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How accurate are GPS Units?


Toadily!
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My very old GPS will get me 15-20 ft. from the cache. How close do the newer ones get you?

(...debating on whether or not to purchase a newer GPS.)

My observations..... with MY GPSr.... is that it seems to get me to within 6-10 feet of the coords I'm looking for.

I need to check my etrex 20 against a surveyors cache in Denver to see how it compares to those coords. My Venture HC was 7 feet off at GZ.

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My very old GPS will get me 15-20 ft. from the cache. How close do the newer ones get you?

(...debating on whether or not to purchase a newer GPS.)

Mine is from 2006, and it get 6ft on the best days. Most of the time, it gets more like 12-15. It has more to do with tree cover, line of sight to the sky, and the number of satellites my GPSr is chatting with.

 

So, I wouldn't stress too much about 15-20 feet. When you get that close, you may as well pocket the GPSr and get to the looking around.

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One thing that you probably haven't thought of...

 

Your GPSr may be off a few feet (less than you think), but the Cache Owner GPSr was probably off a few feet too!

It's a cumulative effect, ya know.

 

Whose was the 'most' accurate? It doesn't make a hill of beans which one, because the two (readings) together rarely agree.

 

That's why you simply cannot count on being dead-nuts on the target.

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My old(ish) Garmin eTrex legend C routinely reports +/- 6-7 feet, and judging by how close it USUALLY gets me, it's not lying. That's on top of a hill, though. In a valley, not so much. It's all about line of sight. Clouds and sunspot activity matter as well.

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Yesterday was my first day out and with a new Dakota 20 also. Didn't find 3 of 4 attempts. Testing today in the yard and it seems to be 15-18 feet off. I am marking waypoints within a 3 acre area and when attempting to navigate back I'm 15-18 feet off.

 

Is this also normal? Do I need to calibrate the compass?

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Yesterday was my first day out and with a new Dakota 20 also. Didn't find 3 of 4 attempts. Testing today in the yard and it seems to be 15-18 feet off. I am marking waypoints within a 3 acre area and when attempting to navigate back I'm 15-18 feet off.

 

Is this also normal? Do I need to calibrate the compass?

 

It's normal.

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Yesterday was my first day out and with a new Dakota 20 also. Didn't find 3 of 4 attempts. Testing today in the yard and it seems to be 15-18 feet off. I am marking waypoints within a 3 acre area and when attempting to navigate back I'm 15-18 feet off.

 

Is this also normal? Do I need to calibrate the compass?

 

It's normal.

 

I thought that my Dakota 20 was not as accurate as my old Etrex H. However when I tried to compare them they were just different! Neither was more accurate but they did give different readings if that makes any sense.

 

The Dakota, even if it were an extra 15ft out which it isn't, would still be the better unit for Geocaching though. Combined with GSAK and a Premium Membership it gives as much information as I need as well as navigating me to the correct spot.

My HTC Desire HD gives even clearer information, although not as accurate as a GPSr, and for most caches is more than adequate.

 

I guess I just love Geocaching!!

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Yesterday was my first day out and with a new Dakota 20 also. Didn't find 3 of 4 attempts. Testing today in the yard and it seems to be 15-18 feet off. I am marking waypoints within a 3 acre area and when attempting to navigate back I'm 15-18 feet off.

 

Is this also normal? Do I need to calibrate the compass?

 

15-18 feet can be considered normal, but it seems like a bit too much. Under good reception conditions, you should be able to get at least within 10 feet. Things to consider here:

  • Firmware, check that you have the latest.
  • Check that you have WAAS/EGNOS enabled.
  • Understand that it can take the unit 15-20 minutes to download all the necessary data from the satellites. While the unit will have GPS reception pretty much right after turning it on, the satellite constellation data it's then using will just be predicted data, based on the last known actual data. The longer it has been since you've last used it, the more inaccurate this predicted data will be, which means the location readings you get will also be inaccurate, even though the "accuracy" reading the unit gives you doesn't reflect this. This effect can be noticeable even after only 24 hours of not having used the unit. Rule of thumb: if you want the most accuracy, turn the unit on and wait at least 15 minutes before using it. Obviously, the unit will need a good view of the sky during this time.

