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Some GPS data from the bunny hill

post #1 of 23
Thread Starter 
People who say that unless you have GPS or radar you can't really tell how fast you're going are correct in my case. The report that said people UNDERESTIMATED their speed is also correct in my case. This possibly explains the discrepancey between mine and Warpdaddy's experience in my earlier evaluation of the SX-10; where I had thought it was falling apart in the stability and control department was quite likely 15 mph to 20 mph faster than I had estimated my speed at.

I went skiing today for the first time this year, the first time with my Fischer WC SCs and the first time with my GARMIN GPS 76CS. It's just a little local hill with about (edit: oops not 250) 200 feet of vertical and none of it steep, but at least I got out skiing for 2 hours.

I had a really good time. I'm not doing any work today! Man I needed that R&R. Having just read the first day accident thread I was careful and didn't push my luck. I noticed how much I sucked at the skiing on my first day. No I didn't fall, didn't catch an edge, only almost fell once at zero miles an hour when I had to admit that I would either have to turn around or skate up that last 5 yards to the next trail; I had GP wax, not high temp. wax. (don't you love skiing uphill on connecting links ). Anyway, I digress.

First I turned on my GPS at the top of the first lift, put it in my pocket and forgot about it. I had estimated my probable max speed on this hill as maybe 45 klicks or 50 klicks at the max. (I would have said 25 to 30 mph in old-speak (pre-metric). The steepest hill was closed today, so I contented myself with the lesser ones. After a few runs, I was still of the opinion that I had not exceeded 30 mph. I talked with some other people on the chair and the topic came around to speed as we watched someone tuck the run. They mentioned that they thought that was not allowed. I said that so long as they were in control it should be ok, because after all, how fast can they go on this little hill. Oh wait I can tell you ecactly how fast! I have a GPS. Surprizingly my GPS said 63.8 kph (39.6 mph)!

I did a couple of runs in a tuck on what I thought was the steepest longest run. Best I could get doing that was about 68 kph, but it was pretty consistant.

I then did a bunch of runs trying to find the speed at which I found my WC SCs were "in the zone" and having fun. I consistantly found that at about 30 mph (50 kph), they worked well, and below that, not so well. These are 165 cm Fischer WC SCs and I weigh about 165 lbs in the skin.

At about 3:35 I thought my lift ticket was up and skied down to the lodge, making a few last good turns. At the lodge I checked my ticket which said 3:53 not 3:35, so I had a few more runs left. I also checked my GPS, which said 69 kph: . Must be a fluke, I thought. I did a few more runs, just trying to make nice smooth turns; I had found that the high g turns right up hill killed too much speed as I skied up the hill, and with 250 feet of vertical, I didn't want to have to ski slow down the second half of the run. Making these smooth turns, not trying to go fast or slow, but just making nice not too hard turns, I was consistantly getting into the 70 kph range.

My max speed for my local speed bump with the wrong wax was 76.8 kph or about 48 mph, and it felt like a fairly normal speed for me. On a bigger hill I would typically be skiing around this speed, and not feel like I was "speeding".

Based on my data today I would also have to say that I like to ski between 30 and 60 mph.
post #2 of 23
Hm. First off, I haven't used a GPS made in this millenium. So I'm pretty far behind the technology curve and there's a good chance I'm wrong (please correct me!)

The GPS units I've used in the past pretty much suck at measuring speed. There seems to be a lot of factors for that. First, you need to make sure you're getting readings almost once a second in order to properly calculate the rate from distance and time. Second, altitude used to be fairly inaccurate leading distances over small slopes to be inaccurate leading to an incorrect speed calculation. Has that stuff been fixed? (I kind of remember the second item changed in late '98.)

However, what you describe sounds exactly like what I would expect such an experiment to show.
post #3 of 23
Thread Starter 
I have found the Garmin 76CS to be very accurate in calculating speeds. It does not solely rely on altitude readings from the barometer to calculate speeds. It can calculate speeds on flat roads. They are commonly used by motorcyclists to measure speeds. They are regularly used in navigation systems in cars and motorcycles. Their position could be off by a consistant error of a few metres, but this error cancels when two positions are subtracted in the numerator of the speed calculation.
post #4 of 23
Quote:
Originally Posted by Ghost
Their position could be off by a consistant error of a few metres, but this error cancels when two positions are subtracted in the numerator of the speed calculation.
How do you know that the error cancels out when the positions are subtracted? i.e., at position A, couldn't the GPS place you, say, 5 meters too far north, and at position B place you at say, 3 meters too far south? Then, according to it, you just travelled 8 meters too far. A distance of a few meters could become signifigant given the short sampling times of skiing a 200 foot hill. I'm not very up on GPS technology and how the whole thing works, etc., so maybe I'm way off here and things really do cancel out.
post #5 of 23
Thread Starter 
Quote:
Originally Posted by KevinF
How do you know that the error cancels out when the positions are subtracted? i.e., at position A, couldn't the GPS place you, say, 5 meters too far north, and at position B place you at say, 3 meters too far south? Then, according to it, you just travelled 8 meters too far. A distance of a few meters could become signifigant given the short sampling times of skiing a 200 foot hill. I'm not very up on GPS technology and how the whole thing works, etc., so maybe I'm way off here and things really do cancel out.
Experimentation. When looking at positions compared to maps, the position error is usually consistant. Yes, there will be some error, but the accuracy is pretty good, better than say a car speedometer.

