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Normal GPS accuracy?


Granpappa
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I started Geocaching about 5 years ago. I only found a few easy caches, then didn't pick it up again until recently. It seems like my GPS unit is only accurate to within 10-15 feet. Is this normal? I guess that''s why people talk about "Ground Zero", right?

Edited by Granpappa
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Just now, Granpappa said:

Is this normal?

Yes, this is quite a good accuracy. You don't get anything better, no matter what device you use.

 

1 minute ago, Granpappa said:

I guess that''s why people talk about "Ground Zero", right?

Exactly. Ground Zero is the area where your GPS is barely of any use and you need to use your "geo-senses" to search for the container.

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It can also cause frustration when the different devices on different days are off in different directions.

 

So your gps could be 15 feet off to the left, the cache owner's gps could have been 15 feet off to the right when they placed the cache. So even if you are 100% certain you are at exactly the right coords, you could actually be 30 feet away from where the cache physically is. It's why some people talk about "geo-senses", which really just means past experience. Which can also be why it can be very frustrating when you come across a new way of hiding a cache that you haven't experienced before but others have. Lots of "so easy" and "obvious" logs, but you've spent the last two hours checking every nook and cranny in a larger and larger area...

 

 

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In good conditions most GPS units will still have a +/- of 10 feet of error.  Phones used to be much worse but they've gotten MUCH better.  They're right around the same amount of error.

 

In bad conditions or in areas where signal acquisition is obstructed (regardless of the device), the amount of error can be much higher.

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15 hours ago, Unit473L said:

It can also cause frustration when the different devices on different days are off in different directions.

 

So your gps could be 15 feet off to the left, the cache owner's gps could have been 15 feet off to the right when they placed the cache. So even if you are 100% certain you are at exactly the right coords, you could actually be 30 feet away from where the cache physically is. It's why some people talk about "geo-senses", which really just means past experience. Which can also be why it can be very frustrating when you come across a new way of hiding a cache that you haven't experienced before but others have. Lots of "so easy" and "obvious" logs, but you've spent the last two hours checking every nook and cranny in a larger and larger area...

 

 

Yes, I found out what you meant today. I found a nano cache about 25 feet from where the GPS said it was. Must be the compound error that you were talking about.

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On 10/22/2021 at 2:54 AM, Mausebiber said:

Leica:  0,02m = 0,065 feet accurate

 

1077020595_LeicaGPS07.thumb.jpg.8d1af7e18d6ad23c59f4fcc403a6cdfa.jpg

 

 

GPS, GLONASS and GALILEO are all different systems commonly lumped under "GPS". GPS, the US Civilian system, has a positional accuracy of +/- 10 feet, inherent. If Leica can overcome this, systemic, inaccuracy everyone should have one 😎

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5 hours ago, Jimrky said:

 

 

GPS, GLONASS and GALILEO are all different systems commonly lumped under "GPS". GPS, the US Civilian system, has a positional accuracy of +/- 10 feet, inherent. If Leica can overcome this, systemic, inaccuracy everyone should have one 😎

 

Leica can overcome this, because the 3 m is not inherent in the system.  Multiband, differential and CORS GPS systems can do better.  The one pictured above costs about 15,000 dollars and takes about a minute to get the displayed accuracy.

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11 hours ago, Jimrky said:

GPS, the US Civilian system, has a positional accuracy of +/- 10 feet, inherent

 

I think you are very wrong:  https://en.wikipedia.org/wiki/Global_Positioning_System

 

The Global Positioning System (GPS), originally Navstar GPS,[2] is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force.[

 

When selective availability was lifted in 2000, GPS had about a five-meter (16 ft) accuracy. GPS receivers that use the L5 band can have much higher accuracy, pinpointing to within 30 centimeters (11.8 in), while high-end users (typically engineering and land surveying applications) are able to have accuracy on several of the bandwidth signals to within two centimeters, and even sub-millimeter accuracy for long-term measurements.

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1 hour ago, Mausebiber said:
12 hours ago, Jimrky said:

GPS, the US Civilian system, has a positional accuracy of +/- 10 feet, inherent

 

I think you are very wrong:  https://en.wikipedia.org/wiki/Global_Positioning_System

 

According to the same source https://en.wikipedia.org/wiki/Global_Positioning_System#Augmentation

 

"The standard accuracy of about 15 m (49 ft) can be augmented to 3–5 m (9.8–16.4 ft) with DGPS, and to about 3 m (9.8 ft) with WAAS."

