Garmin 78S speed logging question

SeanWeijand

New Member
Joined
May 25, 2021
Messages
8
Location
Vancouver
We have a newly acquired Garmin 78S.

When it tracks our progress it catalogs speed in one decimal place, x.x, up to 4.5 knots.

After 4.5 the next increment is 5. After 5, it is 6.

Is there a way to have it datalog to one decimal place all the time?

Here are some examples from the track.

Index Time Leg Distance Leg Time Leg Speed Elevation Leg Course Position
"216" "2021-05-25, 8:15:28 PM" "139 ft" "0:00:22" "4.3 mph" "2 ft" "170° true" "N49° 16.919' W123° 08.635'"
"217" "2021-05-25, 8:15:50 PM" "157 ft" "0:00:24" "4.5 mph" "2 ft" "180° true" "N49° 16.897' W123° 08.629'"
"218" "2021-05-25, 8:16:14 PM" "161 ft" "0:00:25" "4.4 mph" "0 ft" "161° true" "N49° 16.871' W123° 08.629'"
"219" "2021-05-25, 8:16:39 PM" "152 ft" "0:00:23" "5 mph" "0 ft" "156° true" "N49° 16.846' W123° 08.616'"
"220" "2021-05-25, 8:17:02 PM" "163 ft" "0:00:24" "5 mph" "0 ft" "153° true" "N49° 16.823' W123° 08.600'"

Index Time Leg Distance Leg Time Leg Speed Elevation Leg Course Position
"232" "2021-05-25, 8:21:59 PM" "149 ft" "0:00:20" "5 mph" "10 ft" "136° true" "N49° 16.548' W123° 08.268'"
"233" "2021-05-25, 8:22:19 PM" "132 ft" "0:00:16" "6 mph" "13 ft" "144° true" "N49° 16.530' W123° 08.242'"
"234" "2021-05-25, 8:22:35 PM" "124 ft" "0:00:18" "5 mph" "15 ft" "144° true" "N49° 16.513' W123° 08.222'"

Thanks in advance

Sean
 

AndyM

Paddler
Joined
Mar 20, 2016
Messages
18
The GPS unit itself shows decimals (though it reads a little high - it rounds UP after .5)

In Basecamp (or HomePort) I don't see a way to change it.
You can change units from Statute(MPH) to Metric(km/h), but it still rounds after 4.9km/h (then 5,6, etc)

Not your question, but you can export your track, then read it into some other app that does show more detail.
(eg G7toWin shows 3 decimal places)
 

JohnAbercrombie

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Dec 7, 2011
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Location
Victoria, BC
Sean-
Welcome to WestCoastPaddler!
As Andy said, the basic data is there in the gpx file; different programs will compute the speed and display it differently.

I think the GPS receiver only 'records' the time and lat/lon data - everything else is computed from that, whether in the .GPS unit or in a computer later.

GPS Track Editor (for Windows) is a useful piece of freeware for looking at gpx files.
I just checked and it displays speeds over 5 knots to one decimal place
For downloading the gpx track from the GPSMap78 I use EasyGPS. (Or you can just grab the Current.gpx file from the Garmin folder in the 78...)
For displaying your track coloured by speed, you can use GPSVisualizer (online website).
 

JohnAbercrombie

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You can open a .gpx file with any text editor (Notepad, Wordpad, TextEdit, etc.).
I just had a look at the .gpx file from a paddle.
Here's an example of the data from one track point:
<trkpt lat="48.43893536" lon="-123.29253425">
<ele>-21.400</ele>
<time>2021-04-15T17:09:52Z</time>

Looks like lat/lon/elevation/time?

Everything else is computed from those items.
 

eriktheviking

Paddler
Joined
Jul 4, 2009
Messages
203
Location
Prince George, BC
We have a newly acquired Garmin 78S.

When it tracks our progress it catalogs speed in one decimal place, x.x, up to 4.5 knots.

After 4.5 the next increment is 5. After 5, it is 6.

