How to plant a cache as accurate as possible??? (SporTrak Pro)

[Ruprex]

Hmmm, I'm a bit embarrassed that my cords were off on a cache I placed. How do I avoid such tragedy?

 

I'm using a New SporTrak Pro with the latest firmware (don't have the exact number but it is the latest). I planted a cache on a clear night sky. I had my unit off for a while then turned it on directly over the site. I let it get about 6 or 7 sats and let it sit for about 2 minutes before I logged the coords. I had WAAS enabled while doing so. I was in a bit of a hurry as security patrol was giving me the eye.

 

I also noticed a similar experience the last cache I planted for a family outing. I locked in and saved the coords. Then I did a goto and I was 25 feet off..... What the heck, over? I love this SporTrak for the hunt but it's getting me a bit uneasy for the hide.

 

Have you any suggestions for a newbie like myself? I sure would like to plant caches without being corrected on my coords...

 

Thanks a bunch folks!

 

God was my co-pilot but we crashed in the mountains and I had to eat him.

[+Brown Dwarf]

Might want to leave your unit on for ten to fifteen minutes before you take a reading. Gives it a chance to stabilize. [i keep mine on from the moment I leave the car.] What's happening is that the current causes the components to heat up just a tad -- which affects their electrical properties. Nothing to worry about.

[+briansnat]

25 feet is well within the margin of error. Don't worry too much about your coordinates. If you get a

bunch of complaints, then you might want to go back and take another reading, but otherwise if you're 20 or 30 feet off, that is to be expected.

 

"Paternalism is the greatist despotism" - Emmanual Kant

[+Searching_ut]

The sportrak is a great receiver to use for hiding a cache and saving the coordinates. My personal experience with the unit is that it's about as good as the current technology gets for this purpose.

 

Like others have mentioned, leave the unit on for a longer period of time first when possible. Every GPS I've ever owned has seemed to improve in accuracy if given a little time to get their wits about them. This seems to be especially true of receivers using WAAS. Secondly, try and average the waypoint a little longer if you can. 5 to 10 minutes seem to result in my getting about as good results as you're going to get. Finally, don't get too worked up about being off by 25 feet. Thats actually remarkably accurate, and well within the margin of error you should expect.

 

Have fun caching

 

Jeff

[+Shoebox]

I run into caches more than 100 feet off and at that point people complain and post better coordinates. 25 feet in nothing. I have noticed that when a cache is more than a 100 feet off, the more experienced players will place their gps on the cache and let it sit there for 10 or 15 minutes to get better coordinates and then post them as alternatives to the original cache coordinates. I can usually get within the 25 to 30 foot error expected when I use the coordinates from the revised reading (the ones that sat there for 10 or 15 minutes).

 

Shoebox.

[+RJ]

On the question of how to place a cache with good coordinates. I've been advised and have done this two times, with success. I have my GPS running from the time I leave the car, pick a spot to hide cache, then log the coords with the push of a button. I then walk away 30 or 40 feet, walk back to the site, record the coords with the push of a button. I do this 7 or 8 times. I go home and average out the last 3 numbers in the coordinates, using them as the final coordinates. Works for me!

[+sbukosky]

The suggestion to let the unit sit and stabilize for a few minutes is good. Though, you can sometimes see those coordinates swing around. Sometimes the cache is under cover where a good reading is hard to get. Here I will step out into the open to get a good reading and compare the distance and direction the GPS reads to my actual distance and bearing from it. I will then adjust my north and south coordinates until I'm happy with the results. But, for situations where a good reading is hard to obtain, a good clue is in order.

 

Steve Bukosky N9BGH

Waukesha Wisconsin

[+Kewaneh & Shark]

It sounds to me like you're doing everything right. As far as the Sport Trak taking a good reading: sitting in one spot for 3-5 minutes and letting the GPSr do the averaging should give you a reading accurate enough for what we're doing as cachers. The suggestion of taking multiple readings is an excellent idea too, but if your GPSr is equiped with averaging capabilities (my Meridian Gold and GPS315 both are and I think most Magellans are), I'd let it do the work.

 

If people are finding your cache coordinates to be off by only 20 or so feet, I wouldn't worry about it. That's well within the 35 foot (10 meter) margin of error for most handhelds. Remember, too, that these errors are plus/minus. It's possible that when you place the cache your error is on the plus side (lets say 20 feet north of the 'true' position), and the seekers are finding it with the error on the minus side (like 20 feet south of the 'true' position). Both units are showing the 'true' position to be within that margin or error, but they're outside that margin relative to each other, and consequently, the coordinates look 40 feet off. This could happen if you are placing your caches at night and most of the seekers are hunting in the daytime. You'll each have different satellite configurations and geometries that could result in different coordinates.

