Marking Accurate Coordinates

[MouseFart]

I've had my first ever GPSr for about a week now and still learing to use it. It is the Garmin Legend. A couple of days ago, I used an old stump in an open field to mark as a coordinate, then backed off a couple of hundred yards and pressed "goto". It directed me to within 17' of where the stump was marked, but I was still off 17' and was not happy with this.

 

Just now, I went back out to the stump and in a triangular pattern around the stump (about 10' away on all 3 sides), marked 3 different marks. Went into the house and used a calculator to average the 3 lats and 3 longs, then programmed this figure in as mark #4.

 

Pressed "goto" on mark 4 and this time, it directed me to within 1'7" of the stump. Now this is more of what I was thinking a GPSr should do.

 

Did I just get lucky by doing this or is this how a true cache should be logged once I start placing them in locations that I have in mind?

 

Thanks,

Rick

[+JohnTee]

I've had my first ever GPSr for about a week now and still learing to use it. It is the Garmin Legend. A couple of days ago, I used an old stump in an open field to mark as a coordinate, then backed off a couple of hundred yards and pressed "goto". It directed me to within 17' of where the stump was marked, but I was still off 17' and was not happy with this.

 

Just now, I went back out to the stump and in a triangular pattern around the stump (about 10' away on all 3 sides), marked 3 different marks. Went into the house and used a calculator to average the 3 lats and 3 longs, then programmed this figure in as mark #4.

 

Pressed "goto" on mark 4 and this time, it directed me to within 1'7" of the stump. Now this is more of what I was thinking a GPSr should do.

 

Did I just get lucky by doing this or is this how a true cache should be logged once I start placing them in locations that I have in mind?

 

Thanks,

Rick

 

Rick,

 

You will probably get some more learned replies, however . . .

 

This is an excellent way to mark an accurate coordinate. You could probably only improve on it by trying it several times over different times of day and different days. Now, keep in mind that your GPSr is probably going to get, at best, something like a 7' estimated position accuracy. There's going to be a little leeway in there.

 

There are several good discussions on position accuracy . . . I think you'll find links to them in the Resources pages.

 

I'm also using a Garmin eTrex Legend.

 

JohnTee

[+Lasagna]

With a small enough offset from the actual cache, this will work. A better way would perhaps be to mark a waypoint right at the cache, then go a good distance away and reapproach the cache and mark again. Head off in another direction, and repeat. Repeat until you've got a good sampling. Average these coordinates.

[+Team Neos]

A couple of thoughts. First, chances are that if you went back and did the same thing tomorrow as you did the first day, you might end up with the same results as you did today. Coord vary over time and under various conditions. Second, a lot of people would consider 17 feet "close enough" since GPS can vary by that much. Did you notice what your accuarcy was at the time?

 

That said, when we used the Legends, we averaged our coords the way you did the second time--sort of. Actually, we would each take at least four readings and then average the eight cords together. It's a bit neurotic, I admit, but we do get lots of compliments for the good cords.

 

Some days you can tell about what the final cords will be while you are marking them, because the same numbers keep showing up. Other times, the cords vary by a good amount. When I get cords that vary from each other, I try to remember to take new readings the next time I go out there, and I watch the cache page for comments about the cords.

[+jimear1e]

If you hide it they will come and your logs will tell you what you want to know.

 

I have only been at this a short time and have a few caches hidden. Averaging has been my method and so far no complaints. The more coords you have to glean your average from the better. I have a Magellan Explorist 200 and a Garmin Legend and the fluctuation in coords for me seems to be associated with heavy tree cover.

[+Manco46er]

From what I read, you shouldn't have a problem with coord's. As previously mentioned, 17' will most likely result in a successful find. When I place a cache I'll set my GPS down where I've hidden the cache and let it sit there and wait until I get what I believe to be the best accurate reading I will get. On one of my caches I was having a hard time getting an accurate reading so I moved a little ways to a clearing where I could get a good signal and projected the waypoint. I of course mentioned this in the cache description. I think the most important thing to do is once you have decided on the actual coordinates that you test them several times from alternating directions from the cache. If your tests take you to the cache, you know you've done your job right! Where I cache, generally you are looking at anywhere from 15-25 feet accuracy unless you are standing on top of a mountain.

