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  1. #1
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    Triangle Garm. Accuracy figure of horizontal position fix on GPSMAP 66 satellite screen?

    With the new GPSMAP 65 and 66sr multi-GNSS, multiband receivers now available, many people are interested in what additional positional accuracy this technology actually brings.

    I don’t know what statistical measure Garmin uses to determine the accuracy of the horizontal position fix as displayed on my 66st satellite screen and could not find the answer after lengthy googling, so I thought I would ask here.

    The latest 2020 GPSMAP66st User Manual Appendix page 17 states: “Accuracy of GPS: The margin of error for your exact location. For example, your GPS location is accurate to within +/- 3.65 m (12 ft).

    This statement is featured in most Garmin handheld device manuals. Does this mean that the number is the 50 % CEP (Circular Error Probable) as supported by this 2014 post on the GPSreview site:
    [Only registered and activated users can see links. ] “The declaration of the accuracy by Garmin GPS receivers often leads to confusion. What does it mean if the receiver states an accuracy of 4 m? This readout refers to the so-called 50 % CEP (Circular Error Probable). This means that 50 % of all measurements are within a radius of 4 m. On the other hand, 50 % of all measured positions are outside of this radius. Furthermore, 95 % of all measured positions are within a circle of twice this radius (actually 2.08 times) and 98.9 % of all positions are within a circle of 2.55 the radius. In the given example, nearly all positions are within circle with a radius of 10 m. The determined position is in the worst case accurate to 10 m.”

    Does anyone know if this is correct and Garmin are actually using the CEP statistic?

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  3. #2
    Master asprin624's Avatar
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    This same post is at GPSrChive

    I have been using handheld GPS units since 2007.

    I have always got to what I was looking for every time.
    As for Geocaching the GPSMAP 66 has gotten to the correct location every time.
    I still have to find the cache which could be anywhere in a 3 to 4 meter radius.
    But that has nothing to do with Accuracy of GPS. Most caches are given a location that is close to the cache and not on it directly.

    As for hiking I use the latest open street maps and the GPS has keep me on the trail the whole time.
    I have used eTrex Legend HCx which I have 6 off. Used the Etrex 22x which I have 5 off and my GPSMAP 66.
    They all work and have kept me on the trails I was using.

    I do not know how you are using your GPS that you question its accuracy.

    But if it puts you in the within the area you set out to go to plus or minus 4 meters is that a problem?

    You will not be walking off a cliff because your GPS said to walk 4 metes more are you???
    Last edited by asprin624; 26th January 2021 at 07:15 PM.

  4. #3
    Member + Accuracy figure of horizontal position fix on GPSMAP 66 satellite screen?Accuracy figure of horizontal position fix on GPSMAP 66 satellite screen?
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    Quote Originally Posted by Eyelet99 View Post
    Does anyone know if this is correct and Garmin are actually using the CEP statistic?
    It was historically accepted this was the case and will certainly be same or similar given the numbers I've seen compared back to back with professional grade equipment. You could check with Garmin support but either way it will be very rubbery in a Garmin/consumer device for a few reasons.

    Firstly there will always be some error and a high confidence number would be off-putting and impractical for a typical Garmin customer. My professional Trimble mapping equipment and software is selectable for confidence readings - and even that only has a 68% default, and it is also selectable for 95% or 99% (and can be further selected to consider either an autonomous reading, real-time corrected or post-processed). However the 99% number is significantly larger and can be useful for visualisation and altering strategy when collecting data but would not be practical for general purpose use.

    Secondly consumer devices also have limited rejection of bad signals so I would be wary of reading to much into the number. Pro grade gear is configurable to ignore satellites below specific angles above the horizon (more atmospheric error), above specific noise levels (SNR), or poor geometry (DOP) etc. And the hardware and software is also designed to reject multipath (signal bounce that lengthens the distance and adds error to the position), and a large high grade external antenna on pole can perform much better e.g. stronger multipath rejection, avoid your head blocking the signal etc. This is all good for making the data collected accurate but not so good when you simply want to find a large campsite and everything is stopped waiting for better signal.

    So the Garmin's are optimised for practical productivity rather than precision and the confidence strategy will reflect that e.g. it's rubbery but works most all of the time because that's what you want.

    They will certainly have a software strategy that considers some effects but things like multipath are difficult detect and deal with because the signals can still look ok and for most consumers it will be present because we typically use the devices in forests or amongst buildings etc. So regardless of what the device is telling you regarding the accuracy, a lot of the time the situation will be a lot worse and neither Garmin or you will know by how much. So there would be no point in them trying to give you are higher confidence number, 50% is totally practical in the use case.

  5. #4
    Garmin Expert babj615's Avatar
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    @ Bushwalker8,

    Perhaps you should research the Garmin GPSMAP 65 and GPSMAP 66sr (the models the OP was asking about) before posting misleading and incorrect information.

    These new Multi-GNSS + Multi-Band models from Garmin have many of the features you suggest are only found in 'Pro Grade' gear, including exceptional multi-path rejection and real-time on device atmospheric corrections.

