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Gps Off By 20-30 Feet


west4961

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What were you actually expecting? 20-30 feet, about what might be expected. On a good day if your less than 40 feet that's a bonus, bad day could be 120 feet.

 

It's really not the accuracy figure that's important as much as the integrity of (knowing) the accuracy and how good the actual result is.

 

Cheers, Kerry.

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Hmmmm....  the new 60CS seems to hold from 15 ft to as good as 7 feet for me.  I am AMAZED at just how good it seems.  Very repeatable walking around the park too.  I am sooooo glad I upgraded from the Vista!  :)

Ya know, the REAL truth is that they are all precisely the blessed same. From the EBAY $2.37 model to the 60 umphmfft, what is it? C something?

 

Of course, we don't all have a dog the size of a small moose to back us up....so, ya know, I agree with you. - JamesJM

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Where are you guys coming up with how far off you are? Are you comparing your readings to known markers or to caches?

I'm saying that my GPSr can be off by as much as 50 feet or not at all, in relation to the exact location of the cache. So that takes into consideration all the variables that can occur when using a GPSr. I don't think any are going to be 'accurate.' And what fun would that be, anyway?

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Accuracy depends on many different things combined together, each of them degrading the sum. It is constantly changing, sometimes for the better, sometimes not.

 

How many sats were in view?

Cloud cover, weather?

Trees, canyons?

Standing next to a building or a sherman tank?

GPSr design?

Sun spots?

WAAS correction?

 

What you really should be looking at is your EPE. Someone that knows more about it than me will undoubtedly chime in with the technical side.

 

Whats important in a GPSr to me is WAAS, Helix antennae (most garmins use a patch), battery life and drop-ability.

 

I'll leave submeter accuracy to the govt units. Besides, if every cacher had coords and receivers that put you within 1 foot guaranteed every time....how much fun would the hunt be?

 

I'm not saying that intentionally posting bad coords as good is cool, I have enough trouble with 30 feet sometimes.

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I have a Garmin iQue and use an external, Gilsson helix antenna (amplified) on the top of my "poking stick" so it remains above my head most of the time. This allows me to not block the signals with my body as I am hiking in and also lets me stick the antenna above some of the lower, dense cover, when nearing the cache. Today, I had 6.2 feet accuracy reported, with WAAS enabled and 12 sats locked.

 

Although I am a bit confused as to what JameJM means by

 

Ya know, the REAL truth is that they are all precisely the blessed same. From the EBAY $2.37 model to the 60 umphmfft, what is it? C something?

 

just curious....

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They all MUST be approximately the same in their accuracy. Otherwise, there would be an awful lot of 'former' geocachers out there that just gave up because they couldn't find the caches. Not everyone spends big bucks on this hobby and we still enjoy it and have success. But if you enjoy having and using all that, then that's great! But you don't HAVE to have it. I'd hate for newbies to get discouraged, and think they won't be able to geocache well with their eTrex yellow.

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Maybe that is what he meant, and when you get down to it, it doesn't matter how accurate YOUR GPSr is, more so how accurate the PLACERS was, when they hid the cache. Besides, I don't think a Garmin iQue and a $20 antenna on top of a stick I cut from my backyard is a heck of alot of stuff. I do have a back-up GPSr, an eMap, that my wife uses, and it works too......it was just replaced because I wanted the combination of a GPSr/PDA that the iQue offers. I thought JamesJM was suggesting that they are all equally accurate (or inaccurate).....I guess it depends on your point of view.

 

Personally, I'd rather be within 6 or 7 feet (supposedly), than 30 plus feet, just so I feel I am starting in close to where the dang cache is. (and if the placer of the cache was of by 30 feet, and then I'm off by 30 feet, and it's one of these camo'ed micros I'm finding in my area.....things start to get difficult.....oh well, I (and the discussion of accuracy, etc.) could go on ad nauseum, but somehow I may have already reached that point. :rolleyes:

 

edited for typos...

Edited by hiispy4u
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Although I am a bit confused as to what JameJM means by

 

Ya know, the REAL truth is that they are all precisely the blessed same. From the EBAY $2.37 model to the 60 umphmfft, what is it? C something?

 

just curious....

First remember that all gpsr units are receving the same signals... (Unless you have one that can pick up military signal/etc etc.)

So, greater cost = better reception isn't really true. Patch atenna work fine, provided they are receving enough good signals (which is truee of all units).

All have the error go up under tree cover and such as the signal has to pass threw all the junk above you. And all units are effected my 'user error'. If you keep it upside down in your pocket, its not going to be receving as well as if you keep out at arms length in properly oriented for your specific unit.

