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fun things to try with your GPS



 
 
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  #1  
Old October 7th 09, 01:21 AM posted to rec.skiing.alpine.moderated
Humsquirto
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Posts: 5
Default fun things to try with your GPS

On a boring groomer day last year, I turned on my GPS, reset my track,
and did a speed run, just to see how fast I was going. My GPS (an old
Magellan Sport track) has a "maximum speed" indicator (I'm sure most
others do too), and when I got on the lift I saw that I maxed out at
72mph! I'm pretty sure the GPS speed indicator is accurate, as I
often compare it to my car speedometer, and it is always the same. I
was indeed hauling a$$, but was still totally in control, and felt I
could have pushed it a bit more (not too much though).

Not one to encourage dangerous behavour, but is something worth trying.

Ads
  #2  
Old October 7th 09, 04:03 AM posted to rec.skiing.alpine.moderated
lal_truckee
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Posts: 1,348
Default fun things to try with your GPS

Humsquirto wrote:

72mph! I'm pretty sure the GPS speed indicator is accurate, ...


I profess a slight doubt...

  #3  
Old October 8th 09, 10:35 PM posted to rec.skiing.alpine.moderated
Alun Evans
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Posts: 21
Default fun things to try with your GPS

On Tue 06 Oct '09 at 21:03 lal_truckee wrote:

Humsquirto wrote:

72mph! I'm pretty sure the GPS speed indicator is accurate, ...


I profess a slight doubt...


This generated a lot of discussion amongst people I know too, here's
some facts:

http://www8.garmin.com/products/etrexVistacx/spec.html

GPS accuracy:
Position: 15 meters, 95% typical*
Velocity: 0.05 meter/sec steady state

i.e. 0.05 (meters per second) = 0.111846815 mph

Remember that the position error is constant between the velocity
measurements, thus cancels out.

Though this assumes a good signal the entire time, and you should
check the individual track log points.

I've seen 600mph+ readings sometimes with bad GPS signal and massive
non constant positional errors.




A.

--
Alun Evans

  #4  
Old October 8th 09, 10:35 PM posted to rec.skiing.alpine.moderated
downhill
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Posts: 644
Default fun things to try with your GPS

lal_truckee wrote:
Humsquirto wrote:

72mph! I'm pretty sure the GPS speed indicator is accurate, ...


I profess a slight doubt...

I would drink to that doubt.

There is a GPS option available for many data acquisition systems most
drivers find that you get more repeatable results from a lateral
accelerometer and a wheel speed indicator non driven wheel. There is a
fair amount of error in the gps system, my thoughts also when you get
height changes z axis the calculations are not that accurate vs on a
road where you just measuring x y axis changes

I can show you some funny errors such as going off track on to grass and
show a speed of 203 from the gps calulation, he was doing 95 before
leaving track. yesterday I was chasing a 158 mph high on one lap the
calculated max speed of car at 9000 rpm is about 144--- 145 13 mph over
is one big draft or a gps error

  #5  
Old October 8th 09, 10:36 PM posted to rec.skiing.alpine.moderated
Humsquirto
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Posts: 5
Default fun things to try with your GPS

On Oct 6, 9:03 pm, lal_truckee wrote:
Humsquirto wrote:

72mph! I'm pretty sure the GPS speed indicator is accurate, ...


I profess a slight doubt...


Please explain why you think I am lying.

  #6  
Old October 8th 09, 11:35 PM posted to rec.skiing.alpine.moderated
Bob F
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Posts: 1,296
Default fun things to try with your GPS

Humsquirto wrote:
On Oct 6, 9:03 pm, lal_truckee wrote:
Humsquirto wrote:

72mph! I'm pretty sure the GPS speed indicator is accurate, ...


I profess a slight doubt...


Please explain why you think I am lying.


I saw nothing to indicate he thought you were lying.

