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  1. #21
    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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    I think you may be confused by the basics; your underlying assumptions are very wrong.

    Firstly the earth is not round and the lines of latitude and longitude are not fixed, they are centred in many different places in the centre of the earth as a best fit for each of the various local datums and can be up to hundreds of metres out. So, no matter where you are their exact location depends on which datum you are on.

    And you still havenít said which local datum you are using so itís hard to be specific.

    If you are using global datum WGS84 then itís not accurate at all, its averaged over the world and not directly meaningful in any one area. Everything is continually moving under it. OK for basic navigation but no good for accuracy. Itís a virtual GNSS reference frame about which the satellites orbit, and on which all the accurate local datums are linked.

    Have a good read of this: [Only registered and activated users can see links. ]
    An extract here: ďThe International Hydrographic Office and many international navigation systems quote the datum they use as simply WGS 84. As we have pointed out, this is, strictly speaking, insufficient for a datum. The version of WGS84 is commonly not quoted nor is any reference epoch.Ē

    And this: [Only registered and activated users can see links. ]
    Extract: ďBecause Earth is an imperfect ellipsoid, local datums can give a more accurate representation of some specific area of coverage than WGS 84 can. OSGB36, for example, is a better approximation to the geoid covering the British Isles than the global WGS 84 ellipsoid.Ē

    Havenít you ever wondered why your country that developed GPS still spends enormous resources refining and adjusting your current Ė official - US local datum NAD83? And why all the other countries do the same with their datums? Itís more accurate.

    And it also seems you have missed in the link I posted previously that even in your area NAD83 has a number of different flavours Öone for each of the North America plate, Pacific plate and Mariana plate. Why? Because the coordinates are static, fixed to the plates and move with them.
    And your new datum that you will be moving to in a few years - and that is better aligned to GNSS - will still also be split into separate datums, one fixed to each plate for the same reason.

    Same with most datums, both our GDA94 and GDA2020 are plate fixed, the coordinates are static on the ground and move with us. Read here very carefully: [Only registered and activated users can see links. ]
    Extract: ďGDA94 is a Ďplate-fixedí or Ďstaticí coordinate datum based on the International Terrestrial Reference Frame 1992 (ITRF92), held at the reference epoch of 1 January 1994.Ē
    And funnily enough the coordinates of the GDA survey marks I use here are still measuring the same within 1cm around 25 years after I first used themÖ..

    And it certainly does matter which coordinate system you chose and they are definitely not all linear and equidistant. One local example here, look at the NZMG which is mathematically twisted around NZ and needs special calculations. Itís now superseded but maps still exist and are used: [Only registered and activated users can see links. ]
    Extract: "NZMG is not based on a geometric projection (transverse Mercator is based on a cylinder). Instead it uses a complex-number polynomial expansion. This has the advantage of exhibiting minimal scale distortion over New Zealand; however it is a projection unique to New Zealand and so can be difficult to use or program into computer software or positioning devices (eg, GPS receivers)."

    And there are also many other aspects you apparently havenít considered, for example how earth curvature is treated as itís also a significant factor in calculating the distance between points.

    I think you are also confused with basic ďAccuracyĒ vs ďPrecisionĒ. Google it or read here: [Only registered and activated users can see links. ]

    Accuracy Ė this topic - is how close you are to a known true point. Precision is how closely your data points group together, regardless of where you are.

    So for a Garmin the accuracy relates to how close you are to the known pointÖwhich has to be the ground coordinates it gives you and the spot where you are on the ground. There is no other known or true point, itís not giving you anything else to measure against including satellites. It doesnít matter if the coordinates are in either datum, or are right or wrong, itís simply telling you the distance it thinks you may be from that point on the ground where you are. So you would normally expect the accuracy to be the same under the same time and satellite conditions and your test is not really testing anything.
    Last edited by Bushwalker8; 30th January 2021 at 11:47 AM.

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  3. #22
    Garmin Expert babj615's Avatar
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    Bushwalker8,

    I have no doubt you are very knowledgeable about terrestrial surveying and mapping techniques and systems, but the original question posted was asking about the Garmin 'Accuracy' value and how it is calculated.

    All of which you speak is related to different methods used to map the Earths surface. None of the surveying information you have shared has any bearing on the GPSr 'accuracy' calculation. The Garmin GPSr 'Accuracy' value reflects the devices confidence in its calculated distance from multiple satellites, as they are the only field of reference used by the GPSr for determining its true position. Never does the Garmin GPSr reference any terrestrial signals or landmarks for this purpose.

    Only after the GPSr has calculated its position in relation to a constellation of satellites can it provide the 'desired or expected coordinates' by applying the mapping system selected on the device.