 

That "+/- 10 feet" match pretty well also my experience about modern civilian GPS accuracy.

Edited by arisoft
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10 hours ago, fizzymagic said:

The one pictured above costs about 15,000 dollars and takes about a minute to get the displayed accuracy.

 

4 hours ago, Mausebiber said:

When selective availability was lifted in 2000, GPS had about a five-meter (16 ft) accuracy. GPS receivers that use the L5 band can have much higher accuracy, pinpointing to within 30 centimeters (11.8 in), while high-end users (typically engineering and land surveying applications) are able to have accuracy on several of the bandwidth signals to within two centimeters, and even sub-millimeter accuracy for long-term measurements.

 

For geocaching purposes, accuracy of 3m (10ft) is as much as anyone can expect. We aren't high-end users.

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5 hours ago, niraD said:

For geocaching purposes, accuracy of 3m (10ft) is as much as anyone can expect. We aren't high-end users.

 

Yes, true, but I did respond to the statement below:

 

Quote

You don't get anything better, no matter what device you use.

 

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9 hours ago, arisoft said:

That "+/- 10 feet" match pretty well also my experience about modern civilian GPS accuracy.

 

Again, modern civilian GPS accuracy is 2 cm or 0,065 feet.  They are in use every day to validate Street layout, Building positions and much more.

You are talking about consumer GPSr like Garmin Oregon which is in a cost range of +- $500.-

 

727404310_LeicaGPS04.thumb.jpg.6d98cf7d2d3da42d90328461922458c9.jpg

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In addition, in the context of geocaching, the standard coordinate format is 3 digit precision on the decimal minutes value. That in itself is only so accurate, and necessarily means that no device will pinpoint a geocache "gz" any better than 3 digit precision provides. Even if you buy a $15,000 GPS, it won't help you geocache any better than a 5 year old outdated smartphone :P

Edited by thebruce0
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26 minutes ago, Mausebiber said:

Again, modern civilian GPS accuracy is 2 cm or 0,065 feet.  They are in use every day to validate Street layout, Building positions and much more.

You are talking about consumer GPSr like Garmin Oregon which is in a cost range of +- $500.-

 

Yes, in civil engineering professionals are using RTK (Real-time kinematic positioning) and DGPS for land surveying. Technically speaking, this application is not a pure GPSr as it requires a reference station. Anyway, it is better to talk about consumer GPS as you suggested.

 

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42 minutes ago, thebruce0 said:

In addition, in the context of geocaching, the standard coordinate format is 3 digit precision on the decimal minutes value. That in itself is only so accurate, and necessarily means that no device will pinpoint a geocache "gz" any better than 3 digit precision provides.

 

So, how accurate are the 3 decimal minutes?  You have seen this many times by watching your GPSr.  It could be 100% accurate, right at the spot and it could be many feet off.

The better the GPSr the better the chances that you are at GZ.

 

 

 

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2 minutes ago, niraD said:

See http://www.markwell.us/geofaq.htm#Feet

 

For latitude, 0.001' is always 6.074 feet.

 

For longitude, it depends on your latitude. Near the equator, it's about 6 feet. Near 35°, it's about 5 feet. Near 49°, it's about 4 feet. And so on.

 

He was trolling over the use of the term "accuracy" instead of "precision."   Accuracy of any single measurement is how far it is away from the perfect truth; what you are talking about here is, technically, precision, because (again technically) a 3-digit measurement can be perfectly accurate (with vanishing probability, of course).  I think he is just trying to show off how smart he is, a behavior with which I am quite familiar, seeing as how I do it all the time!  :blink:

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4 minutes ago, fizzymagic said:

He was trolling over the use of the term "accuracy" instead of "precision."

Yep. And I believe my use of the words is sufficiently appropriate...

"...3 digit precision on the decimal minutes value. That in itself is only so accurate..."

ie: 3 digit precision provides that ~6' rectangular zone within which the geocache can be, thus the coordinates can be only so "accurate" to the geocache location.

 

28 minutes ago, Mausebiber said:

So, how accurate are the 3 decimal minutes?

They're only as accurate as the accuracy that 3 decimal precision minutes can provide (see above) =P

How accurate a gps device is to the provided coordinates depends on many factors, but these days, as argued regularly, most any device provides sufficient gps accuracy to go geocaching given the precision that the standard DMS geocache coordinates can provide :)

 

Of course, nothing is stopping cache owners who have GPS devices with reportedly superior accuracy from providing more precise gps coordinates in the description of the geocache, for anyone who wishes to test their own device against those alternative coordinates...

