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

Originally Posted by Bushwalker8

The answer is simple, they are two different sources or error and you need to add both together to understand the total accuracy or distance to a known point.

There are indeed multiple sources of error to take into account when calculating the distance to a known point and the accuracy of that calculation, including:

1. Current GPS reported position errors.
2. Map errors present at the time the current map location is determined.
3. GPS location errors present when the 'known point' position was calculated.
4. Map errors present at the time the 'known point' map location was calculated.
5. Map errors in all calculation due to changes (movement) in the Earths crust over time.
6. ...and the list goes on...

However, these are all referencing the activity of land surveying, and have nothing to do with the GPS receivers reported position accuracy.

Originally Posted by Bushwalker8

The device gives you one, a horizontal accuracy number based on its calculations to where it thinks it is in the 3D reference frame. The OP’s question related to this calculation to better understand the accuracy.

Yes, 'The device gives you one,' is correct.

The devices 'Accuracy of GPS' data field displays the margin of error for the calculated location 'in a 3D reference frame', a reference frame that is based entirely on it's relationship to the satellites being used to make that calculation.

After reviewing this data field on many Garmin GPSr devices, a common theme has become apparent. The GPS Accuracy data field always displays one of the following text options based on the satellite constellations in use:

1. 'GPS' (when only GPS is enabled)
2. 'GPS + GLONASS' (when GPS and GLONASS are enabled)
3. 'GPS + Galileo' (when GPS and Galileo are enabled)
4. 'GNSS' (when Multi-GNSS is enabled)

Can you see the common theme?

They are always indicating the reported position accuracy based on the satellite constellations enabled.

This data field is exactly what it says it is: "Accuracy of GPS".

This data field has no relationship to the accuracy of the maps or map datums being used.

Originally Posted by Bushwalker8

However, the device also transforms that 3D position into the 2D horizontal coordinates of where it thinks it is in your chosen local datum. But it’s often actually somewhere else because the transformation is simplistic, and it does not calculate additional plate movement over time. The known point is actually further away.

The 'GPSr' is not 'somewhere else because the transformation is simplistic'.

The GPSr is exactly where is says it is.

The map data displayed may be incorrect for the current GPSr location due to any number of potential map data errors.

Again, the 'Accuracy of GPS' date field has no relation to the accuracy of the map being used.

The [Only registered and activated users can see links. ] states:

"If you are navigating and comparing the GPS coordinates to a map, chart, or other reference, set the map datum in the GPS unit to the same datum as the map to ensure the most accurate navigation."

The [Only registered and activated users can see links. ] states:

"You should not change the position format or the map datum coordinate system unless you are using a map or chart that specifies a different format."

If we look at a Garmin GPSr that did not provide on-board mapping capabilities, we can see that the [Only registered and activated users can see links. ] states:

"You may select each datum applicable to your region until you find the datum that provides the best positioning at a known point."

Originally Posted by Bushwalker8

So that additional error needs to be added to the device provided accuracy number to give you the total distance to the known point if you want to understand the real accuracy of your position.

Once more, the GPSr reported position accuracy has no relationship to map data errors.

One must be able to understand that these are two different things. The GPSr calculated position in reference to the satellites being used will have some amount of inherent error, as will the maps used to display where that position falls. The GPSr uses the selected 'Map Datum' to position the map data under the devices calculated position as precisely as possible.

Originally Posted by Bushwalker8

So if the question was how does it calculate the accuracy, then one short answer is that it does it insufficiently because it leaves some of the error out.

Only if one assumes this value is supposed to be including map errors in the first place, which I do not believe it does, as I explained above.