What you really get when your buying a more expensive unit is different cases/arrangements (ie bigger screen, better resolution, or more memory in mapping units), base maps/ ability to add more maps, charts (tides, moon, best fishing/etc), altimeter or maybe a compass.

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Now this is getting interesting, so the accuracy of my GPSr when receiving 12 sats, with WAAS enabled and the latest algorithms, not using the patch antenna, but an ampified one that is above my head, reporting 6.2 feet of accuracy is really just the manufacturer's way of making me feel better, when a cardboard box with radioshack parts, built to receive signals, with no amplification, for $2.37 is just as accurate. Now I get it, and how was that for a run-on sentence? :rolleyes:

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First remember that all gpsr units are receving the same signals... (Unless you have one that can pick up military signal/etc etc.)

So, greater cost = better reception isn't really true. Patch atenna work fine, provided they are receving enough good signals (which is truee of all units).

All have the error go up under tree cover and such as the signal has to pass threw all the junk above you. And all units are effected my 'user error'. If you keep it upside down in your pocket, its not going to be receving as well as if you keep out at arms length in properly oriented for your specific unit.

What you really get when your buying a more expensive unit is different cases/arrangements (ie bigger screen, better resolution, or more memory in mapping units), base maps/ ability to add more maps, charts (tides, moon, best fishing/etc), altimeter or maybe a compass.

EXACTLY.

 

That explains why my GPSr (and everyone else's too, if they're honest with themselves) can be dead on at times, and other times many feet off (from the cache's listed coordinates). Too many variables to narrow it down to one thing, including the GPSr itself.

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If it makes you feel any better, last time I took the GPSr out, my accuracy when I started was 120 feet. :rolleyes:

 

20-30 is the norm. If you get better, you're doing good. Besides, you don't want to be looking at your GPSr any closer to the cache, cause that's when you start to trip over the log that the tupperwear is in! lol.

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This thread began as a topic of accuracy, so I thought, not specifically finding a cache with your available resources and specific circumstances. To read all the posts, ulimtately West (who asked the question) seems like you're out of luck and stuck with what you got. Oh well.....

 

20-30 is the norm. If you get better, you're doing good. Besides, you don't want to be looking at your GPSr any closer to the cache, cause that's when you start to trip over the log that the tupperwear is in! lol.

 

edited to add above!!! :rolleyes::D

 

Good point!

Edited by hiispy4u
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This thread began as a topic of accuracy, so I thought, not specifically finding a cache with your available resources and specific circumstances.

Oh, but that's where you're confused...

Accuracy is directly related to your circumstances. It also varies according to unit.

If you're doing a mile hike on flat open space - pick some wide open spot in nevada or whatever - and say your accuracy is 20 feet.

Then add a stream... Water affects accuracy... so then your accuracy is 22 feet.

Now add trees... which affect accuracy because your signal has to go through the leaves to get to your unit... then your accuracy is down to 25 feet...

Now the ground is hilly and you have to go up and down these things... Your accuraccy is down even more...

 

So yeah, your circumstances very much affect your accuracy..

 

How you hold your GPSr affects how it works.. If you hang it around your neck and don't hold it out, you can even completely lose the signal (yeah, mine does this alot!)... GPSrs need to be held out from your body a little and kept relatively flat (think bottom parallel with the ground) to get the best readings...

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QUOTE (hiispy4u @ Apr 29 2004, 11:47 AM)

This thread began as a topic of accuracy, so I thought, not specifically finding a cache with your available resources and specific circumstances. 

 

Oh, but that's where you're confused...

Accuracy is directly related to your circumstances. It also varies according to unit.

If you're doing a mile hike on flat open space - pick some wide open spot in nevada or whatever - and say your accuracy is 20 feet.

Then add a stream... Water affects accuracy... so then your accuracy is 22 feet.

Now add trees... which affect accuracy because your signal has to go through the leaves to get to your unit... then your accuracy is down to 25 feet...

Now the ground is hilly and you have to go up and down these things... Your accuraccy is down even more...

 

So yeah, your circumstances very much affect your accuracy..

 

How you hold your GPSr affects how it works.. If you hang it around your neck and don't hold it out, you can even completely lose the signal (yeah, mine does this alot!)... GPSrs need to be held out from your body a little and kept relatively flat (think bottom parallel with the ground) to get the best readings...