  #7  
Old October 20th 09, 12:56 AM posted to rec.skiing.alpine.moderated
Humsquirto
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Posts: 5
Default fun things to try with your GPS

There is a GPS option available for many data acquisition systems most
drivers find that you get more repeatable results from a lateral
accelerometer and a wheel speed indicator non driven wheel. There is a
fair amount of error in the gps system, my thoughts also when you get
height changes z axis the calculations are not that accurate vs on a
road where you just measuring x y axis changes


Downhill and alun,

thanks for the info, that is more of what I was hoping for with this
discussion. I like the mention of the z-axis change - makes sense. I
still however profess that 70 mph is absolutely attainible. Downhill
racers typically max at upper 80's to 90 mph, I don't think reaching
75% of this speed in ideal conditions (long wide straight run with a
consistent yet steep slope, iced over groomed run, no bumps, tuck the
whole way, not worrying about ruts, turns, gates, etc.) is that hard
to believe. Even with a 10% error, that is still mid 60's, which is
splitting hairs. I posed the question to the folks at the USA speed
skiing association, who said that under the right conditions that was
certainly doable (difficult, yes, but impossible, no).

What if the GPS was actually close to accurate? Errors are not
automatic. So maybe it was closer to 60 mph, but even that means that
70 mph is still possible. Next time I will take my Trimble unit.

Ask yourself, how fast do you think you could go? 60+ mph is not that
unreasonable.

  #8  
Old October 20th 09, 01:38 AM posted to rec.skiing.alpine.moderated
VtSkier[_3_]
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Posts: 344
Default fun things to try with your GPS

Humsquirto wrote:
There is a GPS option available for many data acquisition systems most
drivers find that you get more repeatable results from a lateral
accelerometer and a wheel speed indicator non driven wheel. There is a
fair amount of error in the gps system, my thoughts also when you get
height changes z axis the calculations are not that accurate vs on a
road where you just measuring x y axis changes


Downhill and alun,

thanks for the info, that is more of what I was hoping for with this
discussion. I like the mention of the z-axis change - makes sense. I
still however profess that 70 mph is absolutely attainible. Downhill
racers typically max at upper 80's to 90 mph, I don't think reaching
75% of this speed in ideal conditions (long wide straight run with a
consistent yet steep slope, iced over groomed run, no bumps, tuck the
whole way, not worrying about ruts, turns, gates, etc.) is that hard
to believe. Even with a 10% error, that is still mid 60's, which is
splitting hairs. I posed the question to the folks at the USA speed
skiing association, who said that under the right conditions that was
certainly doable (difficult, yes, but impossible, no).

What if the GPS was actually close to accurate? Errors are not
automatic. So maybe it was closer to 60 mph, but even that means that
70 mph is still possible. Next time I will take my Trimble unit.

Ask yourself, how fast do you think you could go? 60+ mph is not that
unreasonable.


Not necessarily about your particular questions, but:
A friend of mine has the GPS app for his I-Phone. He
also has a dedicated GPS (he's a toy freak). He finds
that both the I-phone and the dedicated GPS show speed
quite accurately when compared to his speedometer.

Having said that, he finds this is so over a distance
and when he accelerates or decelerates there is a lag
before the GPS receivers w adjust to the new speed, so
what I would suggest is that your GPS may give you
a accurate max speed reading but that at any given
instant, the reading could be wildly off. For example,
it is several seconds before my friends GPS will
register the fact that the car has stopped, still
recording 25 or 30 mph while the car is stopped.

So... I would tend to believe you that in the
conditions that you indicate that you really are
going 70 mph (or so with a fairly small error)

As for me, I believe that I top out at somewhere
around 50. I have another friend who likes to
"speed ski". He does so early in the morning
(before crowds and bumps form) and only when he
is alone or almost so on the hill. I suspect he hits
downhill racer speeds. Big guy, big skis, lots of
skill. Has been stopped by patrol, who listened to
his story and said that under the conditions he
described, going fast is OK. Neither one of us has
had our speed measured.
..