    Do you really believe that a device that only requires 4-5 satellites to calculate an absolute position would actually use signals from SV's located at or near the horizon when it has access to a couple dozen additional SV's that are in far better positions for horizontal positioning?

  6. #5
    Member + Accuracy figure of horizontal position fix on GPSMAP 66 satellite screen?Accuracy figure of horizontal position fix on GPSMAP 66 satellite screen?
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    The OP's question relates to accuracy and if the indication is CEP.

    In the GPSMAP66sr other than the real time weather forecasts which are a different beast entirely, the inclusion of L5 is not real time atmospheric correction. It’s a good incremental step forward as the different wavelengths help in the same way that L1 & L2 have traditional done for pro grade dual frequency equipment.

    And multi GNSS is great but the main benefits are quicker time to fix and more often able to get a reasonable position, the improved accuracy is incremental as it’s still code-based resolution and timing errors.

    To get real step up in accuracy requires correction and processing against a known reference point, close by, and either from a true real time incoming data stream processed within the device or post processing later with sophisticated software. And it also requires breaking beyond the inherent limitations of code-based positioning which means working in carrier phase.

    Garmin acknowledge this by providing an optional specialized RINEX/Carrier capability in the GPSMAP66 for sub-metre positioning.

    I have a number of Garmin multi GNSS devices including GPS66i that I use for general navigation and display of JNX historic background maps when I am mapping with pro gear, and have been analyzing back-to-back comparisons of the results for many years as the Garmin equipment improves looking for opportunities however I have not seen suitable results yet.

    I obtained the GPS66i in part because of it's RINEX / carrier phase capability and the promise of lightweight low grade accuracy mapping however my experience has been that it has not proven sufficiently accurate or practical as I had hoped.

    And whilst it is technically multi GNSS (GPS & Galileo only, no Glonass the frequency interferes with the Iridium) it is still noticeably more susceptible to multipath and error than my older Rino 650's evidenced not only by the spiky jumping tracks but also in direct comparison of the static points to the Trimbles post processed data and known survey references at the cm level.

    Don't get me wrong, I love all the devices and take them everywhere and particularly the GPSMAP66i with it's large clear bright screen, JNX & remote emergency capability.

    But these devices are definitely not in the same accuracy and mapping league as pro grade equipment, you get what you pay for.

    And if you really think the device only needs 4 or 5 satellites for a fix there is a serious disconnect with the accuracy discussion, you may want to play with the charts here to understand the relationship between the number of satellites and DOP: [Only registered and activated users can see links. ]

    So to get back to the point of the OP question, unless you are utilizing the Garmin Rinex/Carrier functionality (not for the feint hearted) you won’t be anywhere near approaching sub-metre and the number of metres the device is indicating will most likely be 50% CEP.
    Last edited by Bushwalker8; 25th January 2021 at 04:53 AM.

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    Quote Originally Posted by asprin624 View Post
    This same post is at GPSrChive

    I do not know how you are using your GPS that you question its accuracy.

    But if it puts you in the within the area you set out to go to plus or minus 4 meters is that a problem?

    You will not be walking off a cliff because your GPS said to walk 4 metes more are you???
    I too have been using GPS for over a decade, starting with a Geko. I think you misinterpreted my question: as a hiker and mountaineer, I am generally happy with the accuracy of the horizontal position fix obtained by my 66st which I use for navigation with a variety of installed topo maps for different countries, and haven't walked off any cliffs!

    That said, like other OPs on several forums going back over a number of years, I remain curious about what this Garmin accuracy number actually represents.

    Quote Originally Posted by Bushwalker8 View Post
    The OP's question relates to accuracy and if the indication is CEP.

    So to get back to the point of the OP question, unless you are utilizing the Garmin Rinex/Carrier functionality (not for the feint hearted) you won’t be anywhere near approaching sub-metre and the number of metres the device is indicating will most likely be 50% CEP.
    @Bushwalker8 That is helpful information, thank you.

    I emailed this question to Garmin support some two weeks ago but have yet to receive any response. If I do, I will post it here.

  8. #7
    Administrator Accuracy figure of horizontal position fix on GPSMAP 66 satellite screen?
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    Well I have been using GPS's now for 20yrs, I still have an use my Garmin Etrex, I have only ever seen 4mtr accuracy, since it only uses the US "military sats" and found out it was SA "selective availability" and the Glossnos Sats are Russian, so do you think they are more accurate ???. If your a professional Surveyor your equipment is offset for SA and works to mm acc. I have taken info from Topo/Geological map systems for years either from paper or digital maps and walked to an abandon mine and when been within 4mtrs of the hole it says "arriving destination" so I don't see why you are so worried about accuracy, am not going to send a missile through your window huh.
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  9. #8
    Garmin Expert babj615's Avatar
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    @ Bushwalker8,

    Please do not confuse the GPSMAP 66s/st or your GPSMAP 66i capabilities with the GPSMAP 66sr. As you stated, your GPSMAP 66i 'Multi-GNSS' only uses GPS + Galileo. This is still an advantage, as twice as many SV's are always better. But these devices only use the L1 Band.

