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

@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how much the land has dipped, rolled and stretched in that time), or you could let the Garmin device apply a simple and out of date offset that adds error.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…..