 

This gets a bit worse (or better, depending on your point of view) when one unit is WAAS enabled and the other isn't. A WAAS enabled unit has it's error cut in half to about 17 feet (5 meters), but the posted coordinates are only as accurate as the least accurate GPSr (the non-WAAS GPSr). I have to remember that fact when I'm searching for caches and my WAAS enabled unit is telling me I'm 6 feet from the cache. Chances are that I'm farther away than that. (Accuracy does have a downside.)

 

Something that I've rarely seen mentioned in these forums is a GPS issue called multipath. I know the reason it's not brought up is because it's generally a surveying issue and not one for handhelds, however multipath does affect our handheld GPSr's the same. Multipath has to do with the GPS signals reflecting off of solid objects, such as nearby bulidings or cliff faces. You said you had a clear sky with seven sats, but if there were/are any large nearby objects, they could cause a problem. Multipath was my immediate thought when you said your goto was 25 feet off right after you took the shot. (BTW: Nearby high voltage electrical lines can disrupt the signals too.)

 

I realize I'm using a lot of hypotheticals and 'it could be...' situations but I hope this helps.

 

On a more more simple note: it could be as simple as copying down the wrong coordinates. A few months ago a local cache had some bad coords and when I found it I posted the new ones. The cache owner emailed me to say that my 'new' coords looked like they were about four miles(!) off. When I checked them, I saw that I had posted the coordintes for my house. Oops.

 

Keep on Caching!

- Kewaneh

[Dale_Lynn]

I simply move away for cache..... And come back in to it!.. I do this three different times from three directions.... Usually readings are so close I don't even have to do the math to average them....

 

This is after GPS had been on for 10-15 minutes or more and have wandered around looking for the best "hide"...

 

Dale

 

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I'm Diagonally Parked, In A Parallel Universe.

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[+Harrald]

I take between 10 and 15 waypoints over 2 days (if I can).

• First I'll let the GPSr sit on the spot for a few mins to get a good lock.

• Then I'll walk about 100 feet away in as many different directions as possible.

• Every time I return to the cache I take another waypoint.

• If you can, do this over two days (at different times of day)

• Take those numbers and average them.

Using this system you compensate for different satellite constellations. It's more than most people are willing to go through.

[+evergreenhiker!]

Like others said, it's best to have the unit on for 10 minutes or so before taking a final reading. I usually take a lot of reading...after each reading, I'll go out a fair bit and walk back testing the coords. Time after time, I've always ended up using one of the last coords I've taken. I used a Garmin Etrex Summit. Main thing is do not take a quick reading and just go with that. Others have mentioned going back over several days and different times and then averaging out the coords.

[Kerry.]

Primarily be there at the best possible time.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+Last Lap Gang]

Then I do the average. I will not take a read until I have at least four satelittes locked and a range of accuracy under twenty feet on my Garmin Etrex Venture. We are new and planted four in a day which was a mjor accheievement and also a major worry in the first couple of weeks. Now we keep an eye on them and did go back to repair one as a racoon found it. Accurate coordinates were a big worry of mine too. But like most are saying if you are within the magic 20 then you should be alright.

[AdirondakMud]

From the University at Colorado

The GPS Navigation Message consists of time-tagged data bits marking the time of transmission of each subframe at the time they are transmitted by the SV. A data bit frame consists of 1500 bits divided into five 300-bit subframes. A data frame is transmitted every thirty seconds. Three six-second subframes contain orbital and clock data. SV Clock corrections are sent in subframe one and precise SV orbital data sets (ephemeris data parameters) for the transmitting SV are sent in subframes two and three. Subframes four and five are used to transmit different pages of system data. An entire set of twenty-five frames (125 subframes) makes up the complete Navigation Message that is sent over a 12.5 minute period.

 

So would that mean that you should allow a minimum time of 12 minutes for a fix?

[+Team StitchesOnQuilts]

Ruprex, in addition to all of the great advice given so far, I would also recommend the Magellan Hula.

 

When you're ready to take coordinates, grab the Magellan firmly in one hand, and swing it around your body, really fast. Swing it back. Do this a few times. Then set the unit down next to your cache, and let it settle.

 

I think you might be having problems with the averaging function of the Magellan, and that the Hula will help overcome that.

 

If you want a demo of the Hula in person, along with a better description of the "whys" (hubby explains it much better than I do), just contact me - I'm near San Jose.

 

Shannah

[+Cachetrotters]

quote:

---

Originally posted by Team StitchesOnQuilts:

... I would also recommend the Magellan Hula. I think you might be having problems with the averaging function of the Magellan

Shannah

---

 

Not sure how someone could have a problem with an automatic averaging function, but I think the implication is that there is a problem with the GPSr. There are no problems with the Magellan averaging function. If there are, I beg you to provide credible information as evidence of such.

