Have you ever wondered how fast you slide down a slope?

[BurnHard]

I don't think couloimann's post is the definitive answer. Definitely interesting and true for HIS GPS, but it can very well be that different GPS' have different software implementations.

Incidentally, I am an embedded software engineer working on various GPS units. Indeed, 'normal' GPS' are extremely inaccurate in vertical position. Normal being most consumer GPS. Aviation GPS have a new feature called WAAS which improves vertical accuracy drastically. However, due to consumer GPS' vertical inaccuracy I would think most, if not all, are implemented to compute X-Y speeds only.

I am working on a small private project, implemting and proramming a circuit board with GPS, Accelerometer, and Pressure sensors. The GPS sensor I'm using has the new WAAS feauture and will by itself be much more accurate in the vertical direction. Accuracy will be further improved by error-checking the data against readings from the prsesure sensors. This should give me EXTREMELY accurate position and speed measurements in 3D space.

I'll let everyone know how it goes on my first run.
Geek out

[BushwackerinPA]

you relieze that endlessseason radars people every year at snowbird.

My best 70mph

[Ghost]

I just discovered you can upload png files to photobucket....Bigger pictures!

[skiing-in-jackson]

Several points here...

(I didn't read the entire thread)

Rideit answered some velocity questions. I believe that GPS measures speed in a point to point fashion and does not take into consideration the exponentially (?) increasing speed that actually occurs across or down a given slope. The unit is simply measuring an accumulated average of the number of pings it receives from the available satellites.

Plenty of times I've clocked myself (without even trying) at 55+ MPH. I've even clocked my 8 year old son at 48+ MPH on numerous occasions. Amphitheater Baby!

That said.. I've also clocked myself at 478 mph on skis. This is referred to as the "swing effect". Or, if you don't have good satellite reception, the GPS unit will suffer in accuracy as it "blacks out" between available pings. The points, separated but an elongated blackout, read an artificially high speed.

Did that make sense? I hope so.

If it didn't, I'm, as rideit stated "I'm talking out my ass"