The compass calibration has nothing to do with all of that.

Edited by dfx
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Yesterday was my first day out and with a new Dakota 20 also. Didn't find 3 of 4 attempts. Testing today in the yard and it seems to be 15-18 feet off. I am marking waypoints within a 3 acre area and when attempting to navigate back I'm 15-18 feet off.

 

Is this also normal? Do I need to calibrate the compass?

 

Another thing you didn't mention about your testing procedure is how you are actually marking the waypoint. If you just hit "Mark Waypoint" and call it good you're going to have a fairly inaccurate reading...I'd say 15-18 feet off would be a fair estimate. Good enough to get back to your car or deer stand but for hiding a cache you want your coordinates to be a little better than that.

 

What you need to do is average a waypoint (your Dakota should have this function, my Oregon does and they are very similar). Go to your spot, tell the unit to average a waypoint, then set it down stationary and leave it for awhile. The GPSr will start taking continuous readings and will show you a steadily increasing estimated accuracy. Leave it sit there for 5 minutes or so until the estimate gets as close to 100% as it will get. Then mark that reading.

 

An averaged waypoint will be far more accurate than an instantaneous reading and you should absolutely use averaged waypoints when hiding a cache.

 

For even greater accuracy you should also go out and obtain coordinates on different days and at different times of day. Not everyone does this, of course, but you should at least get an averaged set.

 

Bear in mind, too, that the hider may not have been this careful about getting accurate readings....not all cachers understand all the functions of their GPSr's, especially newbies, so they may think that a simple "Mark Waypoint" is good enough. So you get coords that are 20-30 feet off from the actual cache.

Edited by Chief301
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... [*]Check that you have WAAS/EGNOS enabled. ...

 

I live near Toronto, Ontario, and with WAAS on it does not even get a signal, even leaving it on for an hour in an open area. I turn WAAS off, and it works like a charm.

 

Sounds like you have some more serious problems with your GPS. With my Oregon, I can easily get a WAAS lock almost anywhere between here (Burlington) and Mississauga and beyond. And when it doesn't have a WAAS lock, well then it just doesn't. No harm done.

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Yesterday was my first day out and with a new Dakota 20 also. Didn't find 3 of 4 attempts. Testing today in the yard and it seems to be 15-18 feet off. I am marking waypoints within a 3 acre area and when attempting to navigate back I'm 15-18 feet off.

 

Is this also normal? Do I need to calibrate the compass?

 

I have a new Dakota 20 and that seems in line with mine. Yes, you should calibrate your compass.

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My very old GPS will get me 15-20 ft. from the cache. How close do the newer ones get you?

(...debating on whether or not to purchase a newer GPS.)

Within reason I don't worry about how close to the cache it gets me, I usually put the GPS away when I get down to 20 or 25 feet anyway. It's more important to me to not lose the satellites when under heavy tree cover.

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My very old GPS will get me 15-20 ft. from the cache. How close do the newer ones get you?

(...debating on whether or not to purchase a newer GPS.)

Within reason I don't worry about how close to the cache it gets me, I usually put the GPS away when I get down to 20 or 25 feet anyway. It's more important to me to not lose the satellites when under heavy tree cover.

 

On all 11 caches I've found so far, (yes, I'm a noob), I've used an app on my android phone and when it has told me I'm one metre away that's pretty much how far away the cache was every time.

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I just bought an eTrex HCx on ebay. The former owner lived in Alaska.

 

My son and I went geocaching with the unit yesterday and were unable to find the first cache. A clue provided online was found over 160' away from where we zeroed. Despite this we were still unable to find the cache.