 

don

[Kerry.]

quote:

---

Originally posted by AdirondakMud:

.... An entire set of twenty-five frames (125 subframes) makes up the complete Navigation Message that is sent over a 12.5 minute period.

 

So would that mean that you should allow a minimum time of 12 minutes for a fix?

---

 

12 minutes for a fix? No. That 12.5 minutes is time taken to download a complete almanac and if the unit is used regularily then it's being updated all the time.

 

Straight out of the box a GPSr should be left sit for that length of time and also if not used for some time so as to update the almanac.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Kerry.]

quote:

---

Originally posted by Cachetrotters:

Not sure how someone could have a problem with an automatic averaging function, but I think the implication is that there is a problem with the GPSr. There are no problems with the Magellan averaging function. If there are, I beg you to provide credible information as evidence of such.

---

 

Automatic averaging is just that automatic and one has absolutely no control over it and that can be an issue.

 

There's no real issue with the GPSr just the averaging function, which leaves a lot to be desired as a practical function. An averaging function maybe (not that averaging is really a benefit these days either ) is ok as an "option" but not a function that can't be switched off.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+Team StitchesOnQuilts]

quote:

---

Originally posted by Cachetrotters:

 

Not sure how someone could have a problem with an automatic averaging function, but I think the implication is that there is a problem with the GPSr. There are no problems with the Magellan averaging function. If there are, I beg you to provide credible information as evidence of such.

 

don

---

 

OK, I'm not that great at Markwelling, but I'll do my best. I think the last conversation about this happened before you joined. I learned about it from the forums, myself.

 

The basic issue is that the Magellan averaging feature is accurate down to a certain minimum speed. I have forgotten what that speed is. However, if you take the time to stop, scratch your head, and look around, you are stopped, but the averaging feature is assuming that you are moving.

 

This contributes to error. We have done the Magellan Hula many times, and it's amazing how often the arrow moves a dramatic distance afterwards.

 

We cache with two small children, so moving quickly really isn't an option for us, especially when we're at the end of the walk and actively searching for the cache.

 

The Hula clears the averaging function, without us having to turn the GPS off and start over.

 

Let's see, where are the previous threads?

 

GPS takes me past caches

 

That's probably the best one, because it references the previous one.

 

Now, granted, we haven't updated our SporTrak Pro software in a couple of months - maybe it has improved since then.

 

Shannah

[+Cachetrotters]

I reviewed the thread and remember following it in mid-December. I also checked the previous thread. I see that some can have a problem with the averaging if they are not aware of possible quirks.

It's just that I have not noticed anything with either of my Maggies that I would associate with that function. So, in practical terms, technique or lack of it is the main player when you end up 30 ft the other side of a cache.

 

As for averaging, do you think it is because the system accuracy is so good these days that averaging may only reveal position changes that are below the resolution of the display?

 

don

[+writer]

There's a basic problem - GPS using only one signal (consumer versions, not military) and without WAAS is only supposed to be accurate within 15 meters, or just a touch under 50 feet. You can average, but that still doesn't guarantee that the potential causes of error won't repeat themselves over the relatively short time you are at the cache. And that error can effectively be doubled because there are two devices taking readings at two times.

 

Yes, there are caches where the coordinates seem accurate. There is nothing saying that there *will* be error in readings, just that you can't guarantee that there won't be.

 

Two things can help. One is to let the unit sit for a few minutes. The receivers not only take readings of a single point, but also look at the points over time and calculate position based on movement. That is why as you are moving to a waypoint and then stop, you can see the numbers still changing.

 

The second thing is to look at the arrangement of the satellite positions. If the ones you have a lock on are bunched up in one part of the sky, or are in a line, you won't get as good a reading as if the satellites are spread out. But that won't change over a few minutes, so you may have to settle for what you can get, unless you're willing to head back to take the reading at a later time.

[+15Tango]

My GPS doesn't have averaging, so what I do is scout out the cache location, mark it as a waypoint, then go home, assemble the cache, then go back when I have a different satellite picture overhead, and if I'm within a couple of feet, I'll plant the cache and post it, and if not, I'll still plant the cache, mark it as a waypoint, and come back later with still a different satellite configuration, and take on average of the (now three) sets of coordinates I have. Half the fun of geocaching is getting to within 30-40 feet and then trying to decide what nook of what tree contains the cache. I've found caches that were right on, but they've been suspended by a cord in a hollow tree, so I've missed them after searching the general area for quite a while before "the force" kicks in and I find the cache.

 

Think Globally, Act Locally