Thursday, July 22, 2010

Garmin 405 accuracy.

GPS Accuracy

I've read much about how the Garmin 405 has a highly sensitive antenna, and is great even in heavy tree canopy or tall buildings. I'm mostly impressed with it in my limited sample size of runs so far, but today's run left a little bit to be desired.

If you really care about route data and you will be running in a city, zoom in the above and pay attention to the first and last kilometer of the run. Those do not match reality. For both the out and back my route took me up Pender, to Burrard and down Cordova. At no point did I do crazy Parkour over skyscrapers and on the living roof of the new convention center, as the map would suggest.

That said, the total distance is close to being correct, and everything after I hit the seawall is close enough to make me happy.  I still think it is more accurate than the Nike+iPod gadget overall (OK, maybe not for *this* run). I believe the problem is tall buildings. Around the lake is under trees, and that seems fine. But, from my work to the seawall is all in skyscrapers. Also, my habitual route keeps me on the South side of the street when heading west, and the east side of the street when heading north. As most of the GPS satellites are in a southernly positon in the sky from up here in the Great White North, that puts me in just the wrong position to get a direct signal. My next time out I plan to keep as clear of a line of sight to the southern hemisphere as I can and see if my accuracy improves.

Foot pod?
Another option is getting the Garmin accelerator foot pod to compliment the GPS data. For years while running with the Nike+iPod accelerometer I've had this recurring thought: an accelerometer foot pod, coupled with a GPS (and maybe some fuzzy logic algorithms) could provide almost perfect distance and pace data. I found that the Nike+iPod system is very accurate if two conditions are met:

 1) You have calibrated your foot pod on a track
 2) You run a very consistent pace

The Garmin I find very accurate unless you are in tall buildings, like the run above. But, if you could properly pair the two systems you could make it almost perfect. When you have a clear line to the satellites you could be constantly calibrating the accuracy of the accelerometer. When you lose sight of the satellites you have a recently and perfectly calibrated foot pod to cover the gaps. You would know your pace before the GPS went dark, and just after. You would have historical data for the different paces you run at, and how that matches the data coming off the foot pod. You could reconstruct the missing GPS data  almost perfectly (at least in terms of pace and distance). Plotting this on a map, you could have the route a different color when you think the GPS signal is not good enough to indicate that you probably didn't run that exact path, but pace and feedback distance would be correct on the watch and in the online data.

Is that how it works?
I don't know if this is how the Garmin 405 works when you pair it with their foot pod, but I'm going to bet it doesn't. There is a big technical hurdle that I don't think Garmin could overcome, related to processing power on the watch. If you look at the above map, I don't think I ever actually lost the GPS satellites. What happened (I assume) is that I was in the satellite shadow of a big building, and the watch was picking up the reflection of the signals off of a building across the street. How would the watch know if it was getting bad data compared to good data? Well, it would have to look at pace and location data and know how to do the right thing. An aggressive algorithm could mess things up, smoothing out speed work laps as errors and such. I think it would be tricky to get right, and might be more than a watch can handle (Or maybe not. I think we landed people on the Moon with significantly less power than this watch has).

In absence of that knowledge, I'm hesitant to fork out the cash for a footpod just yet. Sure, I'll get one at some point, if only so I can gather distance info if I'm running on a treadmill in the Winter. But I'm not going to get too excited about it. All the manual says about it is:
Your Forerunner is compatible with the foot pod. You can use the foot pod to send data for your Forerunner when training indoors or when your GPS signal is weak or you lose satellite signals.
As a technical writer, I appreciate the minimalism.  This covers the basics. I'm sure it is true. It leaves enough ambiguity around "weak signal" to allow developers to totally change how it works without reprinting the manual. Perfect. The ambiguity isn't going to keep me from buying the product and there is plenty there to satisfy the incurious. That is what I would have done as a tech writer. As a geeky consumer that blogs about running data, I want to know more. How would the foot pod have changed my run data on the run above?

At some point I will get the foot pod. When I do, I should have several, maybe dozens, of runs on this exact route logged. That should give me plenty of information to see if the foot pod improves accuracy fot this type of run.

Why not?

So here is my proposal. Garmin, why don't you send me a foot pod for testing? I'll give it a glowing review and suggest everybody buy one (assuming it improves accuracy). Heck, I love the 405 so much I think everyone should get one anyway. Buy a foot pod too in case you are on a treadmill. If it improves accuracy in the run above, I'll shout it from the rooftops.

So what do you think? Thanks Garmin. I'll be watching my mail. Also, I'm sure the accuracy would be improved significantly if you threw in an extra ANT+ USB stick. It couldn't hurt, right?


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