  4. #23
    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 answer is simple, they are two different sources or error and you need to add both together to understand the total accuracy or distance to a known point.

    The device gives you one, a horizontal accuracy number based on its calculations to where it thinks it is in the 3D reference frame. The OPís question related to this calculation to better understand the accuracy.

    However, the device also transforms that 3D position into the 2D horizontal coordinates of where it thinks it is in your chosen local datum. But itís often actually somewhere else because the transformation is simplistic, and it does not calculate additional plate movement over time. The known point is actually further away.

    So that additional error needs to be added to the device provided accuracy number to give you the total distance to the known point if you want to understand the real accuracy of your position.

    So if the question was how does it calculate the accuracy, then one short answer is that it does it insufficiently because it leaves some of the error out.

  5. #24
    Garmin Expert babj615's Avatar
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    Quote Originally Posted by Bushwalker8 View Post
    The answer is simple, they are two different sources or error and you need to add both together to understand the total accuracy or distance to a known point.
    There are indeed multiple sources of error to take into account when calculating the distance to a known point and the accuracy of that calculation, including:

    1. Current GPS reported position errors.
    2. Map errors present at the time the current map location is determined.
    3. GPS location errors present when the 'known point' position was calculated.
    4. Map errors present at the time the 'known point' map location was calculated.
    5. Map errors in all calculation due to changes (movement) in the Earths crust over time.
    6. ...and the list goes on...

    However, these are all referencing the activity of land surveying, and have nothing to do with the GPS receivers reported position accuracy.



    Quote Originally Posted by Bushwalker8 View Post
    The device gives you one, a horizontal accuracy number based on its calculations to where it thinks it is in the 3D reference frame. The OPís question related to this calculation to better understand the accuracy.
    Yes, 'The device gives you one,' is correct.

    The devices 'Accuracy of GPS' data field displays the margin of error for the calculated location 'in a 3D reference frame', a reference frame that is based entirely on it's relationship to the satellites being used to make that calculation.

    After reviewing this data field on many Garmin GPSr devices, a common theme has become apparent. The GPS Accuracy data field always displays one of the following text options based on the satellite constellations in use:

    1. 'GPS' (when only GPS is enabled)
    2. 'GPS + GLONASS' (when GPS and GLONASS are enabled)
    3. 'GPS + Galileo' (when GPS and Galileo are enabled)
    4. 'GNSS' (when Multi-GNSS is enabled)

    Can you see the common theme?

    They are always indicating the reported position accuracy based on the satellite constellations enabled.

    This data field is exactly what it says it is: "Accuracy of GPS".

    This data field has no relationship to the accuracy of the maps or map datums being used.



    Quote Originally Posted by Bushwalker8 View Post
    However, the device also transforms that 3D position into the 2D horizontal coordinates of where it thinks it is in your chosen local datum. But itís often actually somewhere else because the transformation is simplistic, and it does not calculate additional plate movement over time. The known point is actually further away.
    The 'GPSr' is not 'somewhere else because the transformation is simplistic'.

    The GPSr is exactly where is says it is.

    The map data displayed may be incorrect for the current GPSr location due to any number of potential map data errors.

    Again, the 'Accuracy of GPS' date field has no relation to the accuracy of the map being used.

    The [Only registered and activated users can see links. ] states:

    "If you are navigating and comparing the GPS coordinates to a map, chart, or other reference, set the map datum in the GPS unit to the same datum as the map to ensure the most accurate navigation."

    The [Only registered and activated users can see links. ] states:

    "You should not change the position format or the map datum coordinate system unless you are using a map or chart that specifies a different format."

    If we look at a Garmin GPSr that did not provide on-board mapping capabilities, we can see that the [Only registered and activated users can see links. ] states:

    "You may select each datum applicable to your region until you find the datum that provides the best positioning at a known point."



    Quote Originally Posted by Bushwalker8 View Post
    So that additional error needs to be added to the device provided accuracy number to give you the total distance to the known point if you want to understand the real accuracy of your position.
    Once more, the GPSr reported position accuracy has no relationship to map data errors.

    One must be able to understand that these are two different things. The GPSr calculated position in reference to the satellites being used will have some amount of inherent error, as will the maps used to display where that position falls. The GPSr uses the selected 'Map Datum' to position the map data under the devices calculated position as precisely as possible.



    Quote Originally Posted by Bushwalker8 View Post
    So if the question was how does it calculate the accuracy, then one short answer is that it does it insufficiently because it leaves some of the error out.
    Only if one assumes this value is supposed to be including map errors in the first place, which I do not believe it does, as I explained above.
    Last edited by babj615; 1st February 2021 at 09:20 PM.