    The GPSMAP 66sr (and GPSMAP 65) 'Multi-GNSS' capabilities include using FIVE GNSS constellations simultaneously. Those are GPS + GLONASS + Galileo + NavIC + QZSS. Further, these devices use both Li and L5 bands for GPS and Galileo (GLONASS does not support additinoal bands for civilian use). This allows for real time atmospheric correction and multi-path rejection. It is my understanding that these devices do not support WAAS or EGNOS because their own on-board corrections provide better results.

    The 66sr also supports RINEX logging, so you may want to investigate further the capabilities of a true 'Multi-GNSS' + 'Multi-Band' receiver!

    Quote Originally Posted by Bushwalker8 View Post

    And if you really think the device only needs 4 or 5 satellites for a fix there is a serious disconnect with the accuracy discussion, you may want to play with the charts here to understand the relationship between the number of satellites and DOP: [Only registered and activated users can see links. ]
    Apples and Oranges.

    We both know that three satellite signals provides the receiver with two possible location solutions when using a fourth satellite clock signal exclusively for the purpose of synchronizing the other signals. In this scenario, one solution is usually very near the Earths surface while the other is either far below or high above the Earths surface, and the GPSr can 'guess' the position near the surface is the correct location. Adding one more satellite signal (total of five now) allows the receiver to verify which of the two previous solutions is the true location, and 'guessing' is no longer necessary.

    I never suggested that accuracy would not improve when more than five satellite signals were available.

    And I never suggested the Garmin consumer grade devices would provide accuracy identical to survey grade equipment.

    However, the new Garmin 'Multi-Band' devices are a significant step forward in consumer grade satellite positioning technology.

  10. #9
    Member + Accuracy figure of horizontal position fix on GPSMAP 66 satellite screen?Accuracy figure of horizontal position fix on GPSMAP 66 satellite screen?
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    There is no confusion between the 66i and 66sr, to be clear I have used the 66i as an example as I have actual experience with one and it helps to illustrate to both the market segment positioning of these devices and more significantly gives an example of a relatively new multi-GNSS unit that in my heavily forested environment here has tested less accurate than an older device.

    In case you missed it my Rino 650 is GPS only, and it demonstrates less multipath and better point accuracy than the multi-GNSS 66i with GPS & Galileo. Regardless of the estimated accuracy number displayed. I wish it was not the case.

    So twice as many SV’s whilst highly desirable are not “always better”, it depends on how good the device is and what it does with them. There is often a difference between simplistic theory and hype and actual real-world performance in specific conditions.

    With the 65 & 66sr I would hold off regurgitating the “exceptional multi-path rejection” and “significant step forward” superlatives until I actually had one in my hands and properly tested it.

    Dual frequency is not new, I've had in my Trimble devices for decades and it definitely helps. However, the devices have to make certain calculations and assumptions so it is not perfect and there are also many more factors than ionosphere at play. Hence even that device is geared for differential correction.

    Reviews of the 66sr & 65 I have seen to date indicate the improvements are clearly there but are incremental rather than earth shattering, and as the mood here suggests that whilst nice to have don’t really provide a significant benefit for most people.

    Interestingly in one example a GPSMAP65s reviewer noted that the track improvements were more significant in urban environment than forest, and in southern Californian forest actually recorded some instances of the 65s tracks noticeably in error compared to a number of older units. So whilst not a scientific review there is some similarity to my experience with the 66i and the 65 & 66sr may also not be perfect in these environments. [Only registered and activated users can see links. ]

    And a general practical note regarding accuracy, don’t get overly hung up and forget that any underlying maps also have an error and particularly so with the growing trend to utilizing open-source data including Topo Active. This data has not been collected to any standard, and almost certainly not with accurate mapping devices. Forest tracks can also deviate over time. So for many common usage situations a 25% or better device accuracy could be somewhat irrelevant. E.g. you might now see with 25% greater confidence that the track on the map is around 30m in error….
    Last edited by Bushwalker8; 27th January 2021 at 01:44 AM. Reason: Corrected GPSMAP66i description

  11. #10
    Garmin Expert babj615's Avatar
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    Quote Originally Posted by Bushwalker8 View Post
    There is no confusion between the 66i and 66sr, to be clear I have used the 66i as an example as I have actual experience with one and it helps to illustrate to both the market segment positioning of these devices and more significantly gives an example of a relatively new multi-band unit that in my heavily forested environment here has tested less accurate than an older device....
    I didn't miss anything, but it appears you may have.

    ONLY the GPSMAP 65/s and GPSMAP 66sr are Multi-Band devices.

    Your GPSMAP 66i is not a Multi-Band receiver.

    Any testing you perform with the GPSMAP 66i provides zero reflection of the capabilities and performance of the GPSMAP 65/s and GPSMAP 66sr.

    Quote Originally Posted by Bushwalker8 View Post
    And a general practical note regarding accuracy, don’t get overly hung up and forget that any underlying maps also have an error...
    I do not get hung up on accuracy. I always follow the trail under my feet, not the line on the GPSr display. 8^)
    Last edited by Boki_Srb; 26th January 2021 at 06:30 PM. Reason: merged 2 posts

 

 

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