50s, clear, dry
mind/body - still feeling solid
easy effort
I've been walking Jaxon to school in the morning so it's been an easy spring board to get the run in from there. The chill is in the air, as was apparent yesterday at 14K, but it sparks some sort of motivation for me. A mere 7:03 pace today with 1000 ft of climbing and felt great doing it. I hope to hit somewhere between 90 and 100 for the week (290 for August and 2122 for the year) and start a slight taper from there for The Bear. On Thursday Tim Long and I are going to do 41 on the Colorado Trail starting at 6 AM at Indian Creek TH. My wife will pick us up at Wellington lake road. Come along and get some good myles in for a fall race.
On another note, I've been toying around with my watch and yesterday as I sat on the top of Mt Bierstadt with Tim (and his Timex GPS watch) we talked about GPS vs. barometric elevation. I had thought barometric was more accurate but didn't know why... I emailed Rick Merriman this morning and here is his explanation that makes total sense:
GPS is always the most accurate if it's getting a good signal. The reason is because the signal is simply sending stored data to the watch. The watch isn't calculating anything it's just receiving data that has been stored in the satellite sending it.
Your barometric pressure watch takes a pressure reading, then calculates what the altitude will be. So, since pressure changes in a spot, then the altitude reading will change slightly as well. These changes are very little though. The pressure on top of Mt. Bierstadt will be different today than it was yesterday so it's possible that if you went up again your altimeter would say something different than 13,923. However, it will be less than 100' difference way up at 14,000'. Barometer watches offer some important things that GPS watches cannot. 1) They don't need a signal, barometric pressure is everywhere so the watch can always calculate. 2) Battery power on GPS is always an issue, just about 8 hours, while the barometer watch uses a basic watch battery lasting about 2 years. So power and signal are pretty important and GPS watches can have issues with these 2 things.
So my take on this is that barometric is better for trail runners who are running in the trees for a long time. If you've got a clear view of the sky for a shorter run then GPS may be the way to go. For me, barometric make more sense.
7 comments:
Alright, I couldn't resist.
Tim's explanation is partially correct regarding consumer grade GPS. The stored data the satellite is sending to the watch is based on a model of the earth called a GEOID. The GEOID model was created using microwave remote sensing satellites that vary in accuracy from 10 to 30 meters depending on the wavelength the satellite is sending(10m in the U.S.).
So combine the inaccuracy of the model with number of satellites your GPS can or can't see and you have a given error. The more relief in the terrain, the greater the error over a given distance. Software programs will also use a smoothing algorithm to eliminate gross errors in the vertical.
As Tim said pressure is always around and the altimeter uses very little battery power.
For a better explanation:
Layman's terms: http://gpsinformation.net/main/altitude.htm
A little more technical: http://www.ngs.noaa.gov/PUBS_LIB/gislis96.html
I knew you'd chime in Christian
So just to be clear, for the purpose of long distance trail runners wouldn't a barometric watch ultimately be more accurate and user friendly (ie real time data)? If you designed a watch for trail running what would it include?
I can't resist either!
The answers are not quite right. First off, the receiver (watch) does actually have to calculate the position (both height and latitude/longitude). It does this by what is in a sense triangulation using range values to multiple satellites - although you need 4 because you also need to solve for the time which is essential for GPS to work. There are lots of ways to do this and you can never find out how each company does it since they use proprietary algorithms. Christian is right that the height is relative to an Earth model so there is some error. Also, the height is always the most poorly determined since you only have information from one side as opposed to lat/long where you have information from both sides. Then there is more error in the vertical due to atmospheric water vapor and charged particles (ionosphere) that can be another 20 meters or so.
You ask about the best. The ideal setup would be GPS and altimeter. If you did it right you could leverage both and get better height readings from the GPS and in-turn use the GPS to correct the barometric altimeter.
You are lucky I am doing Wasatch, otherwise you would get to hear about this for 7 hours on Thursday!
It would take 7 hours to explain some of that...
In Rick's defense, for the sake of brevity I'm sure he made some generalizations there. But the fact remains, High Gear wants to make a watch most suitable for the trail runner and these kinds of comments will give them the ammo they need to invest the time and money.
Good points Nick.
Now for the ideal watch. Like Nick I would include both GPS and an altimeter. But, given the current technology I would go with the altimeter only option. The sensors are smaller, cheaper, provide acceptable accuracy (especially for real-time), and take less power than GPS so it's more likely to last throughout your 100 mile effort.
I've been using an older Suunto T6 and I really like it. Without any extra accessories I get all the normal stopwatch/timer options, plus the altimeter. I can add the HR belt, foot pod accelerometer for distance, GPS for distance (not sure how they use this for vert) and bike accessories. Each one of the pieces is powered by its own battery so the watch doesn't take the brunt of powering everything and the size is acceptable. There are definitely some changes I would make to the operation and options available, but for the most part it does what I want.
I've used my fair share of watches and GPS and I'm still searching for the Holy Grail...
Yea, I did generalize a bit, but I'm certainly not an expert on GPS technology. I appreciate the more tecnical version of how the GPS works. What I try to keep in mind with the difference between GPS and barometric pressure is that, I believe, a GPS watch has a "receiver" and a barometer watch has a "sensor." What most important though, is both technologies have their strenghts and weaknesses and understanding those is what will allow a person to decide which one is best for them. Bottom line is that neither watch recorded the same altitude as what's marked for Mt. Bierstadt.
Scott: Interesting info on barmetric and GPS watches. I wish a better battery could be made. My GPS turned off 11 hours into Leadville. Unacceptable that a watch can't be made that can go 24+ hours. Maybe there is such a watch?
Good luck at the Bear. Sounds like your training is there!
I hope I can meet you for another run one of these days...maybe a Saturday outing. If my foot holds, I may do Pikes on Sunday.
Wyatt
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