GPSrChive Discussion Forum

*** Moved Here From Another Post That Had Gone Off Topic ***

Just now, I was able to export a track as GPX on my 66sr. However, the resulting data has much lower apparent accuracy. For example, Lat/Lon in the GPX from BaseCamp have 15 digits to the right of the decimal point; 66sr export has 10. Elevation from BaseCamp has 15 digits, and 66sr export has 2. At least the additional elevation precision from BaseCamp is probably meaningless, though; all of the values are essentially zero after the hundredths place, +/- some noise, so I think this is some kind of conversion error. I would be happy to post one or more of these GPXes from the unit, or the original FIT files, in case anyone is curious, but I really doubt it makes any meaningful difference for the comparison I was trying to do.

The GPX data file that is exported from a Garmin Fit data file in Garmin Connect is identical to the GPX data file saved on the 67 in addition to the FIT file.

Matt16598 wrote: ↑Wed May 31, 2023 12:02 am Just now, I was able to export a track as GPX on my 66sr. However, the resulting data has much lower apparent accuracy. For example, Lat/Lon in the GPX from BaseCamp have 15 digits to the right of the decimal point; 66sr export has 10. Elevation from BaseCamp has 15 digits, and 66sr export has 2.

Since BaseCamp did not record the actual track data, and the GPSMAP 66sr did, how can BaseCamp be more precise? It can not. This is why I always like to have the raw GPX files copied directly from the device, untouched by any software, when performing any meaningful testing.

I am not suggesting that it always makes a difference, or even frequently makes a difference, but allowing any software to touch/manipulate the raw data always dilutes any test results provided.

I always keep an 'untouched' archive of all important device data that is copied manually from each device to my PC. These files can be loaded over and over into any number of third party software for future inspection and testing without ever changing a single byte of information in the source material.

MarkHL wrote: ↑Wed May 31, 2023 12:42 am The GPX data file that is exported from a Garmin Fit data file in Garmin Connect is identical to the GPX data file saved on the 67 in addition to the FIT file.

They most certainly are not.

Before diving into the weeds, does the claim that the tracks were edited mean that translating from .fit to .gpx is "editing"? If so, I think that's nonsense.

Check my arithmetic. I believe latitude of nine digits of precision and longitude of nine digits of precision will specify an Earth-surface position with error of less than 2mm. In other words, the distance between H###.######,H###.#####0° and H###.######,H###.#####1° is always less than 2mm. At the poles, that distance would be zero. Please check my math. I'm not an expert. If I'm right, that's way beyond the accuracy of the devices in question. And I think both the .fit and .gpx formats are each capable of storing more than nine decimal digits. So in terms of error, the format doesn't matter.

As for the difference, the .gpx format is a text format. You can open a .gpx file and read it with a text editor like Notepad or Word. A .fit file is compressed into binary format. A text editor would display it as gibberish. So the same information needs more storage to be stored in .gpx format. I would hope that any translation algorithm would be invertible after the first pass. If it weren't then translating back and forth between .fit and .gpx several times might lose precision, but I don't see how even that hypothetical loss would be relevant here.

Some Garmin documentation says positions stored in .fit are slightly more precise than the same position stored in .gpx. I've asked around some but never gotten a coherent answer to exactly what that means or how that's even possible.

What is unfortunate is that the Garmin GPX does not display the type of GPS in the data, it would be nice to do so.
My GPS Cross Twonav displays it, as well as its serial number.

mimichris wrote: ↑Wed May 31, 2023 7:05 am What is unfortunate is that the Garmin GPX does not display the type of GPS in the data, it would be nice to do so.

I cannot agree with this sentence, the gpx file contains information about the software in which the file was created.
Check in your gpx files.

Przekątny wrote: ↑Wed May 31, 2023 10:23 am

mimichris wrote: ↑Wed May 31, 2023 7:05 am What is unfortunate is that the Garmin GPX does not display the type of GPS in the data, it would be nice to do so.

I cannot agree with this sentence, the gpx file contains information about the software in which the file was created.
Check in your gpx files.

I expressed myself badly, I am not talking about the software but about the type of GPS, for example the GPSMAP67 and its serial number. I did not find in the original Garmin files in GPX.

mimichris wrote: ↑Wed May 31, 2023 10:54 am I expressed myself badly, I am not talking about the software but about the type of GPS, for example the GPSMAP67 and its serial number. I did not find in the original Garmin files in GPX.

Maybe I expressed myself wrong, the device has software that saves the device name in the file, Garmine device number is not saved in the gpx file, only the device name is saved.

The FIT file format is much better in that regard, it includes info on the device that captured the data, see screenshot.
A GPX file recorded on the GPSMAP 67 only includes the name of the device (GPSMAP 67), and that in a field that is not really meant identifying detailed info on the device being used.

FIT file device info

Schermafbeelding 2023-05-31 125835.png