Garmin ForeRunner 305 accuracy

-- Last Updated: Jan-16-09 10:39 AM EST --

Just curious if others who own and use this unit on a regular basis can comment on its accuracy for paddling? And specifically on its "edge" readings, meaning would you trust the max speed spikes for instance? Also, what do you think about its "smoothing" feature in the Garmin Training center?

On my first use of the 305 in a boat yesterday I thought it was very accurate, but I also thought that the speed displayed lags behind the actual speed, either accelerating or decelerating while kayaking. In the car it is actually way off even on mild acceleration/braking and can never display the correct speed unless I'm on cruise control for a few seconds for it to catch-up.

On the other hand, when looking at the kayaking log it seems to have very reasonable readings - the spikes (speed or heart rate) seem to reach appropriate max or low and there are no errand readings (neither too high nor too low to be unreasonable).

So I think the unit has internal "smoothing" which prevents it from properly displaying quick speed changes but that it still captures the right data internally, and that can be revealed by tirning off the smoothing feature in the Training Center software. Or am I wrong? Also, is there an option to fine-tune the smooting strength (did not see it in the manual/software)?


I can hardly operate my Etrex Vista HCX
so I think it will be a stretch for me to determine if it is in fact “accurate”. The numbers are very pretty though and I like the way they move up and down.

The elevation is worthless
as it insisted that I paddled in some 30+ feet waves on the Potomac river -:wink: But I think the speed and heart rate were accurate (I checked the heart rate a few times at home just out of curiosity as I thought it was too high but it was correct).

The speed seems acurate as well overall and seems to do well at slow speeds (though very bad at high speed), unlike my car GPS that fluctuates too much at slow sopeeds but is dead-on at high speeds…

GPS is also pretty accurate - I used it to create a trace of my morning commute and it showed my illegal 3-lane change on the last exit!

Check the battery setting
I’m not sure if it is the case or not, but most garmins have a battery setting that either runs continuously, or will basically stop recieving a signal and turn on every 5 or 10 seconds (depends on the model, check your manual) if it was in battery save mode it will always lag behind as it is comparing your position now from a position it had 10 seconds ago rather than tracking you at all times.

Thanks for the tip
I got to try that setting next time - the 305 has it. The manual says it can do each second and that is more accurate but can only hold up to 3.5 hours worth of data.

Keep it in perspective too
Most handheld gps recievers say they are accurate to 5 meters, but that is under “ideal” circumstances, which is perfectly placed sattelites and coverage, most handhelds, even if showing an accuracy of 5 meters are really only good to about 10-15 meters, so where speed is distance/time if your distance is only good to even 10m, when you’re paddling, that can be quite a difference.

Good point . And some more "math"
Let’s say I paddle for 10 meters with speed of 4.5mph (2m/s = 7200m/h = 7.2km/h). And that 80% of my measurements have an error of less than 2 meters each (seems typical accuracy in good reception areas from eyeballing my GPS traces from driving to work when overlaid on roads in Google Earth).

Each point may be off by as much as 100% error (2 meters), so my speed may show as 0 miles per hour or 9 miles per hour at any given point. Not good and I should not rely on individual measurements.

The average acuracy is much better though. Taking a measurement each second, I will have about 10 measurements for the 10 seconds paddled. Therefore we can say that my maximum average error would be no worse than 0.2 meters. At speed of 4.5 miles per hour (2 meters per second) would equate to a less than 10% error max, on the average, at any time. Not bad at all for a 10 second interval.

Similarly, if the above reasoning is right, then I can probably rely on catching my maximum speed for any 5 second run (e.g. surfing a wave) to within 20% of the true speed with 80% probability.

If we assume the errors are randomly and “evenly” distributed to the + ot - side (e.g. it will think that I’m left from where I am about as many times as it thinks I’m to the right of where I actually am), graph them and then “smooth” or approximate the get an average speed that is pretty darn close to the actual even for a short measurement interval such as 10 seconds.

The average speed over a longer period of time, say 1 hour (3,600 seconds) will be equal to the actual speed for any practical purposes. That works for me.

You said it!
Lol, getting a little more into it that i would have wanted to, try to stay away from probabilities and such

Got t’be careful wt you say 'round here
People will critically read every post here and jump at any discrepancy :wink:

So staying away from probabilities is probably a good idea LOL. Seriously though, most of the times when I post something here it is as much for reaching a conclusion for myself when I’m not clear as for anything else…