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GPS accuracy - Reality Check


henryat1140

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My son and I have been looking for caches this summer. We have Identical Vista HCX's. Our typical experience is: enter the waypoints, get in proximity, then walk in the general direction. Aha, the feet are getting smaller (both GPS units), and smaller, finally they are single digits, and we believe we're within a few feet of GZ.

 

We start searching, and consulting the GPS's. Invariably, after a few minutes they do not agree - sometimes they disagree by 30 or 40 feet, and in opposite directions. His will say "36 feet ->" and mine will say "24 feet <-". I'm not exaggerating the difference. After another one or two minutes, his will say something entirely different and mine will too. And the GPS keeps changing, one minute pointing this way, the next an entirely different way.

 

Now, a lot of the caches we look for are in tree cover, and I would expect some variation - but 30 or 40 feet?

 

My son eventually usually finds the cache - he's very good at it.

 

My overall question is: when I got into this, my idea was to put the coordinates in and the unit takes you to within a few feet of the cache - I would define few as 15 feet or so. In reality, we seem to be dealing with accuracy of more like 30-50 feet.

 

Our software is up to date, and I have no reason to believe the units are malfunctioning. They generally work 'in tandem' really well until we get down to the actual finding. We are probably average in battery use and replacement - anyway it doesn't seem to be a malfunction, but rather a case of degraded performance, (or unreasonable expectations onm my part).

 

What is your experience? Am I expecting too much? Is the tree cover mostly to blame? (Our units brag about performance in such conditions).

 

In your experience how close does your GPS take you?

 

Thanks for responding.

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depending on terrain and environmental factors, it's nto all that unusual to have accuracy of only 30 or so feet.

 

20 should be considered very good reception.

 

and remember that your accuracy gets added to the hider's so you can be off by quite a lot.

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Exactly right! I have found that when you get close to GZ and the arrow seems to be erratically jumping around, hold still for a few minutes - it may settle down and give a good reading. If that doesn't work. move away maybe 100 feet and start walking back. Note where the arrow and distance point, put the GPS away and go to that spot. I have had good luck doing these methods. Good luck!

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20 - 30 feet is the normal expected accuracy in the majority of situations.

 

Keep in mind that the cache placer's GPS unit has the same (or perhaps greater) margin of error, so added with the margin of error for the finder's GPS unit, this is what you end up seeing.

 

50 feet is probably on the high side. But 30 is probably within reason especially under tree cover even with the "H" chip.

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On top of a very tall bald hill, or on the top floor of a parking garage, with a clear sky, no solar flares, and an optimal satelite constelation you can probably get precision down to <3-ft. In normal conditions 10-20-ft is good. In heavy tree cover expect 30-40-ft. When surrounded by tall buildings or rock cliffs precision can be as bad as 50-ft or more. These numbers go up if you're not using a high-precision receiver. Also, remember that just because your receiver is showing 6-ft precision that doesn't mean the person who hid the cache didn't have 30-ft precision when he shot the coordinates.

 

I typically "follow the arrow" to ground zero, do the "drunken bee dance" for a minute or two to let the reception settle down, then I put the unit on the ground or a stump and use it as a central point in my search radius. I periodically check to make sure I'm still in the right place, adjusting my search radius accordingly. Almost all receivers have a "settling out" time that occurs after reaching ground zero that can chance position by many feet.

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Apparently, you've never heard the saying, A man with one watch knows what time it is. A man with two watches is never sure.

 

Just because you have the same units and are in the same general area, it doesn't follow that you will have exactly the same set of satellites locked (with identical reception characteristics), and the same amount of WAAS correction. In fact, it would be amazing if you did.

 

A 10 feet difference between the two could be explained by something as simple as blocking a satellite with your body for a few seconds. Forget about it, and just look for the cache.

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My overall question is: when I got into this, my idea was to put the coordinates in and the unit takes you to within a few feet of the cache - I would define few as 15 feet or so. In reality, we seem to be dealing with accuracy of more like 30-50 feet.

 

Yep, that is quite normal for all geocachers, regardless of which unit they have.

 

Remember that the person who hid the cache also had a unit that had a level of inaccuracy. Combine that with your unit and you have the potential to be 30+ feet off.

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Son chipping in here....

 

One thing I do to assure me is to go to the satellite page of the VistaHCx, and check the accuracy level. Sometimes it's 12ft, and other times it's 30ft. So the unit can tell us how accurate it is able to be, changing especially when we're under the heavy cover of August. I've remarked a few times in my logs, how much easier it seemed when the leaves were off the trees, in January and February.

 

I also believe the hider's accuracy might vary maybe even more than our WAAS enabled recievers, so a 50ft radius may be about as good as we can expect in the summer. :)

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Funny thing, take our TomTom out and its generally spot on.

 

I watched several guys using their Etrex's and searching, all showing a different GZ, then I walked up and laid down the TomTom and was within 2' of said cache.