[+Renegade Knight]

You got unlucky with the first single reading and the GPS worked normally for the second trial.

 

There should not have been much difference between your single coordinate and the averge of 3. For averaging to truly work you would need to revisit the site every time you check on your cache and take another coordinate on the cache and average those. Over the course of say, a month you might have enough information for a good averge.

 

For another GPS with averaging built in you could leave it there overnight (not a good idea, for one thing your batteries won't last that long) and over that 24 hour period it should have a good coordinate.

[+Renegade Knight]

A little explanation.

 

The purpose of averaging is for the GPS to get as close to the real coordinate as possible.

 

Your experiment was about finding the spot you just marked wiht the GPS again, which has nothing to do with the real coordinate except by coincidence.

 

At any moment in time your GPS can be 20' off of that real coordinat. As long as the conditions that cause the error don't change abruptly (and they typically don't) you should be able to mark the stump, and come back to the stump (5 min later) and be fairly close to the stump even though the coordinate you are using is 20' off actual. That's becaues your error isn't changing so the GPS comes up with the same 'wrong' location twice.

 

Take that same coordinate that got you back to the stump today and try it again in a couple of days. It should take you to another spot because a few days is enough for the conditions causing your GPS error to change. Or even go back at night.

[+Team JSAM]

When I am hiding a cache I stand on top of the cache for about a min take a reading then walk at least 100 ft away and go back, I do this from several different ways until I get the same coords at least twice. Then I walk about 250 ft away and watch to the cache with the coords I am going to post just to make sure it takes me there.

[+briansnat]

Pressed "goto" on mark 4 and this time, it directed me to within 1'7" of the stump. Now this is more of what I was thinking a GPSr should do.

 

Sure, but you could go back there tomorrow and find that your original coords (17 feet off) are more accurate than the ones that were only 1' 7" off today.

 

When I first started hiding caches I meticulously averaged coordinates the way you did on your second try, sometimes taking 20 readings. I've since started just making certain I had a good sat lock and good sat alignment and taking one reading and going with that. I find that my coordinates are just as good as they were when I averaged a number of readings.

 

You have to remember that even under the best conditions your GPS is only accurate 10-30 feet. When you take into consideration that the people searching for the cache will have the same accuracy, I think the quest for pinpoint accuracy is a waste of time.

 

As a case in point, I have caches where some finders have praised my accurate coordinates and other finders have complained that they were 50 feet off.

 

Almost counts in horseshoes, hand grenades and cache coordinates.

[+boda]

I agree with Briansnat. You can spend a lot of effort and not get much more accuracy in return. I use a GPSr with an averaging feature and do that over time as I plan and develop the cache. The coordinates never vary over 12 feet over time, but they do vary some. Cachers have said that the coords were right on.

 

A friend and I set out a series of caches up a long fire road. It is all uphill for 4.5 miles (and downhill all the way back, thank goodness). We didn't want people to get up there and have a difficult time due to bad coords, nor did we want to go up again just to redo them. We used 2 GPSr. We each took readings, compared them, then backed off and came back to check them again. We then went up the mountain to set the next cache. As we came back down, we checked them again for accuracy. We adjusted one cache, and later cachers have had no problems.

 

Good luck.

[+Markwell]

I agree with what everyone else says (even though my website says differently in the FAQ).

 

If you average your waypoints that you acquired in an hour's period of time, you could be averaging bad waypoints. Finding their average won't necessarily make it a better reading.

 

Finding the average waypoint over several days and visits at different times of the day and under various conditions MIGHT help you get the best possible coordinates, but usually not more than would be necessary for finding the cache.

 

The only thing that I've seen even anecdotally improve coordinates was having WASS enabled.