How Are You Sure That Your Coordinates Are Right

[+TexasGringo]

When placing a cache...how do you get your coordiantes to be sure that you are close?

 

I have a Garmin 60CSx and I press "Mark" and then "Average" for about 100 averages. Then I walk back to my car...turn the GPS unit off and then back on to reset it...and then follow those coordinates to see where they go. I have noticed that it varies a little...maybe that is the GPS error?

 

I'm afraid that someone will waste time on a trip to nowhere...based on bad coordinates.

Edited July 29, 2006 by GURU4HIRE

[+PMaholm]

When placing a cache...how do you get your coordiantes to be sure that you are close?

 

I have a Garmin 60CSx and I press "Mark" and then "Average" for about 100 averages. Then I walk back to my car...turn the GPS unit off and then back on...and then follow those coordinates to see where they go. I have noticed that it varies a little...maybe that is the GPS error?

 

I'm afraid that someone will waste time on a trip to nowhere...based on bad coordinates.

 

I've been using the "average" feature on my 60CSx for the caches I'm working on I usually let it average for a couple minutes or so. When you go to "Mark" a new waypoint there should be a button that says "Avg" that'll do it. It'll even tell you the predicted accuracy. Hope this helps!

 

-PMaholm

[+wiseye]

After you have avg. your waypoint, do not turn the unit off! You are losing the accuracy that you just built up by having it on. Just walk away and then follow the arrow back to your waypoint, you should be pretty close. When using the avg. function your unit should give an estimate of error or ETE when this gets down to 5-10' you are pretty accurate. save the waypoint then. The longer you average the more accurate the waypoint will be.

[+Team GPSaxophone]

I don't have an "average" feature on my Legend, so I let the GPSr sit on teh cache spot for a minute or two before taking coords. I also walk in a circle around the cache area (for example, if it's hidden in a tree I walk around the tree) and mark waypoints as I walk. I average these coordinates by hand along with the first one I took. It's accurate enough to get people within the expected 20' accuracy of most GPSrs

[+Lil Devil]

After I hide the cache, I mark the coords. Then I walk away about 100 feet in some random direction then walk back to the cache, where I mark a waypoint again. I repeat this a few times, walking off in a different random direction each time. When I get home I plot them all on a map. Usually most of them are only 1 or 2 thousandths off, with maybe one of them 60 feet off I'll throw away that outlier, then average the rest of them.

 

Using this method I've never had anyone complain about my coordinates

[+CharlieP]

Although averaging for a few minutes is likely to give better accuracy than a single reading, the average may still be off 40 feet or more. If satellite geometry is not good at a particular time, it is necessary to wait and take readings at a time when geometry has changed enough to make a difference. Testing has shown that a large number of averaged readings taken over a period of less than 30 minutes will not be as accurate as a lesser number of averaged readings taken at times separated by at least 30 minutes.

 

I try to take several readings (mark waypoints) over a period of less than 5 minutes or so, marking only when the GPS shows an EPE less than 20 feet, preferably 15 feet or less. Then this process is repeated 30 minutes or more later. This process may be repeated a few times. Then I average the waypoints on the map page visually. I have tested this method on known positions and if reception is good, it is usually within about 5 feet, if three or more sets of readings are taken, each set separated by at least 30 minutes.

Edited July 30, 2006 by CharlieP

[+JDandDD]

When possible, I visit an area 3 times. The first is a scouting mission to see likely spots and at that point I take averages of each likely position. The second time I visit it to choose the actual cache spot from the candidates and I take another averaged reading. Finally, when I return with the cache I take yet another averaged reading. Then I look at the points on the map to see what level of agreement there is and set the final from that review.

 

If that isn't possible I take about a two minute averaging of the spot. However, I always visit the spot at least twice so that let's me check the accuracy on two different days.

 

JD

[+Ed & Julie]

After I hide the cache, I mark the coords. Then I walk away about 100 feet in some random direction then walk back to the cache, where I mark a waypoint again. I repeat this a few times, walking off in a different random direction each time. When I get home I plot them all on a map. Usually most of them are only 1 or 2 thousandths off, with maybe one of them 60 feet off I'll throw away that outlier, then average the rest of them.

 

Using this method I've never had anyone complain about my coordinates

 

That's exactly what I do. I'll take 6-10 sets of readings, throw out the two furthest out, and average the rest.

[+Team Neos]

We used to use Legends, and used the method Lil Devil described, although we only walked about 30-50 feet away before returning to make a new mark with the two units. We would each make at least four marks--more if they were noticeably different.

 

Last spring, we had the Legends and our newer GPS units (60cs and 76cs) with us while on a trip to SC. We were helping a cacher there place his first cache. He didn't have a GPS yet then, so we wanted the numbers to be very good to carry him through until he did get his GPS (since he wouldn't be able to go back out there and check them another day for awhile).

 

Our new GPS units have averaging, so we decided to use four GPS and compare the numbers we got. To figure out the cords we did the usual thing with the Legends, while we let the two with auto-averaging sit on top the cache and do their readings. We marked the auto-averages and then used those two units to do the 'walk away and return' thing a couple of time each. And then we let them sit on the cache again to get another set of averages.

 

When we looked at all the readings, there was almost no variation. One set of numbers kept repeating for the location, and when we averaged out all the cords, that was indeed the magic number. We left the SC cacher feeling sure the coords would hold up, and indeed several people have said that good coords took them right to the cache.

 

It was interesting to see four units come up with almost exactly the same data, by either method.

[+Z_Statman]

Averaging is great and does fine tune GZ, but, remember that the civilian response to the GPS network is only about 3-10 meter accurate so being off 20' can be "good."

 

Also, I have a Meridian Color and my GZ is pretty much always about 3-5 meters off from coordinates provided by a Garmin.

 

Bottom line - get close and work the "geosense" from there on.