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STARWATCHERDE
09-20-2009, 04:54 AM
looking into recorded pace data of both 305 and 310xt devices I recognize that, even running constant pace, measured data are varying awful. The graph looks completely jagged. As "Ascent" (my Mac Software) smoothes the graph it looks "fine" on the first view. Now with 310 the data are even more exaggerated.
This leads me to two questions:

1. am I the only one observing this strange data recording behavior?

2. how does the pace calculation work, what methodology is used, in what intervals are the measures taken, does the watch internally calculate with more data points than shown later after an export ?

During the run the displayed average pace looks ok, but the single data are so weired... which fits to another observation, namely the current pace, which also looks more way off than reflecting true pace (GPS measured, with footpod the current speed seems perfect!) - the reason why our complete group (wearing mainly 305s) do not look at current pace.

Thanks in advance - Klaus

R.MCLACHLAN
09-20-2009, 07:43 AM
The average Pace essentially smoothes the data to give a more accurate and even pace. The individual data points can be plus or minus at minnimum 5-7 metres so any pace between individual points will appear very erratic, it is only when averaged over a longer distance that the true pace will be revealed. You really need to use a properly calibrated footpod to get more accurate instantaneous pace readings.

EKUTTER
09-21-2009, 10:51 AM
Also note that if you are using a foot pod, you can set the "speed" mode to either GPS or footpod. That is the speed that gets displayed on the unit for current speed and gets recorded as "speed" in the TCX file. The distance (and lap speed and avg speed) is always based on GPS if there is a GPS signal. GTC will graph the speed based on what your "speed" source was set to. Most other programs (Training Peaks, Sport Tracks) will calculate the speed from the distance (ie GPS). The footpod speed is generally fairly smooth (but inaccurate over time), the GPS speed quite jagged (but generally accurate when averaged over time).

The footpod speed accuracy is based on the footpod calibration and I have found the that the real calibration at the beginning of a run when I am fresh can be 5% different to the end of a run when I am fatigued. It is also different from day to day based on a whole bunch of stuff like temperature and fatigue. Don't assume you can calibrate it and forget about it.

It would be nice if Garmin added an "auto calibration" mode so that it would always be adjusting the calibration throughout a work out when it has good GPS signal.

STARWATCHERDE
09-21-2009, 04:17 PM
thanks, folks!
I bought the footpood for my treadmill training (on rainy days). But it looks like being worth a try to use it also outdoors for current pace.

Where I am really struggling with is the interpretation of pace data within Ascent (Mac Software). Looks like there is no pace smoothing at all - but this could be a software issue.

philipshambrook
09-21-2009, 06:31 PM
It would be nice if Garmin added an "auto calibration" mode so that it would always be adjusting the calibration throughout a work out when it has good GPS signal.

That's a worthy feature request. :D

R.MCLACHLAN
09-23-2009, 05:44 PM
Footpod accuracy is very speed dependant. However if calibrated properly across a range of constant speeds it can be very accurate given that you set the correct calibration factor for the specific speed you intend to run. i.e. the calibration factor for 10km/hr is different from 16km/hr. The default calibration factor is a compromise giving approx 95% accuracy.

There is an Excel spreadseet tool that allows you to calculate the speed based factors - It can be found on the "wristtop trainers" group on Yahoo. Look for "T6 swiss army knife". It was written for the Suunto T6, but the principles remain the same.

I agree constant auto calibration would be good if it was possible to implement it.

EKUTTER
09-23-2009, 06:31 PM
Footpod accuracy is very speed dependant. However if calibrated properly across a range of constant speeds it can be very accurate given that you set the correct calibration factor for the specific speed you intend to run. i.e. the calibration factor for 10km/hr is different from 16km/hr. The default calibration factor is a compromise giving approx 95% accuracy.