I've had the GPS since this summer, and have compared its readings to car speedometers, bicycle speedometers, walking speeds, time and distance measurements, and am quite confident that it is within a couple of mph.

BTW my car speedo is a little optimistic.
post #6 of 23
Thread Starter 
Leaning all the time:

Garmin states the velocity is accurate to 0.05 m/s. That's 0.18 kph or 0.11 mph.
http://www.garmin.com/products/gpsmap76cs/spec.html

I've also learned that it uses a Doppler shift of the carrier signal to calculate speed. And it seems that only horizontal speed is displayed, so your actual on-snow speed could be higher.. http://gpsinformation.net/main/gpsspeed.htm
post #7 of 23
Ghost, a GPS does a very accurate job of calculateing AVERAGE speed. The problem with maximum speed is, any signal loss causes a zero speed indication. The GPS assumes you have not moved. When the signal is re-aquired, the distance/time is calculated and you get an unrealisistic high speed. If it makes you feel good, fine; otherwise, you should understand it has potential error. BTW, it is a valid way of calibrating you car's speedometer as you generally have a good signal on the open road. Altitude errors are easily within 50 feet that you meausured. The 76CS uses a combination of barametric pressure and sattelite position, neither of which has a precision of better than 50 feet.
post #8 of 23
Thread Starter 
For those who want to know....the owner's manual
http://www.garmin.com/manuals/GPSMAP...nersManual.pdf
post #9 of 23
Thread Starter 
Quote:
Originally Posted by Cirquerider
Ghost, a GPS does a very accurate job of calculateing AVERAGE speed. The problem with maximum speed is, any signal loss causes a zero speed indication. The GPS assumes you have not moved. When the signal is re-aquired, the distance/time is calculated and you get an unrealisistic high speed. If it makes you feel good, fine; otherwise, you should understand it has potential error. BTW, it is a valid way of calibrating you car's speedometer as you generally have a good signal on the open road. Altitude errors are easily within 50 feet that you meausured. The 76CS uses a combination of barametric pressure and sattelite position, neither of which has a precision of better than 50 feet.
Thanks Cirquerider for the education .

It does not make me feel good at all . What would make me feel good was knowing what the max speed actually was. It should calculate your velocity based on change in position divided by change in time. However, I'm quite prepared to believe that it's messed up. That way my original speed estimates aren't so far out of whack.

So I gather unless you are able to hear the little beeps (unlikely) you won't know if it has actually lost satellite signal or not. And you should reset the average speed, and thus get your average speed for the run.

How about track logs?

I seem to have a number of 1 point track logs which seems to indicate that signal was lost, but there are a few long logs. Here for example is a part of one. My software only lets me cut and paste one at a time: .

164 11/12/2005 2:24:08 PM 1275 ft 9 m 0:00:12 3 kph 81° mag 17 T 546744 4809277

165 11/12/2005 2:24:20 PM 1273 ft 3 m 0:00:01 10 kph 70° mag 17 T 546752 4809280

166 11/12/2005 2:24:21 PM 1271 ft 72 m 0:00:08 33 kph 58° mag 17 T 546755 4809281

167 11/12/2005 2:24:29 PM 1216 ft 32 m 0:00:02 57 kph 62° mag 17 T 546808 4809330

168 11/12/2005 2:24:31 PM 1199 ft 18 m 0:00:01 64 kph 59° mag 17 T 546833 4809349

169 11/12/2005 2:24:32 PM 1188 ft 82 m 0:00:05 59 kph 44° mag 17 T 546847 4809361

170 11/12/2005 2:24:37 PM 1139 ft 27 m 0:00:02 49 kph 48° mag 17 T 546892 4809429

171 11/12/2005 2:24:39 PM 1119 ft 42 m 0:00:18 8 kph 88° mag 17 T 546909 4809450

172 11/12/2005 2:24:57 PM 1092 ft 14 m 0:00:27 2 kph 191° mag 17 T 546950 4809459

173 11/12/2005 2:25:24 PM 1089 ft 5 m 0:00:54 0.3 kph 253° mag 17 T 546950 4809445

174 11/12/2005 2:26:18 PM 1092 ft 7 m 0:00:26 0.9 kph 231° mag 17 T 546945 4809443

175 11/12/2005 2:26:44 PM 1090 ft 3 m 0:00:15 0.7 kph 164° mag 17 T 546941 4809438

