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

The accuracy field is based on where the device it thinks it is in the frame, but it’s reported as a 2D horizontal number. The number relates directly to the ground coordinates it provides you, and importantly those numbers are in whatever datum you have chosen so the datum is intrinsically linked and a significant part of your overall error.

GNSS devices report horizontal accuracy and typically with a confidence level. A common method is the already mention CEP or Circular Error Probability 50%.

Note very carefully the word Circle, it is obviously 2D. It doesn’t say Spherical or anything else in 3D for the reference frame or individual satellites.

In other devices and software it’s much clearer that the accuracy number is horizontal, the software even draws it for you in 2D.

You appear to be confused with other things like satellite specific User Range Error which is quite different to the User Accuracy we are discussing here. Here is a simple overview with a picture that may make it clearer for you: [Only registered and activated users can see links. ]
Note that the User Accuracy is a 2D circle on the ground, around the coordinates the device is giving you, which will be in the datum you have selected.

This has nothing to do with land surveying, it’s the basic ABC’s of GNSS, maps and navigation.

You have quoted recommendations from Garmin advising to use local datums so that should tell you it’s fundamental and important because of potentially large discrepancies in error. And if you do so the Garmin will add a simple general offset for the datum to take out the worst of the error but it's definitely not precise because the datums are far more complex. I gave an earlier example NZ datum that shows they can require far more computation.

And if the datum is important then it follows that if the datum moves then that is also important.

So if you want to understand the accuracy number, the topic of this thread, then you should understand that the Garmin doesn’t track that movement of the datum over time. So it’s reported position – the location of person in the middle of the circle in the picture referred to above - increases in error over time. The plate moves, and with it goes you, the datum and the map. They are all connected, the Garmin isn’t because it's reference frame is left behind in your wake. The coordinates it’s giving you are no longer correct.

No-one has said the drift is in or should be added in the accuracy number. The point being made is simply that it's not, the Garmin doesn’t track it and so you should be aware of it.

Garmin clearly understands their datum transformation is initially simplistic, and that their device isn't tracking it's subsequent drift over time. They provide the raw data in the RINEX file to allow you to do the processing yourself to correct the position. The correction includes a far more rigorous and accurate datum transformation computation, and adjustments for drift using precise reference station data. That is the only purpose for the RINEX.

You don’t have to take up the correction option, it’s optional. But if you don’t then accept you won’t be correct and you won’t know by how much or in which direction.

The points here relate to the underlying basics and do not include the “map errors” you refer to which is an additional consideration.

And I have no idea what point you are trying to make by listing the GNSS combination selections, obviously whichever combination you have selected the device will still provide an accuracy number. And if it thinks the chosen constellations give it a better horizontal position on the ground then the accuracy number will reflect that.

You still haven't said what datum you are using.