or Connect
EpicSki › The Barking Bear Forums › On the Snow (Skiing Forums) › General Skiing Discussion › Cheap GPS to keep track of speed?
New Posts  All Forums:Forum Nav:

Cheap GPS to keep track of speed?

post #1 of 24
Thread Starter 
I was thinking about buying a GPS mainly to keep track of top speeds. I don't want to spend more than $100, and I don't really know what to look for...
post #2 of 24
I think you should at least get one that keeps track logs. The max speed feature doesn't seem all that reliable.

Here's a start on your search http://www.garmin.com/
Click the link to their "purchase assistant".
post #3 of 24
I dont think those things are that accurate w.r.t to speed.. I was clocked at 60+ mph yesterday.. I ski fast but that seems a bit unpossible to me..
post #4 of 24
Quote:
Originally Posted by coug
I dont think those things are that accurate w.r.t to speed.. I was clocked at 60+ mph yesterday.. I ski fast but that seems a bit unpossible to me..
Has anyone on here ever tried comparing a real-time radar gun reading with a gps readout while skiing? Like coug, I'm also quite skeptical of gps readouts for this kind of application.

I *have* compared a gps reading to my car speedometer on a highway (and they are quite close), but I just really wonder about what happens when you add the incline component on a ski run.
post #5 of 24
This is recycling some stuff nicely.
post #6 of 24
Well Bob, Try that car speedometer reading and GPS reading while driving down the Teton Pass. That should give you some idea if there is a difference when you factor in incline.
post #7 of 24
GPSs are very accurate with speed and altitude. The issue is when the signal to the satellites gets blocked. If you are on an open trail and you GPS is out in the open or has an external antenna, you should get good readings.
post #8 of 24
Thread Starter 
How is GPS affected by slope incline?
post #9 of 24
Quote:
Originally Posted by Utah49
Well Bob, Try that car speedometer reading and GPS reading while driving down the Teton Pass. That should give you some idea if there is a difference when you factor in incline.
U49:

Teton Pass tops out at 10 PERCENT grade, which is nearly flat when compared to most ski runs. That's why I just don't know.

I'm not saying they *aren't* accurate on a ski slope, I just haven't been convinced they are. Radar guns I believe.
post #10 of 24
My post was a touch tongue in cheek. 10% grade is pretty steep for a Hwy. I read somewhere that interstate Highways don't exceed a 6% grade.
post #11 of 24
The real time speed displayed on a GPS is accurate for 2-dimensional movement. The problem comes in the data logging for maximum speed. Any signal loss causes gross inaccuracies. You can set a GPS on a stationary surface, and it will record distances and speed that simply don't exist and report them in the log. When traveling at high speed on a slope with changing exposure to sattelites in the sky (slope aspect, trees), the problems become very different from the highway. A GPS simply reports its current location, generally within 15 to 20 meters of accuracy. Within that margin of precision, the GPS floats as its position relative to sattelites changes and signal interferences (reflection, blocking) cause timing and position errors. The precision of a GPS is strongly related to its quality cost and antenna, with professional units being capable of very accurate positions. The recording of maximum speed just happens to be one of the most error prone measurements that it records.
post #12 of 24
I couldnt find any data on the Garmin Rino 120 I used yesterday... But the typical accuracy of these things are within a few feet which leaves room to a lot of measurment error.. To illustrate, at 60MPH you are travelling at 1.49 feet/sec now add 5 feet to your current reading from the previous and you are pretty fn fast.. that which would be max deviation and you get a pretty screwed up reading...
post #13 of 24
Quote:
Originally Posted by sir jman
How is GPS affected by slope incline?
They are not affected by incline. They read accuracy down to about 3 feet, not 15-20 meters as Cirque mentioned. That was the case about 5-10 years ago, but the Gov't relaxed it's accuracy requirements, and now allows them to be incredibly accurate. Some can get down to the foot. They triangulate between 4-20+ satellites. not just 3, so they can get very accurate altitude readings. It's the same system that airlines use for altitude readings, and planes go up and down on an incline the same way skiers do.
post #14 of 24
haha, my friend got one of those watches from REI.com . Well, I got excited cause im stupid and tried to see how fast I could go on it. I got it up to 82.2 mph, and even if its not reading accurately, i still went fast as hell, plus i was on a slope with little trees around us, pretty wide, and open. Tested it with a car driving down the moutain with trees all around us on a decently steep pitch (upside down mountain) and it read perfectly with the car.
post #15 of 24
Thread Starter 
Quote:
Originally Posted by JohnH
They are not affected by incline. They read accuracy down to about 3 feet, not 15-20 meters as Cirque mentioned. That was the case about 5-10 years ago, but the Gov't relaxed it's accuracy requirements, and now allows them to be incredibly accurate. Some can get down to the foot. They triangulate between 4-20+ satellites. not just 3, so they can get very accurate altitude readings. It's the same system that airlines use for altitude readings, and planes go up and down on an incline the same way skiers do.
good point
post #16 of 24
Quote:
Originally Posted by coug
I couldnt find any data on the Garmin Rino 120 I used yesterday... But the typical accuracy of these things are within a few feet which leaves room to a lot of measurment error.. To illustrate, at 60MPH you are travelling at 1.49 feet/sec now add 5 feet to your current reading from the previous and you are pretty fn fast.. that which would be max deviation and you get a pretty screwed up reading...
but if you are reading your speed over 1000 feet, and it's off by 2-3 feet, it's not much of an error. yes, a 1 second top speed could be a bit flakey, especially if the signal gets lost fot a moment.
post #17 of 24
Quote:
Originally Posted by JohnH
but if you are reading your speed over 1000 feet, and it's off by 2-3 feet, it's not much of an error. yes, a 1 second top speed could be a bit flakey, especially if the signal gets lost fot a moment.
I agree, the error is probably +-5 feet and hence you would expect it to average out over the length of a slope.. It would be good measure of average speed but not instantenous/max speed.. But if it has a data log, then you can filter the data and get pretty close to the actual speeds, I think..
post #18 of 24
Accuracy is good these days. I use mine for geocacheing and locating benchmarks all the time. I even plot sites for planning and report purposes. I still don't believe the max speed. I have gone hiking and had maximum speeds that are impossible to achieve on foot reported. Great tools, when used the right way. Under $100? I'm waiting to see that too.
post #19 of 24
Navman Sport.Tool A300 - when I picked it up it was $75 after rebate. We discussed it here: http://forums.epicski.com/showthread...ghlight=navman
post #20 of 24
Quote:
Originally Posted by Cirquerider
Accuracy is good these days. Under $100? I'm waiting to see that too.
Hi cirqueman! I got a garmin wrist mounted unit on ebay for $93. Seems to work. hopefully the technology will phase the threads out.