 

On our second cache of the day, I zeroed on a group of trees 255' away from the actual cache (my son found it by accident when he got bored and started wandering. A third cache was not found; it was indicated to be on a rocky formation, and the GPS continually indicated it was over 90' away in some deep woods.

 

Following the failure to find the first cache, I toggled through the menu and found the WAAS was not enabled. I enabled it and kept the unit on until we reached the second cache location (about ten minutes). We spent approximately a 1/2 hour in the area, and as stated, the cache was 255' from the stated position.

 

This morning I discovered the unit was set for the incorrect time zone. I'm not certain if this caused the disparity. My old blue legend always got me within a few feet of a cache. I will have to borrow it back from the new owner and see if the readings compare.

 

Any thoughts? Is it possible the unit is set up incorrectly? Is there anything I can do with the unit to make the accuracy a little closer than a couple hundred feet?

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A while back, I read about about a little test you can perform that helped me get some perspective on the accuracy of our GPSr and how that can change over a fairly short time. Turn "tracking" on if your unit has the abiity (it's the feature that lets you track your trail over a distance so you can backtrack) and set the GPSr down on a spot that's fairly open to the sky. Let it just sit there for 10-15 mintues and then take a look at the "trail" your GPSr made while just sitting all by itself. You will probably see a zig-zag star-like pattern on your screen. GZ moves around as the satellites change position and configuration, atmospheric conditions affect the readings, tectonic plate movement and so on. It helps if you remember this when you can't seem to get a good lock on GZ. Maybe walking away and coming back from a new angle will help.

 

edited fer speling

Edited by Brooklyn51
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In the units setup I found the GPS set to "NAD27 Alaska" in Map Datum. I toggled down to and set it for "NAD27 CONUS" assuming it was for the continental US. I have no idea if any of this will effect the accuracy of the unit. i still get readings that are 80' to 130' off of two other GPS units that more or less match each other.

 

Of the settings that are user-selectable, which ones will effect the accuracy? How can I get this thing narrowed down a little closer to dead-on? Is there a benchmark anywhere that can be used? Or assuming that the other two units used are close, how do I dial mine in to get a similar reading?

 

Help! :blink:

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Even on a good day, GPSr are only accurate to within 9-10 feet anyway and that's typically with a fairly good GPSr, good line-of-sight to the horizon, and good signal from at least 5 satellites. I've lost count how many times I've been standing at GZ with my GPSr indicating 0ft to the cache and it's not there. Best bet is to put it away when you're "within" 10-15 feet and turn on your georadar. Start looking for the clues. Good luck!

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Receiver quality probably the single factor that makes one Commercial GPSr more accurate than another but it isn't the only factor. Believe it or not your "older" GPSr has gotten more accurate over the years. This is because GPS is a system and your GPSr is only one cog in the greater system. You can read more about GPS accuracy at GPS.GOV

 

If improved accuracy is your only reason for buying a new GPSr then I'd only buy a new unit if your "older" GPSr isn't using the SiRF Star 3 chipset.

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I'm still sort of new at this but I figured I'd chip in with my 2 cents.

The GPS sats are military, and part of the agreement made for making them available for civilian use is that accuracy is restricted. Military units can get within a meter, but it's sort of pointless to use them because no one placing the cache uses that sort of accuracy.

 

TL;DR- within 10 feet of a cache for civilian GPS is better than fine.

Edited by overskill
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I'm still sort of new at this but I figured I'd chip in with my 2 cents.

The GPS sats are military, and part of the agreement made for making them available for civilian use is that accuracy is restricted. Military units can get within a meter, but it's sort of pointless to use them because no one placing the cache uses that sort of accuracy.

 

TL;DR- within 10 feet of a cache for civilian GPS is better than fine.

 

It was called "Selective Availability" and was turned off May 2, 2000..... on May 3rd Geocaching was born!

 

http://www.geocachin...ut/history.aspx

 

P.S. Before it was turned off typical Selective Availability errors were about 50 m (164 ft). A lot more then the 10-20 feet we complain about now.

Edited by KI4HLW
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