Thread: Accuracy figure of horizontal position fix on GPSMAP 66 satellite screen?

Originally Posted by Bushwalker8

Apologies my typo and thanks for pointing it out, it was meant to say Multi-GNSS & I’ve corrected the post.

Thank You.

Originally Posted by Bushwalker8

And this multi-GNSS aspect of the 66i is relevant to 65 & 66sr discussion:
1. The newer devices promote additional GNSS systems as one of the two new main features, and you have also stated the additional SV’s are “always better”. In my situation that has not shown to be the case so raises a question yet to be answered on the newer devices.

Since the GPSr always selects the 'best positioned' SV's to calculate the users position, I can not imagine any scenario where having less SV's for the GPSr to choose from would be beneficial. Just because the GPSr can see a couple dozen SV's does not mean it is using them all to calculate the devices location...

Originally Posted by Bushwalker8

2. It also raises a question in the rigor of the estimated accuracy figure and how it is derived – the current topic.

If you have had different experiences and with one of the new devices please share it. And if you do be very careful how the datums are considered as this is another issue that directly relates to the topic of accuracy.

Garmin devices and software have very simplistic handling of map datums and do not accommodate all variables of plate movement, and particularly over time as everything is continually moving.

This brings up an interesting point.

Certainly the Earths surface is in a constant state of movement, most of which is imperceivable to the human eye, but not always (for example, large magnitude earth quakes). Once upon a time the Earth had a single super-continent that broke up and separated over a very long period of time into the pattern we are currently familiar with. Any selected location (or waypoint) on Earth today would be found in an entirely different location a billion years ago (by a time traveling surveyor, for example).

Wikipedia states: "Surveying (or land surveying) is the technique, profession, art, and science of determining the terrestrial or three-dimensional positions of points and the distances and angles between them."

So, one can imagine how plate tectonics would play a part in why a surveyors results might change over a period of time, as they are measuring the relationship of multiple points on the Earths surface against each other.

A simple definition for 'datum' is "1. a piece of information." or "2. a fixed starting point of a scale or operation."

There are many different datums used for mapping the Earths surface, some of the more common being 'NAD 27', 'NAD 83' and 'WGS 84'.

Garmin GNSS receivers calculate their position using trilateration, where they reference their relationship only to the known locations of a number of SV's. At no time does the GPSr reference any location on Earth to determine it's position.

Map Datums only come into play when translating a GPSr position into a usable map location. Legacy GPSr (and still some today) do not have internal mapping capabilities, and provide the user only with coordinates for their current location. Those coordinates will be translated into the Map Datum specified on the device, allowing the user to plot their location on a paper map that was created using the same map datum.

A mapping GPSr does this in real time for a digital map specified by the user.

Originally Posted by Bushwalker8

So ground coordinates you obtain from elsewhere or collect yourself will have already changed or will change position over time, and more importantly any datum transformations in the device will be further distorted by the simplistic transformation and the date for which it was set.

Normally this would not be an issue with consumer accuracies around 3 metres or more and Pro gear and software are designed to deal with it. However, with advancements in constellations and receiver’s Garmin is now nudging the boundary where these anomalies are relatively approaching similar magnitude and possibly now at times even the exceeding the displayed accuracy estimated in some areas.

I might argue that the coordinates do not change position, but rather the surface of the Earth under them.

The GPSr still finds itself in the same location as when the 'waypoint' was originally marked, however, the 'landmark' found at that location may be different due to movement of the Earths crust.

Here the GPSr reported location is still correct. It is the 'Map' that has changed, and it is the map makers responsibility to integrate these changes into newer versions of their maps.

Originally Posted by Bushwalker8

As an example, we have recently updated our national datum here for GNSS consistency for the second time due to plate movement, this time the error difference was around 1.8m.

So, was this 1.8m of plate movement uniform and consistent throughout the entire surface of the Earth?

No, of course it wasn't.

This is a perfect example of why changes in the Earths surface due to plate tectonics can not possibly be integrated into any uniform coordinate system, and why it is the maps that must be updated.

Originally Posted by Bushwalker8

So for a device like the 66st which was estimating an accuracy of 1.8m this would be misleading and the reality with respect to the ground/map would actually be more like up to 3.6m.

The GPSr estimated accuracy here is not misleading, as it has no direct relationship to any map chosen by the end user.

The GPSr estimated accuracy refers to the expected level of error for position calculations in relationship to the SV's used for determining its location.


To summarize,

1. A GPSr calculates its position based on the spatial relationship between itself and multiple SV's orbiting the Earth.
2. The coordinates reported by the GPSr are a translation based on the datum and format specified by the user.
3. Map datums use a static grid system to reference locations on the Earths surface (latitude and longitude, for example).

Thus,

1. The relationship between specific landmarks on the Earths surface are never static.
2. The coordinates for any and all landmarks on the Earths surface will change over time.
3. Waypoints marked using a GPSr are a static record of the devices location, not the position of the Earths surface.

@Bushwalker8 and babj615,

Your technical info is super and very informative even if you two do not see eye to eye on issues.

That being said the GPSMAP 66 series GPS units are for recreational use they lack being about to attache an external antenna to them. In fact there are only a couple of GPSMAP series GPS units that let you add an external antenna to improve the accuracy of the GPS units.

Most users are satisfied with the GPSMAP 66 series with updates come from time to time to improve them.

This does not taking away from what you both have pointed out but will have little or no effect on the average recreational user of these GPS Units....

Even the forum GPSrChive that is dedicated to Handheld units has not responded to the original post on their forum.