If you use it for more than one run, it becomes obvious what is true and what is unit induced error. Do six identical runs, it shows 46-48 mph on five of them, 77 on another. Don't bother to post the 77 mph run here. The realists will eat you alive.

In theory, the incline should produce results slightly slower than true speed. The occasional way "out in left field" reading is software freak out.

I gotta pair of 200's waiting at home to do some speed trials on.
post #21 of 24
Hmmm Newfydude....So patrollers are out to set speed records now huh?
post #22 of 24
Quote:
Originally Posted by JohnH
They are not affected by incline. They read accuracy down to about 3 feet, not 15-20 meters as Cirque mentioned. That was the case about 5-10 years ago, but the Gov't relaxed it's accuracy requirements, and now allows them to be incredibly accurate. Some can get down to the foot. They triangulate between 4-20+ satellites. not just 3, so they can get very accurate altitude readings. It's the same system that airlines use for altitude readings, and planes go up and down on an incline the same way skiers do.
They CAN triangulate 4-20 satellites but they only DO triangulate what is available to them. Last I heard the network is 24 satellites worldwide. 6 -8 is about all you'll get in any one area at a time. Then put a mountain that you are skiing on in the way and you block satellites in that chunk of sky. Add some trees maybe take another one or two. A couple more may be in the same area and not giving good differential or perhaps directly overhead. The main thing is the ones low on the horizon at right angles to travel are the most important for accurate incline travel and also the most difficult to get a consistent signal from while moving on skis. GPS in a plane is both a different unit and functioning with a broader horizon in 360degrees. That is one reason why multi million dollar systems are in place or strictly visual landings are used at tight airports. GPS alone is not reliable and accurate enough as satellites signals are lost in tight spots.

I'm sure they are getting better but speed readings during mountain travel especially during elevation changes are still very suspect in my mind.
post #23 of 24

I agree w L7....

I have (2) GPS's, a Magellan 2000 and a Garmin Etrex Vista. Trying to get an accurate instantaneous speed is a waste of time. I use it in the mountains of CO every year at elevations of around 10,000 ft and I can tell you that the accuracy in real world conditions is pretty good, but not great. Even in the middle of a wide open field, my Garmin never reports accuracy better than about 12 feet, sometimes much worse, depending on satellites available. That's excellent for navigation on foot but it would wreak havoc on speed numbers. You could use it for average speed as said earlier but forget about max speeds. Here's a test for you guys with GPS's. Go out into a field with a wide unobstructed view of the sky, get a solid fix with your GPS unit. Make sure you've got plenty of satellites in view. Now stand perfectly still and set your GPS on "tracking" mode where it will show you a line of movement. Pretty soon the center of the screen will start to turn into a black blob. You haven't moved an inch, but your "location" has been bobbing and weaving all around you because of the inaccuracies of the system. If you want improved accuracy you need to use differential GPS. You also need to mount a good antenna on top of your skid lid.
post #24 of 24
Accuracy of Garmin 76cs (not less than 100 bucks though)
http://www.garmin.com/products/gpsmap76cs/spec.html
New Posts  All Forums:Forum Nav:
  Return Home
  Back to Forum: General Skiing Discussion
EpicSki › The Barking Bear Forums › On the Snow (Skiing Forums) › General Skiing Discussion › Cheap GPS to keep track of speed?