Is there a way to have it datalog to one decimal place all the time?
Getting the additional decimal or even 0.5 knot in the readings are likely only an illusion of accuracy from consumer-level GPS readings. It is one case where I think Garmin is being reasonable- extracting accurate local speeds from GPS readings has fairly large inherent errors. Not so much of an issue to be +/- 1km/hr for a car on the highway but it's a relatively large uncertainty for a paddler or a walker. You can often see this when sitting still and a GPS unit will show a "speed" as the GPS position shifts on successive readings.
 

cougarmeat

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Sep 17, 2012
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847
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Bend OR USA
Once upon a time, long ago, I was taught about the mechanics of significant numbers. The accuracy of a calculation isn’t how far you carry decimal points in the answer, it’s determined the minimum accuracy in the numbers going in. Just because your device shows two decimal places - if it does - that doesn’t necessarily mean it’s accurate to two decimal places. You have to see more details of the numbers going in and the mechanics of the equations. Or - as I do - just add “-ish” to everything. Close enough.
 
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JohnAbercrombie

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There's certainly variation in the position info - thus the 'speed' data on my GPS watch when I'm sitting on the shore eating lunch!

But, the faster I paddle, the less effect those variations will have, I think. To my mind it would make more sense for the Garmin to report the 1/10 knot differences at the higher speeds; the OP reports the opposite with his '78.

Here's a snip of some (Garmin) GPS data from a recent paddle (current-aided in this excerpt from the GPS Track Editor display). No wild variations; it was choppy so my actual speed was probably changing a bit. Still, I always say: My speed was about 'x' knots.... :)

track points.JPG
 

eriktheviking

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Jul 4, 2009
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Prince George, BC
There's certainly variation in the position info - thus the 'speed' data on my GPS watch when I'm sitting on the shore eating lunch!

But, the faster I paddle, the less effect those variations will have, I think. To my mind it would make more sense for the Garmin to report the 1/10 knot differences at the higher speeds; the OP reports the opposite with his '78.

Here's a snip of some (Garmin) GPS data from a recent paddle (current-aided in this excerpt from the GPS Track Editor display). No wild variations; it was choppy so my actual speed was probably changing a bit. Still, I always say: My speed was about 'x' knots.... :)
Oh data... now we're talking. At 5 knots (about 2.6m/s), if you look at the sampling intervals, they vary between 3 and 10 seconds so the distances vary from 5m to 30m (the "leg length" column). If a standard rule of thumb GPS accuracy is around 3m, you can roughly say each leg length is +/- 3 meters, so the short legs are almost dominated by the error and the long ones have an error estimate around 10%. I expect the Garmin software smooths this out and also uses the heading information (best case using an independent heading reading from a magnetometer). I am pretty sure that is why the speed data tends to jump in such discontinuous ways by 0.5 knots or whatever as the algorithm adjusts over an averaging time span longer than many of the intervals.

This came up this past winter for our XC ski racers who were doing socially distanced skiing on the courses. They all have their own phones and watches recording GPS tracks and they were surprised about having different recorded distances from each other at the end (several tens of meters or more). That precipitated a long discussion from coaches about how GPS works etc, which I am sure all the teenagers found fascinating.
 

JohnAbercrombie

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Oh data... now we're talking.
I wasn't very clear in my last post- all that speed (and heading) info is computed by the GPS Track Editor software, not the Garmin GPS unit. The Garmin only saves time, elevation,lat/lon in the gpx file.
So the Garmin (watch) might well have computed different speeds to show me when I looked (in 'real time').
BTW, my Garmin Quatix watch does report speeds in knots to 1/10 knot with no change above/below the 5 knot number.
My recollection is that the GPSMap78 does the same, though there aren't many times I'm moving faster than 5 knots with it running, as I mostly use it on trips. I was surprised to read that Sean's '78 unit switches to integral speed values above 5 knots..
Yes, different software may well be using different averaging & error correction algorithms.
It's amazing how 'spoiled' we have gotten by GPS...for those old enough to remember navigating without it!
 
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JohnAbercrombie

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Joined
Dec 7, 2011
Messages
3,028
Location
Victoria, BC
We have a newly acquired Garmin 78S.

When it tracks our progress it catalogs speed in one decimal place, x.x, up to 4.5 knots.

After 4.5 the next increment is 5. After 5, it is 6.

Is there a way to have it datalog to one decimal place all the time?

Here are some examples from the track.

Index Time Leg Distance Leg Time Leg Speed Elevation Leg Course Position
"216" "2021-05-25, 8:15:28 PM" "139 ft" "0:00:22" "4.3 mph" "2 ft" "170° true" "N49° 16.919' W123° 08.635'"
Sean-
How did you get that data from the 78S?

Does it do the same if you set the Units to 'Nautical'?
 
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