  #9  
Old October 20th 09, 06:16 AM posted to rec.skiing.alpine.moderated
downhill
external usenet poster
 
Posts: 644
Default fun things to try with your GPS

Humsquirto wrote:
There is a GPS option available for many data acquisition systems most
drivers find that you get more repeatable results from a lateral
accelerometer and a wheel speed indicator non driven wheel. There is a
fair amount of error in the gps system, my thoughts also when you get
height changes z axis the calculations are not that accurate vs on a
road where you just measuring x y axis changes


Downhill and alun,

thanks for the info, that is more of what I was hoping for with this
discussion. I like the mention of the z-axis change - makes sense. I
still however profess that 70 mph is absolutely attainible. Downhill
racers typically max at upper 80's to 90 mph, I don't think reaching
75% of this speed in ideal conditions (long wide straight run with a
consistent yet steep slope, iced over groomed run, no bumps, tuck the
whole way, not worrying about ruts, turns, gates, etc.) is that hard
to believe. Even with a 10% error, that is still mid 60's, which is
splitting hairs. I posed the question to the folks at the USA speed
skiing association, who said that under the right conditions that was
certainly doable (difficult, yes, but impossible, no).

What if the GPS was actually close to accurate? Errors are not
automatic. So maybe it was closer to 60 mph, but even that means that
70 mph is still possible. Next time I will take my Trimble unit.

Ask yourself, how fast do you think you could go? 60+ mph is not that
unreasonable.

I have been clocked at 55 on a radar gun in a speed suit & 212cm skis
and it was work. The fastest guy went 65 and he had pro experience. My
experience with people and speed they have a tendency to lean towards
big numbers even if there is error

The fastest master skiers hit 70 max, you can not use world cup people
as a real world example for regular skiers. If you work out like Bode
you might see 60 but I think you going to be in for a surprise. Plus
your talking about using downhill or super-g skis to do this, using
170cm to do this is asking for trouble

The real way of testing your speed is trip lights or a radar gun. Or a
pitot tube that would be the cheapest way. Even simpler measure the
distance and run a watch time the fastest segment. I use gps in my line
of work and you need to view it in context of other data, but it is not
repeatable over a period of hours.
In one of the testing magazines I get there is a new product for
increasing the accuracy of your gps signal input. Does not fit in
formula cars so did not look in detail.

My thought to best experience the speed is to stand up from the tuck
when you want to slow down

Bring this up in the spring I will share some data from ski runs this
year I am going to some trade shows and putting on a tech seminar in feb
and one of the subjects we will be exploring is gps data.

As for comments from speed groups Lime Rock a 1.6 mile course can be
driven in 49 seconds in a formula atlantic car but only by a small group
of people and many have tried.

  #10  
Old November 2nd 09, 04:25 AM posted to rec.skiing.alpine.moderated
Humsquirto
external usenet poster
 
Posts: 5
Default fun things to try with your GPS

downhill,

...........very convincing.

so, maybe not quite 72.

still, I felt like I was going faster than the old AMC Eagle coming
down hwy 50 after a sunday family ski day.

Great info, by the way, I hope you are able to post some good data
after the season.




from downhill:


I have been clocked at 55 on a radar gun in a speed suit & 212cm skis
and it was work. The fastest guy went 65 and he had pro experience. My
experience with people and speed they have a tendency to lean towards
big numbers even if there is error

The fastest master skiers hit 70 max, you can not use world cup people
as a real world example for regular skiers. If you work out like Bode
you might see 60 but I think you going to be in for a surprise. Plus
your talking about using downhill or super-g skis to do this, using
170cm to do this is asking for trouble

The real way of testing your speed is trip lights or a radar gun. Or a
pitot tube that would be the cheapest way. Even simpler measure the
distance and run a watch time the fastest segment. I use gps in my line
of work and you need to view it in context of other data, but it is not
repeatable over a period of hours.
In one of the testing magazines I get there is a new product for
increasing the accuracy of your gps signal input. Does not fit in
formula cars so did not look in detail.

My thought to best experience the speed is to stand up from the tuck
when you want to slow down

Bring this up in the spring I will share some data from ski runs this
year I am going to some trade shows and putting on a tech seminar in feb
and one of the subjects we will be exploring is gps data.

As for comments from speed groups Lime Rock a 1.6 mile course can be
driven in 49 seconds in a formula atlantic car but only by a small group
of people and many have tried.- Hide quoted text -

- Show quoted text -


 




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