 

Actually, you made my point for me...I was being a bit sarcastic in what you quoted me as saying, in response to all those saying accuracy, circumstances, equipment, etc. is all the same and doesn't matter.....I was trying to answer West's question on how I have improved my accuracy, etc. and what he can expect from different units, and possibly using a $20 Gilsson antenna, etc. But to read the other posts, it just doesn't matter..... :rolleyes:

 

One dead horse please, well beaten....as opinions obviously differ....

 

(typos, again.....dang my typing ) and oh yeah, with all that accuracy, I'm still a noob with only somewhere around 11 finds, but that wasn't what West was talking about either, oh well.....

Edited by hiispy4u
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just curious....

First remember that all gpsr units are receving the same signals... (Unless you have one that can pick up military signal/etc etc.)

So, greater cost = better reception isn't really true. Patch atenna work fine, provided they are receving enough good signals (which is truee of all units).

Professional grade units have improved software. Trimble, for example, has several patents on their software with improved processing algorithms. Add improved antennas, multipath filtering, interference shielding, realtime DGPS from a local base station not just from the Coast Guard beacons, better signal masks, etc and you can consistently get submeter accuracy in a handheld unit -- centimeter under good conditions.

 

Of course you are going to pay $$ for it. :rolleyes:

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Assuming nearly every cache is placed with a gps, I would expect that I MIGHT find a cache anywhere within 2x my indicated "accuracy". This can be a fairly large area. With this in mind, I pay more attention to potential hiding spots than I do my gps once I get close. Triangulating seems to help, especially if there is heavy cover. My point is, I doubt a cache location is a good benchmark for testing your gps.

 

Just my .02 worth - I'm a newb with an Etrex yellow!

 

Garry

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Besides, you don't want to be looking at your GPSr any closer to the cache, cause that's when you start to trip over the log that the tupperwear is in! lol.

:rolleyes::D:D Isn't THAT the truth!!

 

Anyway, like I said, no matter what unit you have, we all have had a good amount of success, or we wouldn't still be doing this.

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....

 

How you hold your GPSr affects how it works.. If you hang it around your neck and don't hold it out, you can even completely lose the signal (yeah, mine does this alot!)... GPSrs need to be held out from your body a little and kept relatively flat (think bottom parallel with the ground) to get the best readings...

Now these are some helpful ideas. Thanks! :)

 

The thing that drives me nuts is the 'drift' that set in sometimes. In an open field, completely away from structures and trees, I walked back and forth looking for a cache. With multiple satelites being received and numerous trips across the area, the target never came closer than twenty feet away. when I got to where it said I was supposed to be, it then said I was supposed to be thirty feet away in another direction. Is there a technique for getting the best estimate of the correct spot?

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Wow all these replys and not an ounce of useful information.

 

Try this:

 

Make sure your on the right datum. Should be WGS84. If not you'll be off by 20-30 feet. You can find the settings in the manual. Next - READ the manual. The initail setup explains how to tell the GPS what area of the world it is in. Lastly, make sure you have WAAS enabled, and you are picking up at least one WAAS sattelite. If you don't pick up at least one WAAS sattelite, you will be off by 20-30 feet.

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Wow all these replys and not an ounce of useful information.

 

Try this:

 

Make sure your on the right datum. Should be WGS84. If not you'll be off by 20-30 feet. You can find the settings in the manual. Next - READ the manual. The initail setup explains how to tell the GPS what area of the world it is in. Lastly, make sure you have WAAS enabled, and you are picking up at least one WAAS sattelite. If you don't pick up at least one WAAS sattelite, you will be off by 20-30 feet.

"Wow," that's probably the least useful reply yet. I'd let it go, but after your criticism of the earlier posts, I'll try to constructively criticize yours:

 

1. Map datum. If he is consistently getting within 20 - 30 feet of a cache, I'd say he is using the correct datum. An incorrect datum would likely cause an error in the hundreds of feet.

 

2. Read manual - tell GPS what area of world you are in. This is helpful when using a gpsr for the first time or in a new geographical area to help it lock on quicker. If he has already got satellite lock indicating 20 - 30 foot accuracy, this would not be helpful or necessary.

 

3. WAAS. Not using WAAS is not going to throw your accuracy off by an additional 20 - 30 feet. At best, a good WAAS signal might improve your accuracy by 5 - 10 feet. And as far as picking up at least one WAAS satellite, I think picking up one is most you could hope for. Anyone ever get 2 WAAS signals at the same time?

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Wow all these replys and not an ounce of useful information. 

