Zumo User Forums

When you touch the screen in the upper left area where it shows signal strength bars until it shows your location and accuracy, I see Galileo and GPS.

It also shows 10 Foot Accuracy.

Location shown is six digits to the right of the decimal point.

Position Format is h ddd.dddddd°

Datum WGS-84

What Accuracy do you get?

Jim1348 wrote: ↑Fri Jul 14, 2023 8:25 pm What Accuracy do you get?

I just took my XT outside. It took awhile for accuracy to settle on 13 feet but as I walked around the yard it changed to 14 feet.

The GPS Accuracy displayed when long-holding the speed button on the map varied between 10 & 11 feet during the same walk around the yard.

Accuracy isn't fixed. It varies according to weather, satellite position, your position relative to trees, hills, high sided vehicles, tall buildings, the moon, etc.
2 to 3 m is good. Sometimes (in locations as above) it can be a lot more.

As I understand it (which is not very well) ....
The GPS system fixes your position by calculating the distance from the satellite (by the time it takes to reach it). It can draw a circle of all the locations that are the same distance from the satellite. It can then do the same from other satellites. There will be on point where all of these circles meet. Except they don't.

Just like when you were using a ruler and compass at school, and asked to draw three lines that were supposed to meet up. They rarely meet up at a single point. They meet up near to a point and form a triangle. The bigger the error, the bigger the triangle.

That is how the satnav knows the accuracy of its 'fix' of your location - but it cannot fix you more precisely then (say) 3 metres. Or 10 metres if you are in the hills.

I read a report where they realised the calculating altitude using this method would be fraught with problems. The angle of intersection of the lines would be too acute for it to be reliable enough - a tiny error from one signal could result in a massive calculational error. What is the result of multiplying an extremely large number by an extremely small number ? Is it very big or very small or extremely average ?? The answer is that we try to avoid such calculations.

But they were genuinely surprised at the degree of accuracy that they were getting.

I use my Zumo for on the road navigation so as long as the icon on the map on the screen aligns fairly close to the ahead that I can see that's all that worries me. If I was using it for geo caching for similar I would be concerned about accuracy.

Richard_R wrote: ↑Thu Jul 20, 2023 7:22 am I use my Zumo for on the road navigation so as long as the icon on the map on the screen aligns fairly close to the ahead that I can see that's all that worries me. If I was using it for geo caching for similar I would be concerned about accuracy.

^this^

I find it interesting that a Zumo records tracks to so many digits:
<trkpt lat="39.578400962054729" lon="-82.317653987556696" />

That would imply an accuracy better than 0.0000004 inch!

Reminds me of my early adult years living with the mindset that it's not about whether you're actually rich or smart, it's about the impression that you make. All of those decimals points is Garmin's way of attempting to make an impression.