Using a GPS to track speed

[+Glorfindel]

I have used my Legend to track speed in various situations and am wondering about what issues affect accuracy of the reading you get.

 

In the car is seems to be dead on with my speedometer. Walking in the trail is seems very consistent.

 

I took it up to Bogus last Friday for a fun day of snowboarding and used it to map out the runs and to keep track of Max. Speed. I was shocked to see it reading 57.6MPH after a pretty fast run . I reset it after each run an found it giving very consistent numbers. There were areas though that I knew I was going faster than the part of the trail that gave the 57 reading, but it was in areas with very steep vertical drops. In these it was averaging around 35MPH.

 

My question. Does anyone know if the unit only tracks horizontal forward motion or does it track point A to point B motion? i.e. What would it read for a skydiver, 0MPH or 512MPH?

 

Signatures are dumb.

[Cache Canucks]

I believe that the calculation of speed is based on the time required to make a given change in lon/lat location. Were you to jump off of a 100ft cliff and fall straight down, your GPSr would indicate a speed of '0' because it would detect no change in horizontal position (...despite the vertical distance being covered in an exceptionally short period of time!).

 

[Kerry.]

Speed is horizontal based on XY and also includes doppler principles. In normal vehicle/walking type use it has to be very steep to have a noticeable impact on the difference.

 

A skydiver, straight down, zero.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Cache Canucks]

...if you were intent on knowing how fast you were falling as you plummeted down the cliff face, you could probably calculate that from the Vista altimeter's 'ft/sec' readout (assuming that you were *really* quick at doing the math before you hit the ground).

 

 

[+Glorfindel]

Does anyone know of any circumstanced that would give you a max. speed result that is off in the opposite direction. Being faster that you are actually traveling?

 

--

'It was my lot to take the Road, and I came to the Bridge of Mitheithel, and left a token there, nigh on seven days ago.'

[+Jamie Z]

quote:

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Originally posted by Glorfindel:

There were areas though that I knew I was going faster than the part of the trail that gave the 57 reading, but it was in areas with very steep vertical drops. In these it was averaging around 35MPH.

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I've posted this calculation several times already... but I will again.

 

What you thought was steep really isn't.

 

In a few minutes of googling, I found some vague data about ski slopes in Michigan. The vertical drops for those slopes were generally around 500 feet, with a mile being the typical longest run.

 

Lets just be very conservative and assume that your local ski slope had twice the vertical drop, and the run you were on was half as long as the longest run. Namely, 1000 foot drop in 2640 feet. I propose even the most extreme slopes aren't this steep. (although if someone had some actual data, that'd be useful)

 

This means that the angle of the hill averages about 22.25 degrees. If you were flying down the hill at 60mph, your GPS would indicate a horizontal speed of 55.5mph. Not much difference, even at this unlikely slope.

 

Trust your GPS.

 

Jamie

 

Oh yeah.. to answer "Does anyone know of any circumstanced that would give you a max. speed result that is off in the opposite direction. Being faster that you are actually traveling?"

 

Not physically. The calculation is GROUNDSPEED multiplied by the COSINE OF THE ANGLE. Since the cosine function is never greater than one, your will always be less than groundspeed, except in the case of flat terrain, where groundspeed equals horizontal speed.

 

It's much the same as police radar will never calculate a higher-than-actual speed. In most cases, it calculates a somewhat smaller number.

[peter]

I agree with Jamie that we tend to greatly overestimate the steepness of slopes. Figures I've seen for steep, but still normal, ski runs have been for grades of about 35% (just under 20 degrees), and remember that usually the skiier doesn't go straight down these but instead slaloms back and forth to reduce the effective grade.

For extreme cases, there is a 95% slope (43 degrees) at Charbrieres used for speed record attempts (over 150 mph). Even that slope would only increase your speed from a GPS-indicated 35 mph to 48 mph. So my guess is that you were actually going faster on the sections that were less steep and the consistent 57 mph readings are a good estimate.

 

As to your other question, on readings that are too high, I've seen those occasionally. They've occurred in areas conducive to multipath resulting in the GPS thinking I made an unusually sudden change in position almost instantly. The most extreme case was bicycling in a canyon area and getting a maximum speed reading of 4032 mph.

[+MaxEntropy]

It might simply be the GPS error.

I've reset my trip computer on my Legend, taken a hike then a few miles later I was surprised to see a max speed of 14 MPH. I had no idea that I could walk so fast.

Somewhere along the way, the error found a point behind me, the next time, the error showed me ahead of myself. It calculated that I had to have gone 14 MPH to make that distance in that time thus my max.

 

Mickey

Max Entropy

More than just a name, a lifestyle.

[+apersson850]

Sometimes, it seems that if you are running, it manages to measure the speed of your body added to the forward speed of your hand. This assuming that you are running like people are most, with your arms swaying back and forth all the time.

 

Probably the doppler effect that's used there, since by simply calculating speed from the time taken to go from one position to another, it wouldn't notice anything significant by just these arm movements.

 

I've at least seen rather high "running" speeds, without any particular detour in the tracklog.

 

Anders

[+apersson850]

First, nothing worked. Then, it posted both of my replies, without really acknowledging any of them. Now, I can't remove this one...

[+sledgehampster]

My MAP 330 does a fine job as a speedometer in my 88 Buick Riviera. Digital dash is toast (what do you expect for $500.00). Placed unit on dash, set it on speedometer, set the overspeed alarm on the car (CRT screen still works!) and take off. GPS matches the overspeed alarm every time.

[+Bill D (wwh)]

quote:

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Glorfindel wrote:

Does anyone know of any circumstanced that would give you a max. speed result that is off in the opposite direction. Being faster that you are actually traveling?

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I've had that happen sometimes in my car, with the track history showing a max speed MUCH faster than I'd been driving at. It seems to be due to the GPSr losing signal then regaining it, and getting the calculation wrong when it "jumps" from the last known position.

 

Bill