 

Try this:

 

Make sure your on the right datum.  Should be WGS84.  If not you'll be off by 20-30 feet.  You can find the settings in the manual.  Next - READ the manual.  The initail setup explains how to tell the GPS what area of the world it is in.  Lastly, make sure you have WAAS enabled, and you are picking up at least one WAAS sattelite.  If you don't pick up at least one WAAS sattelite, you will be off by 20-30 feet.

"Wow," that's probably the least useful reply yet. I'd let it go, but after your criticism of the earlier posts, I'll try to constructively criticize yours:

 

1. Map datum. If he is consistently getting within 20 - 30 feet of a cache, I'd say he is using the correct datum. An incorrect datum would likely cause an error in the hundreds of feet.

 

2. Read manual - tell GPS what area of world you are in. This is helpful when using a gpsr for the first time or in a new geographical area to help it lock on quicker. If he has already got satellite lock indicating 20 - 30 foot accuracy, this would not be helpful or necessary.

 

3. WAAS. Not using WAAS is not going to throw your accuracy off by an additional 20 - 30 feet. At best, a good WAAS signal might improve your accuracy by 5 - 10 feet. And as far as picking up at least one WAAS satellite, I think picking up one is most you could hope for. Anyone ever get 2 WAAS signals at the same time?

Well - not ment to criticize anyone, but here it is from Magellen:

 

1) The reason for initialization is to speed up the process for computing the

first position fix when the SporTrak has no last position computed in

memory (i.e.,when the SporTrak is brand new or memory has been

cleared).This gives the SporTrak an approximate indication of where it is

located so it can use its satellite almanac (stored in permanent memory)to

estimate what satellites are overhead and which ones it should be looking

for.

 

2) Position coordinates on your receiver do not match the location on your map.

1.Make sure that your receiver is set up to use the same datum as your map.

The map datum is generally shown in the map legend.See Setup -

Changing the Map Datum under Reference for instructions on selecting the

map datum in your receiver.

2.Check your LAT/LON format.Make sure that the format selected in

COORDINATE SYSTEM (DEG/MIN/SEC or DEG/MIN.MM)is in

the same format as the map you are using.

 

3) How accurate will my GPS receiver be with WAAS?The FAA reports that

expected accuracy can be improved to around 7 meters vertically and horizon-

tally.Our own testing suggests that Magellan GPS receivers will typically

experience accuracy improvements to around 3 meters.You can expect to see this

level of accuracy for 95%of the time that you are receiving WAAS signals.

 

So - like I said.... check your map datum, check for WAAS signal, and read the book.

 

Or - don't listen to me because i'm just a Mr. Know-it-all anyway.

Edited by dzgametec
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night time is the best time for the most accurate sat sigs

No, not so and especially in the case of differential night time is actually the worst.

 

In many respects things are more linked to geometry than simply day/night but if circumstances exist then it can be but it ain't necessarily so.

 

Cheers, Kerry.

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Wow all these replys and not an ounce of useful information.

 

Try this:

 

Make sure your on the right datum. Should be WGS84. If not you'll be off by 20-30 feet.

No, not at all really correct as 20-30 feet is certainly not an indication of a datum issue.

 

For sure check the datum but 20-30 feet doesn't highlight a datum issue one little bit.

 

Cheers, Kerry.

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night time is the best time for the most accurate sat sigs

The middle latitudes of the northern hemisphere have less atmospheric distortion at night (theoretically) but in practical use the difference between day and night is quite small.

 

Nightime inversions can degrade the signal more than any nighttime effect will improve it, although both are slight.

 

If you check your almanac there are generally two times per twenty four hours when you have very good geometry for a couple of hours. One is almost always at night and one during the day, although it does vary.

 

For example, (I bring up the chart on screen as I type this) at my house today my lowest PDOP is between 0001 and 0200 and between 1600 and 1730. My best number of satellites fall on three peaks, between 0100 and 0200, 0500 and 0800, and 1400 to 1700.

 

Call me a wimp, but I choose not to go look for a cache at 0200. :D

Edited by dsandbro
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Twice today I found myself with the GPSr (MeriGold) saying the cache was about 1m, and sure enough it was. I had a WAAS signal, and I assume the person who placed the cache did as well. All I can say is WOW. I guess this is more indication (got good results at a base network marker as well) that WAAS seems to work well in Canada (at least around Edmonton).

 

My first couple cache finds (back in 2001) certainly required a much broader search once I got to the area due to the larger GPSr errors for the placement and search combined.

