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.