It seems to be calculating average velocity for the leg. Would that work?
Next time I will try putting the GPS in a backpack so as to loose signal less frequently.
post #10 of 23
A lot also depends on the sample rate you are set for. I was reading that although the GPS systems can be very accurate, The longer the total sample time with the shortest sample rate would be most accurate. According to many reports, the accuracy can be off by 33% or more for short bursts.
post #11 of 23
I have a Garmin 60cs that I use for hiking. I was thinking of using it to measure skiing speed this winter. By the way, I have had good luck setting the track log distance interval to 0.04mi (200ft) although if you do a lot of turns I don't think the GPS will give you good data. In addition to looking at the calculated output I thought of performing a sanity check of the data by doing the following:

1. Output track log as Ascii file and import into Matlab data analysis program.
2. Calculate distance for each leg of the track log (It may already give you distance).
3. Plot Distance Vs. Time.
4. Throw away any obvious outliers.
5. Fit polynomial to remaining data to smooth GPS errors (or perform some other smoothing operation).
6. Calculate first derivative of polynomial to get instantaneous peak speeds.

I haven't tried this yet so I don't know if I thought this though entirely. It would also be fun to overlay the track log data on a digital map of the ski area (I did this in the summer with hiking track logs from the Indian Peaks). I think I need to take a day off and go skiing this week.
post #12 of 23
Everyone at epic knows that it is impossible to go more than 25mph unless you are a world cup racer.
post #13 of 23
Having used my cycling computer and my Garmin Geko201 side by side, I can state that the GPS measures speed just as well as my cyclo-computer.

L
post #14 of 23
Quote:
Originally Posted by rcahill
It would also be fun to overlay the track log data on a digital map of the ski area
Amen! to that.
post #15 of 23
My g/f just got a Garmin Quest2 and I just ordered a full set of topo maps for Canada so track logs over ski hill maps may be in my future.
post #16 of 23
You can also supposedly do this using the GPS track logs and Google earth. I've never been able to make it work as the laptop and GPS don't seem to be the best of friends and won't talk to each other....

L
post #17 of 23
I would think that the measurements would be more inaccurate when carving down the hill than when tucking, unless making really long gs turns.
post #18 of 23
Track overlay using GPS. 1/2 day at Sierra at Tahoe:

post #19 of 23
Quote:
Originally Posted by Cirquerider
Track overlay using GPS. 1/2 day at Sierra at Tahoe:

What are those big cuts out and straight back in on the upper right? Scooting out to roll a spliff or what?
post #20 of 23
That is an out of bounds gated area with heavy tree cover. The sharp jagged lines are caused by loss of signal This is off Gate 1. The best lines just northeast of the main (Grandview) lift, through gates 3-5 were closed this day. These out of bounds runs are anywhere to the right side of that ridge in an area called Huckelberry Canyon and all the way around the sharp box canyon area around the creek. Area is accessed through gates and safety gear is recommended. Those are mainly North exposures, and get lots of wind-load and snow stays nice for a long time. The downside is this area avys, and there are some flats at the bottom that require a walk-out.
post #21 of 23
Quote:
Originally Posted by Lonnie
Having used my cycling computer and my Garmin Geko201 side by side, I can state that the GPS measures speed just as well as my cyclo-computer.

L
Over what distances and time? How many turns?

As stated above, GPS's are great at measuring Averages. I found that the altimeter watch I ski with is great as long as I'm doing long runs. If I'm teaching on the bunny hill with kids (skiing slow stopping a lot, etc) I can go a whole day and clock less than 100 Vertical feet. The altimeter shows a vertical change from bottom to top of the run at 110.

PS. Hi Lonnie,

I'll be looking for you, Mark and Scott when I'm up there again this season. More on that later.

DC
post #22 of 23
Quote:
Originally Posted by Cirquerider
That is an out of bounds gated area with heavy tree cover. The sharp jagged lines are caused by loss of signal This is off Gate 1. The best lines just northeast of the main (Grandview) lift, through gates 3-5 were closed this day. These out of bounds runs are anywhere to the right side of that ridge in an area called Huckelberry Canyon and all the way around the sharp box canyon area around the creek. Area is accessed through gates and safety gear is recommended. Those are mainly North exposures, and get lots of wind-load and snow stays nice for a long time. The downside is this area avys, and there are some flats at the bottom that require a walk-out.
Good cover story, dude.

For those that claim their GPS is accurate compared to their car speedo or Bike computer, I think the open road and better satellite coverage is very valid for providing an advantage as is the generally level travel.

Who has compared the GPS to a bike computer on a mtn bike while descending on a slope comparable in pitch to a ski slope with trees nearby obscuring satellite reception? Keep in mind many ski slopes would block one whole quadrant of the sky (satellites) and that would often be the south quadrant where a lot of these satellites would likely be positioned (especially if in Canada). I think that comparison of GPS vs bike computer on a ski slope would be much more fair and interesting.
post #23 of 23
Thread Starter 
I've just been using the automatic track log, and have only used up 80% of the memory since I bought the think. I think it has about 10,000 data points capacity. I'm going to clear it and set it to record every 5 m, that should give me about 50 klicks worth of points, or maybe every 2m, that way I will be able to see how round my turns are. When I looked at my track log, some logs had a lot of points in them, but many had only 1. I am assuming that if all the data points are in the same log, the signal wasn't lost. Is this so?
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