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Sorry if someone already mentioned this and I just missed it in going through the replies. GPS technology that consumers use (only one of the signals being broadcast by the satellites and not the second signals that military/high end equipment also detect) only states accuracy within roughly 50 feet. If you are consistently closer to caches than that, don't argue, as you're ahead of the game.

 

Also remember that the person placing the cache has potential inaccuracy in getting a read on coordinates (and that's ignoring weather conditions, staying still long enough to get a reading that has settled down, etc.).

 

Finally, even the 50 ft. specification of GPS technology is, if I'm remembering correctly, a 95% confidence level. So 95% of the time, other factors being equal, you should be within 50 ft. of the actual location. The other 5% of the time, you could be way off. Unfortunately, you never know when it's the 95% or when it's the 5%.

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I use a SporTrak Pro, so maybe I can help a little with strategy. When I get close to a cache I try and move more slowly. Believe it or not the SporTraks seem to need a little more time to make up there minds where they really are. If I am out in the open away from trees or hills that is all it normally takes to get me within 10 feet of the cache. If i am in an area with a lot of tree cover or near rocks I have to work in from several different directions to try and triangulate. This is especially true if there is heavy tree cover. I also find that if I start looking in an area and forget about the GPS for 5 minutes or so it will eventually get better information and point to where I needed to go to start with. Nothing beats experience though. After a while you can often spot what it out of place. Remember the GPS can only do so much. I suppose one day there will be a product out there that will talk to you and tell you exactly where to look, but what is the fun in that. :D

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There have been several past topics on accuracy. To summarize:

What accuracy is not: 1.What your screen EPE or "accuracy" says. and 2. How close you get to a placed cache.

What accuracy is: How close your coordinates are to those of a known place on earth when you are at that place.

I have confidence in my GPSrs because I have gone to 4 of these places on various days and tested. I have been pleasantly surprised, and sometimes stunned by how accurate my 2 GPSrs are. The first thing I will do when I get another GPSr is test it.

I will try to find the past thread that explained how to find these "super accurate" benchmarks, or post it again here if anyone is interested.

Edited by EScout
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...

Professional grade units have improved software. Trimble, for example, has several patents on their software with improved processing algorithms. Add improved antennas, multipath filtering, interference shielding, realtime DGPS from a local base station not just from the Coast Guard beacons, better signal masks, etc and you can consistently get submeter accuracy in a handheld unit -- centimeter under good conditions.

 

Of course you are going to pay $$ for it. :D

Those base stations placed over a known point make all the difference. It uses the difference between the calculated position and it's known position to determine a differential correction for the roving GPS unit.

 

Without that base station they would not be nearly as accurate.

 

Changing subjects: A trick for magellans that seems to work is to step backwards a few steps as you get close. It messes up the moving averate enough to put you on track for the last few feet.

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I'm new to this and to using GPS's, so I finally found my first one(after using the clue) and found that the GPS was off 71 ft. It is a Mer. Plat. After reading the posts it seems that this is a little far off. I was in heavy tree coverage and had three satellites linked. How do I know if one is WAAS?? and is 71 ft. acceptable or should I have my unit looked at? it is new.

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I'm new to this and to using GPS's, so I finally found my first one(after using the clue) and found that the GPS was off 71 ft.  It is a Mer. Plat.  After reading the posts it seems that this is a little far off.  I was in heavy tree coverage and had three satellites linked.  How do I know if one is WAAS?? and is 71 ft. acceptable or should I have my unit looked at? it is new.

71 feet is more than what I would consider to be "normal" variability, but it's not unheard of. If your GPSr is off by 30 feet, and the GPSr of the person who placed the cache was also off by 30 feet, then there you have it.

 

Heavy tree coverage makes a VERY big difference--and with only 3 satellites, chances are your accuracy was poor. I've seen my accuracy go from 9 feet to 150 feet within a matter of seconds after walking under a tree canopy. I'm sure you will find that your accuracy is much better when searching out in an area with unobstructed skies.

 

I'm not a Meridian owner myself, but from owning several different Garmins (eTrex, Legend, and 60CS) I can tell you that they can behave differently in terms of figuring out where you are. Although their overall accuracy is the same, the 60CS is rock solid in averaging your distance from the cache even while moving, while my Legend tends to be rather jumpy until it has a chance to sit for a minute and decide where it actually is. The more you use your particular GPSr, the more you'll learn to adapt to and overcome its limitations.

 

Edit: As far as WAAS goes, I believe that any satellite from 33 and above is a WAAS satellite. Maybe someone who knows for sure can confirm that.

Edited by Team PerkyPerks
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