There is an Excel spreadseet tool that allows you to calculate the speed based factors - It can be found on the "wristtop trainers" group on Yahoo. Look for "T6 swiss army knife". It was written for the Suunto T6, but the principles remain the same.

I agree constant auto calibration would be good if it was possible to implement it.
Having to set the calibration based on your planned speed kind of defeats the main purpose of calibrating it. Especially since the calibration is not just based on speed, but based on temperature and effected by fatigue. This is one of the biggest reasons I gave up my more comfortable Polar RS800 in favor of the 310. I would calibrate my Polar (same tech as the Garmin footpod) over the first couple miles of my run (I knew the exact distance and could adjust the calibration on the fly) but on long runs, the last couple miles would be off by 3 to 5% (again, known distance). The GPS based distance with the 310 is consistent for the duration with just the occasional spike due to RF interference. The only reason I use my footpod now is for the cadence.

Basically, I found you need to be able to calibrate against a known distance on EVERY run to have any confidence in the numbers better than 3%. Even then, the accuracy can degrade over the duration of a run.

R.MCLACHLAN
09-24-2009, 05:56 PM
With the 310xt the main advantage of using the footpod, other than for static treadmill type running, is that instantaneous pace is much more accurate and cadence info is available. The accuracy out of the box is fairly good but can be improved by careful calibration. None of these gadgets is error free - like yourself I use the footpod as a tool within known limitations.

BTW I am interested in how temperature affects the footpod. Fatigue I can understand due to changes in cadence & gait when tired.

EKUTTER
09-24-2009, 06:20 PM
My experience is that the calibration changes from summer to winter as the temperatures fluctuate. The accelerometers could easily be impacted by temp, similar to strain gauges on bicycle power meters (which is a known issue). I don't have any hard evidence for this though.

Another big factor for the calibration is shoe attachment. Run with your laces fairly lose vs tight where they hold the foot pod in place and you will likely notice a sizable difference. This only really becomes an issue if you are moving the foot pod between shoes.

R.MCLACHLAN
09-26-2009, 05:31 AM
I have noticed problems if the pod is not attached fairly securely. It needs to be quite tighly attached to avoid too much movement on the laces as that seems to affect accuracy. Simlarly shifting between shoes requires recalibration because as you point out the position will be different.

fabriceboutique
10-06-2012, 07:22 AM
I do not understant the non accuracy of the pace, i ran the marathon of berlin and i was looking the average speed all the time, i wanted to run 3'49 a km...at the end my garmin 310xt show me 3'46 average but my official time was 2h43...with 3'46 you should do less than 2h40.
more than 3 min error, my official pace was 3'52....how to explain this...if it does not work , my watch is useless thanks

RABYDN
10-21-2012, 08:40 PM
I do not understant the non accuracy of the pace, i ran the marathon of berlin and i was looking the average speed all the time, i wanted to run 3'49 a km...at the end my garmin 310xt show me 3'46 average but my official time was 2h43...with 3'46 you should do less than 2h40.
more than 3 min error, my official pace was 3'52....how to explain this...if it does not work , my watch is useless thanks

given the fact that it was a long-distance run.. what if the "start" and "finish" points were different (i.e., you started your 310 at a different point than the one taken into consideration by the official stopwatch). Or what if you have had to struggle with the pack in the beggining of the race.

also, the Berlin Marathon may have provided 1km waypoints. Did the 310 "match" those waypoints? You can set your 310 to return 1km "laps". Were those laps consistent to the actual landmarks?

the one thing you can do to check accuracy of your 310 (in GPS form) is to go running around a track that you know for sure it is 1km long (or that has 1km lap points) and then see if your device is returning consistent data. Mine has been always consistent.

ANDE22
11-10-2012, 12:47 AM
The person information factors can be plus or less at minimum 5-7 meters so any speed between personal factors will appear very intermittent, it is only when averaged over a more time range that the real speed will be exposed. You really need to use a effectively adjusted foot pod to get more precise immediate speed parts.