 

No GPS is perfect, and it seems to be that you get what you paid for when it comes to the receiver.

 

As long as it gets me to withing 10-20 feet, then I won't complain.

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A very good thing to do when you are caching in those conditions too is to take a look ahead of you (especially if you are in mountains around here as you don't want to literally walk off a cliff) when you get into the 75 to 100 foot range of coords and see what it is pointing to. Start there with the search. Remember your unit is showing you a center point of a circle. It is not really a plus or minus. It is saying that the unit is 95% sure that you are within a circle with a radius of X number of feet that contains the coordinates that you have punched in. The smaller the number, the tighter the circle. Also as you start searching and spinning around, lowering your unit, stopping starting and etc the unit starts to sort of "freak out" and gets lost. It wants to make sure that it is able to draw that circle so it makes the radius bigger (your number goes up) to be 95% sure that it contains the coordinates. Calibrate it, keep one eye on the gps unit and one on the trail and look for geo-beacons. In heavy trees, tall buildings, canyons and etc your unit is gonna drop off a bit, that is normal.

 

RonGerth

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During approach your unit is generally held level and steady, but once you start searching it gets thrashed around a bit, and IMO that will discombobulate it's lock on the satellites some, making it jump around. Sit still with it a minute and it should level back out. I've noticed that about my unit.

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On top of a very tall bald hill, or on the top floor of a parking garage, with a clear sky, no solar flares, and an optimal satelite constelation you can probably get precision down to <3-ft. In normal conditions 10-20-ft is good. In heavy tree cover expect 30-40-ft. When surrounded by tall buildings or rock cliffs precision can be as bad as 50-ft or more. These numbers go up if you're not using a high-precision receiver. Also, remember that just because your receiver is showing 6-ft precision that doesn't mean the person who hid the cache didn't have 30-ft precision when he shot the coordinates.

 

I typically "follow the arrow" to ground zero, do the "drunken bee dance" for a minute or two to let the reception settle down, then I put the unit on the ground or a stump and use it as a central point in my search radius. I periodically check to make sure I'm still in the right place, adjusting my search radius accordingly. Almost all receivers have a "settling out" time that occurs after reaching ground zero that can chance position by many feet.

 

I do this too and it usually works for me. I confess I don't have that many finds so sometimes it takes me awhile to find the caches.

Edited by Tresco
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Son chipping in here....

 

One thing I do to assure me is to go to the satellite page of the VistaHCx, and check the accuracy level.

 

I also use the Vista HCX. When I have reached the general area, I use the compass page to navigate to the cache. I have set the page to show 4 Data Fields and have these set to:

 

Destination

Dist To Dest

Accuracy

Time Of day

 

So there is no need to switch back to the satellite page to check accuracy. At a glance you can add the (Dist To Dest + Accuracy) to get a good idea of how big your search area is. Remember you may need to increase this area to allow for discrepancies between yours and the hiders GPSr.

 

Regards Tarot

Edited by Tarot
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Also remember that the unit updates in position only about once a second, so move slowly when you are near GZ, and you might have better luck in getting a good position. How you hold the unit will also change its ability to get a signal.

 

Since you are comparing two units to the same spot, and have almost the same sat coverage at the time, then the actual position of the cache and the hiders in-accuracy don't realy come into play.

 

Next time this happens, power off the units, and then re-start them, and see if they are closer to the same position. There has been reports of drift with some of these units.

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Son chipping in here....

 

One thing I do to assure me is to go to the satellite page of the VistaHCx, and check the accuracy level.

 

I also use the Vista HCX. When I have reached the general area, I use the compass page to navigate to the cache. I have set the page to show 4 Data Fields and have these set to:

 

Destination

Dist To Dest

Accuracy

Time Of day

 

So there is no need to switch back to the satellite page to check accuracy. At a glance you can add the (Dist To Dest + Accuracy) to get a good idea of how big your search area is. Remember you may need to increase this area to allow for discrepancies between yours and the hiders GPSr.

 

Regards Tarot

 

I don't use time of day for one of my boxes. I use battery life after I hiked into a level 3 cache about 3/4 mile off a trail and when I was about 400 feet my unit died, ran out of batteries. Of course my spares were in the truck a good mile away or better. Lucky for me I had an idea of where the cache was (looking ahead to check route saved me) and was able to get into the area and start searching. Found it but cussed myself all the way back to the truck.

 

Distance to next

accuracy

battery life

elevation if in the mountains what ever it is set to if not

 

RonGerth

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When you think you are close, but no cache, lay the GPSr on the ground and wait until the GPSr gathers all the signals that it will compute. When I do this, I get a pretty good bearing and distance. As the op said "20-30" is pretty good....Just remember, if all we had to do was inter coords and go straight to the cache container....well what the heck!!! that would not be a challenge. Happy caching and good luck....

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