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

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!