  6. #25
    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 accuracy field is based on where the device it thinks it is in the frame, but itís reported as a 2D horizontal number. The number relates directly to the ground coordinates it provides you, and importantly those numbers are in whatever datum you have chosen so the datum is intrinsically linked and a significant part of your overall error.

    GNSS devices report horizontal accuracy and typically with a confidence level. A common method is the already mention CEP or Circular Error Probability 50%.

    Note very carefully the word Circle, it is obviously 2D. It doesnít say Spherical or anything else in 3D for the reference frame or individual satellites.

    In other devices and software itís much clearer that the accuracy number is horizontal, the software even draws it for you in 2D.

    You appear to be confused with other things like satellite specific User Range Error which is quite different to the User Accuracy we are discussing here. Here is a simple overview with a picture that may make it clearer for you: [Only registered and activated users can see links. ]
    Note that the User Accuracy is a 2D circle on the ground, around the coordinates the device is giving you, which will be in the datum you have selected.

    This has nothing to do with land surveying, itís the basic ABCís of GNSS, maps and navigation.

    You have quoted recommendations from Garmin advising to use local datums so that should tell you itís fundamental and important because of potentially large discrepancies in error. And if you do so the Garmin will add a simple general offset for the datum to take out the worst of the error but it's definitely not precise because the datums are far more complex. I gave an earlier example NZ datum that shows they can require far more computation.

    And if the datum is important then it follows that if the datum moves then that is also important.

    So if you want to understand the accuracy number, the topic of this thread, then you should understand that the Garmin doesnít track that movement of the datum over time. So itís reported position Ė the location of person in the middle of the circle in the picture referred to above - increases in error over time. The plate moves, and with it goes you, the datum and the map. They are all connected, the Garmin isnít because it's reference frame is left behind in your wake. The coordinates itís giving you are no longer correct.

    No-one has said the drift is in or should be added in the accuracy number. The point being made is simply that it's not, the Garmin doesnít track it and so you should be aware of it.

    Garmin clearly understands their datum transformation is initially simplistic, and that their device isn't tracking it's subsequent drift over time. They provide the raw data in the RINEX file to allow you to do the processing yourself to correct the position. The correction includes a far more rigorous and accurate datum transformation computation, and adjustments for drift using precise reference station data. That is the only purpose for the RINEX.

    You donít have to take up the correction option, itís optional. But if you donít then accept you wonít be correct and you wonít know by how much or in which direction.

    The points here relate to the underlying basics and do not include the ďmap errorsĒ you refer to which is an additional consideration.

    And I have no idea what point you are trying to make by listing the GNSS combination selections, obviously whichever combination you have selected the device will still provide an accuracy number. And if it thinks the chosen constellations give it a better horizontal position on the ground then the accuracy number will reflect that.

    You still haven't said what datum you are using.
    Last edited by Bushwalker8; 2nd February 2021 at 03:35 AM.

  7. #26
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    Quote Originally Posted by Bushwalker8 View Post
    Garmin clearly understands their datum transformation is initially simplistic, and that their device isn't tracking it's subsequent drift over time.
    BTW if you have any information about Garmin's "datum transformation" and stuff, don't hesitate to share this information here. That'll be interesting because in many Garmin devices the user can define custom grid using different projection templates (Mercators, Lambert, etc.). Unfortunately, it's not documented at all. What sort of math is implemented? What are the limitations, known defects and other imperfections? Garmin says nothing. So it'll be nice if anyone can shed some light on that.

  8. #27
    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 Swall View Post
    if you have any information about Garmin's "datum transformation" and stuff, don't hesitate to share this information here.
    I can elaborate a little more based on my local situation here.

    Itís likely the devices are populated with the reference frame and local datum parameters as they were fixed prior to the devices release and not subsequently updated with firmware. For example, our current datum GDA2020 was released in 2017 and my GPSMAP78sc last firmware update in May 2019 didnít get it.

    My GPSMAP66i did come with GDA2020 but itís been poorly and incorrectly implemented. If you look at any waypoint and switch the datum between GDA94, GDA2020 & WGS84 itís clear the shifts are wrong. They are kind of the right distance and direction between GDA94 & GDA2020 (bearing in mid no decimals are displayed so itís not precise), however the relationships to WGS84 are wrong.

    In this example and using UTM and you can see they have GDA94 still locked at the WGS84 reference frame origin where it was when originally released in 1994 as the GDA94 & WGS84 coordinates displayed are identical. But this is incorrect because GDA94 has moved with the plate and is the reason that GDA2020 was released to re-align with the framework origin. Garmin didnít do that and the error shows us they have simply tacked on GDA2020 and calculate it as a simple offset from out of date GDA94, and as a result the GDA2020 coordinates are incorrectly offset at higher numbers.