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7 minutes ago, fizzymagic said:

He was trolling over the use of the term "accuracy" instead of "precision." 

 

Hello, maybe you noticed that English is not my first language, for me, accuracy and precision are just the same.  Does it really bother you if we have a controversy discussion?

 

 

 

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Quote

For latitude, 0.001' is always 6.074 feet.

 

For longitude, it depends on your latitude. Near the equator, it's about 6 feet. Near 35°, it's about 5 feet. Near 49°, it's about 4 feet. And so on.

 

What I understand is that the distances you mentioned above are from the third digit to the next third digit,  from   10.000 to 10.001

 

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1 hour ago, thebruce0 said:

Of course, nothing is stopping cache owners who have GPS devices with reportedly superior accuracy from providing more precise gps coordinates in the description of the geocache, for anyone who wishes to test their own device against those alternative coordinates...

 

Once upon a time, all geocachers were able to use decimal degrees for the posted coordinates, in the very same hi-precision, as they were entered in the database. The bravest geocachers used those precious coordinates with the magic of GPX, and they were all happy. Suddenly, a nasty frog ripped off all decimal degrees from the cache description and placed a curse on all geocachers to use these inaccurate minutes, and they never lived happily ever after.

 

Edited by arisoft
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2 hours ago, Mausebiber said:

What I understand is that the distances you mentioned above are from the third digit to the next third digit,  from   10.000 to 10.001

That is correct. That is the precision offered by Groundspeak's geocache coordinates.

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On 11/3/2021 at 10:00 PM, fizzymagic said:

 

Leica can overcome this, because the 3 m is not inherent in the system.  Multiband, differential and CORS GPS systems can do better.  The one pictured above costs about 15,000 dollars and takes about a minute to get the displayed accuracy.

 

Yes GLONASS and GALILEO can do better, but GPS tops out at about +/- 10 ft per all sources that I can access. Averaging attempts to give better results but is an overlay not an access of greater accuracy. If you have a source stating that GPS systemic accuracy is better than +/- 10 ft  I'd appreciate a citation so that I can access it, thanks 😁

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Suggest you read this:

https://www.gps.gov/systems/gps/performance/accuracy/

 

I have abstracted two paragraphs of interest:

 

"For example, GPS-enabled smartphones are typically accurate to within a 4.9 m (16 ft.) radius under open sky However, their accuracy worsens near buildings, bridges, and trees.

High-end users boost GPS accuracy with dual-frequency receivers and/or augmentation systems. These can enable real-time positioning within a few centimeters, and long-term measurements at the millimeter level."

(my bolding)

"Using two GPS frequencies improves accuracy by correcting signal distortions caused by Earth's atmosphere. Dual-frequency GPS equipment is commercially available for civilian use, but its cost and size has limited it to professional applications."

 

So, yes, your hand-held GPSr or smartphone is limited to +/- 4 m in good conditions. But those high end systems have been in use for surveying for some time now.    When you are laying out roads, bridges and housing developments you need a lot better accuracy than +/- 4 m.

 

 

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6 hours ago, Michaelcycle said:

So, yes, your hand-held GPSr or smartphone is limited to +/- 4 m in good conditions. But those high end systems have been in use for surveying for some time now.

 

You are right. GPS has been used for surveying longer that geocaching. https://gssc.esa.int/navipedia/index.php/Real_Time_Kinematics tells us that this augmentation system was available at mid-1990s - before "Selective Availability" was discontinued. The hi-accuracy of these devices is based on fixed base station, not on the accuracy of the GPS system.

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7 hours ago, arisoft said:

 

You are right. GPS has been used for surveying longer that geocaching. https://gssc.esa.int/navipedia/index.php/Real_Time_Kinematics tells us that this augmentation system was available at mid-1990s - before "Selective Availability" was discontinued. The hi-accuracy of these devices is based on fixed base station, not on the accuracy of the GPS system.

 

Well, it's more complicated.  You're basically right:  the cm-level precision achieved by survey-grade GPS units is mostly from referencing to a fixed base station at a known location; the GPS errors for other systems within a few km are highly correlated.

