Trip distance: Furuno vs. Garmin & others

Ranger

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Recent trip from A to B. New TZT16F reports trip distance 40.01 NM. Older Garmin 7612xsv reports 42.4 NM. Both using new FA-70 as GPS source.

?????

Return trip from B to A. TZT16F reports 39.75 NM trip distance. 7612xsv reports 42.0 NM. Both still using FA-70 as source. Also, as we drifted away from the dock, stowing fenders and lines and so forth, the TZT16F trip log showed 2 yds for a long time, even as the 7612xsv incremented up to around 300’.

This time I also recorded the track on other devices/apps.
- TimeZero on a laptop (below, in cabin spaces) with BU-353S4 GPS puck via USB reports 42.05 NM.
- AquaMap on a newish Samsung tablet with internal GPS reports 42 NM.
- Plan2Nav (an older C-Map app) on an older Samsung tablet with internal GPS reports 42.8 NM.

Given the cluster at approx 42 NM, I’d guess the other devices are closer to ground truth… and the TZT16F is approx 2.3 NM off over the relatively short track.

And both Garmin and Furuno MFDs (TZT16F is still software v02:05:02.05:02.05) are using the same GPS input source (FA-70).

I see an old 2019 TZT2 thread here reporting similar: Furuno USA Community • View topic - Trip Counter Accuracy (furunousaforum.com)

So… whassup with all that? Actual error in software or execution? Known for a long time, not fixed yet? Explanation? Or…?

-Chris
 
Hmmm... no comment, yet...

I thought maybe useful to also include a track distance comparison between our previous NN3D MFD and TZ. These are northbound vs southbound tracks along the AICW, just a mixture of hand steering and "go straight" autopilot, i.e., not following a pre-programmed route:

Alligator River to Coinjock, 20180329, NN3D, 31.10 NM
Coinjock to Alligator River, 20191110, TZ, 31.26 NM

Portsmouth to Edgewater, 20180403, NN3D, 131.7 NM
Edgewater to Portsmouth, 20191105, TZ, 131.9 NM

Wrightsville Beach to Swansboro, 20180325, NN3D, 47.10 NM
Swansboro to Wrightsville Beach, 20191115, TZ, 46.70 NM

I think this exhibits a high degree of correlation between that MFD and our version of TZ. (But see Edit, below.)

Unlike the not-so-close TZT16F/TZ comparison above. TZ track distances seem to correlate well with the Garmin and with the AquaMap app, and its also much closer to the Plan2Nav report than it is to the TZT16F distance.

So maybe the TZT16F computation method is different from the earlier NN3D? And different from TZ?

Edit: I'm not actually sure this comparison is legit. The NN3D tracks were exported from the MFD, imported into TZ, and I don't know where the distance calculation happens. If the export file contains distances as computed by the NN3d, that's one thing. OTOH if the export file does NOT contain that info, and distances displayed within TZ were actually calculated by TZ... then that's another thing. And in the latter case, of course there might be a high degree of correlation between the two.

Anyway... how to tell which is correct? Or at least "more correct?"

-Chris
 
Our parent company is on national vacation this week. The post is not being ignored but we need their comment. Please be patient as we attempt to get you an answer.
 
Great, thanks.

It also occurs to me if useful I could likely capture 6 simultaneous distance versions of a single track: one from the TZT16F, plus the Garmin, TimeZero, and three different tablet/phone apps.

Likely not necessary, but at least possible...

The very obvious difference in early distance measurement while we were drifting away from the dock -- something like 300' on the Garmin (might have been more, but that tracked with my Mk I Eyeballs, too) versus only 2 yds on the TZT16F -- sorta stands out as weird, too.

Cheers, -Chris
 
The parent company got back with me. Basically, this is very complex because there are so many variables. One example is that most sailboats wish to use speed thru water when many power boats wish to have speed over ground as a measurement. This can lead to devices made primary for that particular market not matching other units that focuses on a different market. The current algorithm used in TZT2/TZT3 is the same one since the original NavNet series one (~2002). Based on feedback we provided in 2019, they have have been considering how they wish to approach any changes to that. They conducted some recent testing based on our current discussions and they now plan to make changes in the next NavNet series. While it is understood these units are not a scientific measuring device nor its primary purpose, they have heard the feedback. They did note that while units like the NavNet tend to underestimate distance, units like TZpro and cellphones/tablets tend to overestimate distance because of GPS dithering. You will find some clicking away short distances just sitting at the dock due to sensitivity. The best recommendation short of using very special purpose devices would be to take an average of several devices and then average them.
 
Johnny Electron":3lhnyso5 said:
They conducted some recent testing based on our current discussions and they now plan to make changes in the next NavNet series.

The best recommendation short of using very special purpose devices would be to take an average of several devices and then average them.


Thanks for researching that. I hadn't known STW/SOG entered into the GPS distance calculations; would have thought distance between GPS point A and GPS point B (et cetera) was independent of speed... but in any case, the whole process has been sort-of "black box" to me.

Anyway, I've been mostly interested in the seeming conundrum, rather than in knowing perfectly accurate distances, so the "averaging" recommendation is useful enough for our purposes. Even if the initial view is disconcerting when 3-4 other devices -- one of which is TimeZero -- cluster around a given number and the Furuno TZT3 deviates more and more as distances increase.

Changes in the next NavNet series? TZTouch4 or similar? Or a programming update in the next TZT3 software release? (I haven't even updated to 3.01 yet... on my to-do list...)

-Chris
 
Next series (like TZT4 or whatever it ends up being called). It is quite a bit away. They have no immediate changes to trip distance planned with TZT3.
 
Got it.

In the meantime, when colleagues and acquaintances ask me for plotter recommendations, I'll be able to say "Furuno... unless the trip distance calculation is important."

-Chris
 
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