@babj615

Not correct, it doesn’t work that way. The coordinate systems work very differently.

For example GNSS positions are based on ECEF, 3 dimensional coordinates fixed from the centre of the earth.

You can see in this in an extract from a GPSMAP66i Rinex file header, the calculated ECEF coordinate location is reported in the XYZ line.


2.11 OBSERVATION DATA M (MIXED) RINEX VERSION / TYPE
RINEXWRITER GARMIN 14-JUL-20 04:27 PGM / RUN BY / DATE
Download navigation file using this link: COMMENT
ftp://cddis.gsfc.nasa.gov/gnss/data/daily/2020/196/20p/ COMMENT
BRDC00IGS_R_20201960000_01D_MN.rnx.gz COMMENT
GARMIN MARKER MARKER NAME
OBSERVER / AGENCY
GPSMAP 66i 5.90 REC # / TYPE / VERS
ANT # / TYPE
-4011478.7500 2922211.7500 -3902601.5000 APPROX POSITION XYZ
0.0000 0.0000 0.0000 ANTENNA: DELTA H/E/N
1 1 WAVELENGTH FACT L1/2
4 C1 L1 D1 S1 # / TYPES OF OBSERV
2020 7 14 4 27 42.000000 TIME OF FIRST OBS
END OF HEADER

Note it’s also says “approx” because the intention of having the RIXEX file is to professionally correct it via post processing. And despite the hype 66sr will also only be considered approximate unless it’s corrected.

The ECEF coordinates simply represent a point in space (or within the earth), and don’t have a direct relationship to any place on the surface. They are essentially meaningless to a device user.

So the ECEF coordinates need to get converted to a surface coordinate system and there are many variations.

You can convert them manually but it’s not practical, you can convert them accurately with professional hardware and software that knows how much movement and in what direction (and also the impact of how much the land has dipped, rolled and stretched in that time), or you could let the Garmin device apply a simple and out of date offset that adds error.

Given the accuracy topic, surface coordinates would then normally be transformed to local datum and coordinate system which is locked to the local land surface and relevant to it’s maps. This is intentional and the coordinates stay with the land and move with it and a point on the ground retains it’s coordinates in the local datum. So as the land moves it’s the ECEF coordinates for it that change.

The datum may get updated from time to time and gain a new name but the coordinates for any point in the earlier datum still remain the same.

And the coordinates of maps in the datum remain valid.


Also Data files are available for local datums that detail dynamic plate movement and are used by pro gear and software for detailed transformation on any date or between dates if historical files.
Here are some simple rolled up annual summary parameters that show our updated local datum movements into this year. The formulas and underlying data in detail files can be calculated for any date:
"GDA2020 (20)","GDA2020-20","GRS 1980",0.00000000000000000,0.00000000000000000,0.00000000000000000,0.00000000000000000,0.00000000000000000,0.00000000000000000,0.00000000000000000
"GDA2020 (21)","GDA2020-21","GRS 1980",0.00000000000000000,0.00000000000000000,0.00000000000000000,0.00000000729057966,0.00000000573757599,0.00000000585247683,0.00000000000000000


And regarding the accuracy number it is clearly misleading if you are trying to return to somewhere on the surface or that is represented on a map and the place has since moved, the error is now greater. Strangely enough most people actually use the device as it was intended to relate to surface positions and maps…..

Since WGA1994 Australia has moved 1.8mts West so now the new maps needed to be coordinated to GDA2020 and with SA of 4mtrs and WGA84 1.8mts out you are now upto 5.8mtrs away from true coordinate on the ground.


have a read of this

Code:

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Originally Posted by Bushwalker8

Lat Lon is a projection, you don’t say what datum you are using. The current local one for your area is NAD83 and like other national datums across the world it’s regularly adjusted over time (epoch) so even it has multiple versions. You should only use NAD27 there if you are using old maps or have obtained coordinates in NAD27.

Here’s a description from your responsible authority the NGS of an NAD83 2011 adjustment which specifically mentions tectonic activity and earthquakes in Western US: [Only registered and activated users can see links. ]

And this is where you are going next: [Only registered and activated users can see links. ]

Garmin devices normally have a selection for NAD83, however the transformation is simplistic, the epoch date isn’t specified and it isn’t updated.

Thank you for those links, I have read through them and multiple pages linked from them. I find they all have one thing in common. They are all discussions about different systems and methods used to map the surface of the Earth.

None of them have any bearing on how a GPSr calculates its distances from multiple satellites orbiting the Earth to trilaterate a three dimensional position relative only to those satellites.

Only after the GPSr has made this determination does it then display coordinates based on the projection and datum specified by the user.

Originally Posted by Bushwalker8

So forget all the nonsense about accuracy, you won’t get it.

Won't get what?

Each GPSr has unique capabilities and will provide 'some level of accuracy' in the position it reports.

I believe the OP wanted to know 'how' the Garmin GPSr 'Accuracy' was calculated, which I suspect Garmin engineers are not going to be allowed to divulge.

However, I do believe that accuracy value is an indication of the GPSr confidence in its reported position relative only to the satellites being used to make the calculation, and has no bearing what-so-ever on the projection or map datum selections chosen by the operator.

In fact, I just ran a simple exercise on my Garmin GPSr where I repeatedly changed the Map Datum settings and compared the coordinates and estimated accuracy displayed for each Datum selected, and the accuracy value never changed, confirming it has no relevance to the map projection or datum configured on the device.

Originally Posted by Bushwalker8

Just use it and enjoy whatever it is you are doing they are great devices to support almost any activity.

I think we can all agree on that!