    Itís the GDA2020 coordinates that should currently at this time be the same as WGS84, and because GDA94 has shifted to the NE, the GDA94 coordinates should appear lower than WGS84.

    So now both datums are in the wrong place in relation to the reference frame and any improved position accuracy in the 66 devices has already been blow away if you use it with an Australian map.

    Another consideration is that the Garmin shift would not account for all the variations around the country. These transformations are calculated using a complicated mathematical formula (7 parameter similarity Helmert transformation), or commonly applied in software by a shift grid. However, the official shift grid has over 5 million points and takes over a minute to load on my laptop so that is clearly not happening in the Garmin. Itís more likely they are simply providing the same average shift amount to any point.

    If you want to have a look at a shift grid there are examples of our local ones here: [Only registered and activated users can see links. ]

    I would be careful trusting datums in your own area until you have tested them.
    Last edited by Bushwalker8; 6th February 2021 at 12:54 PM.

  9. #28
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    Thank you all for the interesting material posted in response to my accuracy question.

    After yet more searching, I eventually found this page on Garmin's website [Only registered and activated users can see links. ]

    which which states:

    "The GPS location accuracy of Garmin handhelds is around 3 meters (10 feet), 95% of the time. This means, at any given time, your handheld will save your location within 3 meters of your actual location."

    This rather suggests that the position accuracy is being stated in terms of R95 rather than CEP?

  10. #29
    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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    Good find, yes in this case they are clearly quoting 3m at 95%.

    And it does appears to be in reference to the dual frequency units as per your original question when taken in context with this statement where they grade dual frequency at +/-3m and older units at +/-5m: [Only registered and activated users can see links. ]

    Bear in mind they have all the data every time, whatever they quote is simply where they decide calculate the circle in the plots.

    It could be a shift to standardizing on 95% going forward with the improved products. They canít continue to claim smaller circles due to the other errors now becoming relatively significant, but increased confidence percentages similar to professional gear is good marketing.

    Edit:
    Taking this a step further it may be that the published/marketing accuracy is 3.0m at 95%, and this in turn could be confirming the device itself is reporting 50%.

    With all else being equal, a lower confidence level would normally accompany a smaller circle.

    And the reviewer here reports the 66sr as having ďa GPS accuracy that is almost always 1.8 m in the ideal caseĒ, and that observation is supported by a number of screen shots.
    [Only registered and activated users can see links. ]

    Iíve just had a quick look at a data point post processed 0.3m at 95%, and at 68% the circle now becomes 0.2m. My software doesnít have an option of 50% but you can see it would be slightly smaller again so the order of magnitude is in the ball park.
    Last edited by Bushwalker8; 7th February 2021 at 10:54 AM. Reason: Additional observation

  11. #30
    Administrator Accuracy figure of horizontal position fix on GPSMAP 66 satellite screen?
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    Ok good posts
    Just to let you know why am not worried about Accuracy, for a start I do Prospecting looking for Gold/rocks & minerals as a hobby, I like to go out bush in Western Australia and look at my own country, have worked over seas but never done a Holiday trip. "why" anyway I have been mapping since 2000 and have converted all sort of files into geo co-ordinated maps and use them to go where I want to be. "That's what we want isn't it ?" so while I doing my mapping I use what ever co-ord system is avail on the software. No the Devices I use are older than GDA 2020 so hey not worried.
    First device is:
    Spoiler: pic1
    013

    I have 2 of these the latest software is V2.14 no further updates " so they are WG84"

    I also have one these:
    This device at the time was one one the series "Before In-car NAVS where introduce to cars."
    So I was able to get My M8's in the area of interest better.
    [Only registered and activated users can see links. ]
    I used OziExploerCE for this and have used it since Beta version.
    Only last year I update to a Tablet and a new phone to run a program called Trilobite Solutions which brings all my data into 1 program and I have been assisting the Developer with update to the program.
    Spoiler: Pic3
    Screenshot 20210102 084541 Australian geology travel maps 2

    Accuracy in Geology is not guaranteed so hey who cares.

    Not downing your need for info Eyelet99 just stating my facts.
    Can't see a link/attachment? Don't post asking why. Just 'Like' the post & hit F5:
    THE LIKE BUTTON IS NOW IN THE BOTTOM LEFT HAND CORNER
    Posts serving no purpose like thanks or to ask about links go to [Only registered and activated users can see links. ] or are deleted, it's GPS Power policy.

 

 

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