success with it.

 

Check out this post with a comment on accuracy.

http://forums.Groundspeak.com/GC/index.php...=0entry770070

[Keystone]

RK speaks the truth. Listen well. There are more modern consumer units with allegedly greater precision (and at least a better coordinate display), but many do fine with older ones like the 315. Sometimes it just takes a bit more concentration, which is fun!

 

Welcome to geocaching!

[+Thot]

The 50' isn't reality.  It's probably closer to 20 if you update your firmware.  Do a search on the Magellan 315 in the GPS and Software section .

I called it the "circle of uncertainty", so yes the outer limits of the circle would be the extremes, but the circle represents the area where it could lie.

 

I’m basing the 50' feet (actually 15 meters) on Magellan’s current ad on their website. It says, “You’ll always know exactly where you are and where you’re going to within 15 meters . .”

 

It seems like it would be unusual for a company to understate the ability of their wares.

 

http://www.magellangps.com/en/products/product.asp?PRODID=2

[+Renegade Knight]

...I’m basing the 50' feet (actually 15 meters) on Magellan’s current ad on their website. It says, “You’ll always know exactly where you are and where you’re going to within 15 meters . .”...

You left out the "or better" part of that line!

[+Thot]

Magellan’s current ad on their website says, “You’ll always know exactly where you are and where you’re going to within 15 meters . .”

You left out the "or better" part of that line!

Yeah, well . . .

 

By the way, thanks for the tip about the firmware upgrades.

[bnolan]

Just enter the coordinates in UTM

 

The resolution of UTM coordinates is 1 meter, which exceeds the accuracy of the GPS.

[+Thot]

Just enter the coordinates in UTM

 

The resolution of UTM coordinates is 1 meter, which exceeds the accuracy of the GPS.

I don't know what UTMs are, but I've only been fooling with this thing for a few hours. So far I don't see a way to enter coordinates in any form except lat/lon.

[+Thot]

Never mind. I figured out how to switch the coordinate system to UTM. Figure I don't have to understand it to enter it.

 

Do you think that the device keeps the higher resolution than it can display? If it just rounds to the nearest hundredth minute then entering it to greater fidelity won't achieve anything.

[bnolan]

UTM is a rectangular coordinate grid. The world is divided into small Zones (I live in zone 12T) Then the next two numbers are meters east and meters north of the zone reference.

 

When you save a waypoint in UTM, the GPS uses that resolution of information. The display is rounded off to the hundredths of minutes. Your GPS was designed back in the days when the government was adding random error to the signals for defensive purposes. Back then, the extra digit would just have danced around randomly, so why display it. When the government stopped adding the random error in May of 2000, all of a sudden there was enough accuracy for someone to hide a treasure and someone else to find it, thus geocaching was born.

 

Many people have reported success using UTM on these forums with older model GPSr units. The UTM coordinates are printed on the cache page right underneath the Degrees and Minutes.

 

Have fun cacheing.

[+Thot]

You are looking at the worst case possible.

 

The 50' isn't reality.  It's probably closer to 20 if you update your firmware.  Do a search on the Magellan 315 in the GPS and Software section . You will be suprised at how many people have a lot of success with it.

Your are right!

 

I have confirmed what you said and better. I located a microcache today that I doubt I could have found if the error had been as much a 40'. In fact the gadget pointed directly at it, which, given how it was hidden, was necessary for me to find it. It takes a little practice, but I'd guess it pointed within 2' of the cache.

 

So, the answer really is, Try it, you'll see."

 

So, I now have to wonder where the up to 50 foot error Magellan specifies comes from.