 

Consumer GPS units get signals from the L1 band (1575 MHz); even other nations' systems use the L1 band (Glonass and and Galileo  both transmit at 1575 MHz for use in handheld devices; they also transmit on other frequencies) .  But multi-band receivers also use signals from the L2 band (1227 MHz) , which is far enough from the L1 band that the propagation delay through the ionosphere is significantly different.  Since the ionospheric delay is responsible for the largest error in the GPS position, using the dual bands reduces position error by more than a factor of 2.  In addition, the coding speed for the L2 system is different than the L1 system, which can result in faster signal acquisition.

 

Supposedly this year some US GPS satellites will start to transmit on the L3 band (1176 MHz) a signal that is optimized for aircraft and other critical systems.

 

Building an antenna/amplifier/decoder system for more than 1 frequency is expensive, so consumer devices, which generally are just fine with 3 m accuracy, don't use it.  Having two antennas in a handheld unit is not going to work well, especially since the L2 antenna is larger than the L1 antenna.  Combine those considerations with differential GPS and survey-grade gps units can be very costly and are usually pretty large.

 

So it is incorrect to say that 3m is the "inherent accuracy of GPS," but it is accurate to say that it is the inherent accuracy of consumer-grade GPS.

 

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12 hours ago, fizzymagic said:

the cm-level precision achieved by survey-grade GPS units is mostly from referencing to a fixed base station at a known location; the GPS errors for other systems within a few km are highly correlated.

 

What about a golf rangefinder?  I see some optical ones with slope compensation that have a range of 500 feet or more.  Assuming a line of sight to the cache area, I could stand on a survey marker, and check the distance to some point.  Compute the Coords, using as many decimal points as you need.  Just get to the survey marker with whatever device works, and measure location/Coords from there.  I don't know if these devices have a compass (pretty important), nor what the typical accuracy is.  At least the result would be steady (precise), not dependent on radio signal calculations.  Something like using a string to measure distance, as some puzzle caches have.  Except it's like 500 feet of string.

 

No big deal, because I don't need less than "10-15ft of accuracy" for Geocaching.  That is, although the subject comes up all the time, nobody's working that hard on pinpoint "accuracy" (not finders, definitely not hiders).  Some cachers insist it's no fun to be accurate, that then there's no challenge.  In a rare case, I'll pace out a cache placement in a saturated area.  Other than that, I look at all the clues on a cache page to get closer (if I had a Geosense, I'd use one of those), when I want no challenge and no fun.  B) 

 

Edited by kunarion
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17 minutes ago, kunarion said:

What about a golf rangefinder?  I see some optical ones with slope compensation that have a range of 500 feet or more.  

 

No big deal, because I don't need less than "10-15ft of accuracy" for Geocaching. 

That is, although the subject comes up all the time, nobody's working that hard on pinpoint "accuracy" (not finders, definitely not hiders).  

 

I haven't used one for hunting (and definitely not golf... ;), but target shooting, we've seen some laser rangefinders were accurate to 2 feet.

 

Yep.  How many people have said here that they couldn't find their own cache.   :)

We had one that crept along 12 feet when we went for JIC maintenance a year later.

A cache rarely gets put back in the same spot unless secured there...  

 - That's why unlike the other 2/3rds, I liked FTF only to see the cache as presented by the CO.

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On 11/6/2021 at 9:38 AM, Michaelcycle said:

Suggest you read this:

https://www.gps.gov/systems/gps/performance/accuracy/

 

I have abstracted two paragraphs of interest:

 

"For example, GPS-enabled smartphones are typically accurate to within a 4.9 m (16 ft.) radius under open sky However, their accuracy worsens near buildings, bridges, and trees.

High-end users boost GPS accuracy with dual-frequency receivers and/or augmentation systems. These can enable real-time positioning within a few centimeters, and long-term measurements at the millimeter level."

(my bolding)

"Using two GPS frequencies improves accuracy by correcting signal distortions caused by Earth's atmosphere. Dual-frequency GPS equipment is commercially available for civilian use, but its cost and size has limited it to professional applications."

 

So, yes, your hand-held GPSr or smartphone is limited to +/- 4 m in good conditions. But those high end systems have been in use for surveying for some time now.    When you are laying out roads, bridges and housing developments you need a lot better accuracy than +/- 4 m.

 

 

 

You've just quoted GLONASS (GNSS) specification and averaging (augmentation) - you do realize this, right? 😎 I don't think we're arguing anything different, but I do think I have it right as I stated it.😁

 

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