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

Originally Posted by Swall

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.

It’s 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 didn’t get it.

My GPSMAP66i did come with GDA2020 but it’s been poorly and incorrectly implemented. If you look at any waypoint and switch the datum between GDA94, GDA2020 & WGS84 it’s 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 it’s 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 didn’t do that and the error shows us they have simply tacked on GDA2020 and calculate it as a simple offset from out of date GDA94, and as a result the GDA2020 coordinates are incorrectly offset at higher numbers.

It’s 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. It’s 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.

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 can’t 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 “a GPS accuracy that is almost always 1.8 m in the ideal case”, and that observation is supported by a number of screen shots.
[Only registered and activated users can see links. ]

I’ve 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 doesn’t 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.

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

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

Accuracy in Geology is not guaranteed so hey who cares.

Not downing your need for info Eyelet99 just stating my facts.

I too do not pay much attentions to how Accuracy the GPS unit I use is.
For one I have hiked all over Europe and the maps have always show me the paths to hike on.
They have always been spot on. I for one will not walk in a tree or fall info a river because of + or - Accuracy with in 4 meters.
Even if I track my movement with my 66s and later look at it.
The thing it show is 80% of the time it tracks along the path in comparison to the the map on my Laptop that is the same map on the 66s.
Even the tracks I download from many sites to use so the path with 80% Accuracy. But have never gotten lost or wound up in the wrong place.

Like Magnetron said Accuracy in Geology is not guaranteed so hey who cares. The same with hiking and Geocaching.

GPS units are for recreational use for the average person so I do not know what the big deal with Accuracy is.

I am sure if you are in the armed forces over sea you will have a completely different GPS unit altogether where Accuracy is of the most important for you.

Here is an example of the 66s tracking my hike.

Spoiler: GPSMAP 66s tracking.

I stayed on the path the whole time. As you can see the Red Track is some places are not on the path.
If I where to give this track to someone else to use they too would stay on the path and not walk into the bushes even if the track showed the path was some place else which it was not.

You would get the same errors using the tracks you download from website like "Wikiloc" and others. This info comes from many different GPS units and smartphones tracks.

So if it is + or - 4 meters no big deal when it comes to Accuracy.

All my units Old and New have best Acc of 4mtrs, the Geo maps are Mosaics of 100K and 250k the Topo is a full 250K of WA or whole Australia and is from Geoscience Aus and is more acc for the Main Roads in OZ which I use to get me there, I have also been using the app TT NDS Mod from here.

Spoiler: Pic

As you can see the track matches the main road enough but tracks off road change all the time due to weather.

The app in my post is my new phone running Trilobite app and I use that alot too, my Etrex is a backup if my phone dies or i drop it on a Rock from my pocket, I have been working with the developer to include all sorts of Maps that I use when bush and the Western Australia available resources is far better than the Eastern states.