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

**Bushwalker8** · 28th January 2021, 02:33 PM

@babj615

Not correct, it doesn’t work that way. The coordinate systems work very differently.

For example GNSS positions are based on ECEF, 3 dimensional coordinates fixed from the centre of the earth.

You can see in this in an extract from a GPSMAP66i Rinex file header, the calculated ECEF coordinate location is reported in the XYZ line.

2.11 OBSERVATION DATA M (MIXED) RINEX VERSION / TYPE
RINEXWRITER GARMIN 14-JUL-20 04:27 PGM / RUN BY / DATE
Download navigation file using this link: COMMENT
ftp://cddis.gsfc.nasa.gov/gnss/data/daily/2020/196/20p/ COMMENT
BRDC00IGS_R_20201960000_01D_MN.rnx.gz COMMENT
GARMIN MARKER MARKER NAME
OBSERVER / AGENCY
GPSMAP 66i 5.90 REC # / TYPE / VERS
ANT # / TYPE
-4011478.7500 2922211.7500 -3902601.5000 APPROX POSITION XYZ
0.0000 0.0000 0.0000 ANTENNA: DELTA H/E/N
1 1 WAVELENGTH FACT L1/2
4 C1 L1 D1 S1 # / TYPES OF OBSERV
2020 7 14 4 27 42.000000 TIME OF FIRST OBS
END OF HEADER

Note it’s also says “approx” because the intention of having the RIXEX file is to professionally correct it via post processing. And despite the hype 66sr will also only be considered approximate unless it’s corrected.

The ECEF coordinates simply represent a point in space (or within the earth), and don’t have a direct relationship to any place on the surface. They are essentially meaningless to a device user.

So the ECEF coordinates need to get converted to a surface coordinate system and there are many variations.

You can convert them manually but it’s not practical, you can convert them accurately with professional hardware and software that knows how much movement and in what direction (and also the impact of how much the land has dipped, rolled and stretched in that time), or you could let the Garmin device apply a simple and out of date offset that adds error.

Given the accuracy topic, surface coordinates would then normally be transformed to local datum and coordinate system which is locked to the local land surface and relevant to it’s maps. This is intentional and the coordinates stay with the land and move with it and a point on the ground retains it’s coordinates in the local datum. So as the land moves it’s the ECEF coordinates for it that change.

The datum may get updated from time to time and gain a new name but the coordinates for any point in the earlier datum still remain the same.

And the coordinates of maps in the datum remain valid.

Also Data files are available for local datums that detail dynamic plate movement and are used by pro gear and software for detailed transformation on any date or between dates if historical files.
Here are some simple rolled up annual summary parameters that show our updated local datum movements into this year. The formulas and underlying data in detail files can be calculated for any date:
"GDA2020 (20)","GDA2020-20","GRS 1980",0.00000000000000000,0.00000000000000000,0.00000000000000000,0.00000000000000000,0.00000000000000000,0.00000000000000000,0.00000000000000000
"GDA2020 (21)","GDA2020-21","GRS 1980",0.00000000000000000,0.00000000000000000,0.00000000000000000,0.00000000729057966,0.00000000573757599,0.00000000585247683,0.00000000000000000

And regarding the accuracy number it is clearly misleading if you are trying to return to somewhere on the surface or that is represented on a map and the place has since moved, the error is now greater. Strangely enough most people actually use the device as it was intended to relate to surface positions and maps…..

**babj615** · 28th January 2021, 11:14 PM

Originally Posted by Bushwalker8

Given the accuracy topic, surface coordinates would then normally be transformed to local datum and coordinate system which is locked to the local land surface and relevant to it’s maps. This is intentional and the coordinates stay with the land and move with it and a point on the ground retains it’s coordinates in the local datum.

How is that even possible?

Latitude is always parallel to the equator and perpindicular to longitude, which always extends from one pole to the other.

When California experiences 'The Big One' and Los Angeles slips several miles west into the Pacific ocean, you are suggesting that a waypoint I captured pre-event will somehow still retain the same coordinates after the event?

Ridiculous!

**Bushwalker8** · 29th January 2021, 02:28 AM

Originally Posted by babj615

Ridiculous!

Only ridiculous if you are assuming accuracy and you or the device cannot manage it.

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.

So forget all the nonsense about accuracy, you won’t get it. Just use it and enjoy whatever it is you are doing they are great devices to support almost any activity.

**babj615** · 29th January 2021, 04:59 PM

You stated:

Originally Posted by Bushwalker8

Given the accuracy topic, surface coordinates would then normally be transformed to local datum and coordinate system which is locked to the local land surface and relevant to it’s maps. This is intentional and the coordinates stay with the land and move with it and a point on the ground retains it’s coordinates in the local datum.

Then I asked:

Originally Posted by babj615

How is that even possible?

Latitude is always parallel to the equator and perpendicular to longitude, which always extends from one pole to the other.

When California experiences 'The Big One' and Los Angeles slips several miles west into the Pacific ocean, you are suggesting that a waypoint I captured pre-event will somehow still retain the same coordinates after the event?

Ridiculous!

And your reply does not answer the question:

Originally Posted by Bushwalker8

Only ridiculous if you are assuming accuracy and you or the device cannot manage it.

I still maintain that:

Originally Posted by Bushwalker8

the coordinates stay with the land and move with it and a point on the ground retains it’s coordinates in the local datum.

...is not possible.

The distance between Phoenix and Los Angeles is a given value today, but after a large seismic event ('the Big One') where the San Andreas fault widens and pushes most of the California coast out into the Pacific ocean, the 'new' distance between Los Angeles and Phoenix will be much greater.

Any Latitude and Longitude reading taken at a landmark in Los Angeles prior to the seismic event will no longer relate to the same landmark after the event.

The coordinates do not change with the land movement.

Originally Posted by Bushwalker8

Lat Lon is a projection, you don’t say what datum you are using.

Does not matter which projection or datum you choose. They are all man-made imaginary grids consisting of uniformly spaced parallel and perpendicular segments. The distance and bearing between any two locations in any projection/datum chosen will always be the same.

In the example I provided, at any point in time before the seismic event, the distance and bearing between Phoenix and Los Angeles had a static value, as would the distance and bearing between Phoenix and Dallas.

However, after the seismic event, the distance and bearing values between Phoenix and Los Angeles will have changed dramatically, while the distance and bearing between Phoenix and Dallas will have remained essentially unchanged.

There is no projection or datum that can account for this change in the Earths surface while maintaining identical before and after coordinates for all locations.

A GNSS user visitng landmark waypoints in each city that were saved prior to the seismic event would find the Dallas and Phoenix landmarks still located at the recorded coordinates, while the Los Angeles waypoint coordinates would no longer be anywhere near the landmark they originally referenced, but they would still be in the same location on Earth as when they were originally marked and saved.

Thus:

Originally Posted by Bushwalker8

the coordinates stay with the land and move with it and a point on the ground retains it’s coordinates in the local datum.

Is not physically possible.