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My Max Speed Reading


roogie

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I've seen a picture of a max GPS speed from a Concorde flight. It suprised me that they would calculate correctly up past 1000. :huh:

That's awesome, I live about 10 miles from the Garmin World HQ and have a friend who works there in the aviation department, he didn't know much about the handhelds.

 

roog

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I live in SoCal, and once while heading home from work, I looked down and the screen on my ST Pro was black. I thought, "what the..." and started to zoom out to see what was going on. After a few presses of the zoom button, I realized that it was showing my position somewhere off the coast of South America, in the Pacific! After a minute or so, it suddenly changed back and showed my position correctly in Southern California. I didn't think to check the max speed reading, I wish now I had. South America to North America in just a few seconds!!! :huh: Although according to the manual, the max speed reading is limited to 951mph.

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I went out jogging one day and checked on my GPS when I was done. I then noticed my max speed (3923 MPH). I wondered what that booming noise was behind me :huh:

 

This is an actual untouched photo, but I think something like what 4x4van described must have happened. Guess the Garmin's go faster though :o

 

dfba8405-b960-4f11-b4ee-29a03886dbfb.jpg

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On a flight from Florida to the Dominican Republic the pilot let me use my Garmin Legend. It was fascinating watching the islands pass underneath and on the GPS itself. The maximum speed, which I have kept on the GPSr was 926 MPH. Now that's what I call HAULIN!!

926 MPH! That jet must have made quite the sonic boom hopping those islands.

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On a flight from Florida to the Dominican Republic the pilot let me use my Garmin Legend. It was fascinating watching the islands pass underneath and on the GPS itself. The maximum speed, which I have kept on the GPSr was 926 MPH. Now that's what I call HAULIN!!

I don't think that the GPS was reading accurately - unless you were on a concorde.

 

Most (non concorde/Tupolev Tu-144) commercial air liners cruise at 550+/-.

 

Speed of sound at cruising altitude is about 660 MPH/573 knots.

 

Of course, you may/probably know this, but just want to clarify.

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I've got a screen shot from a cache where my GPS hiccuped.

 

1373334_200.JPG

 

Screen shot taken with g7towin back in 2003.

Now that's geo dashing. Pretty fast for a guy with wet shoes.

Considering that that's about 5.6 x the speed of sound, you'll definitely never hear me coming.

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My ski buddies and me take our GPSr's with us when we ski each weekend and we have reached 60 mph (we hope we don't fall). We have decided that this is not healthy competition.

ROTF! My daughter likes to go down a hill near my parent's house on her scooter. She doesn't go very far up so I am not too worried about her going too fast. I, on the other hand, just had to go all the way up and clocked myself at 27mph. I am really glad I didn't wipe out. LOL! It was darn fun! Definately not something to challenge someone to a race with though.

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My ski buddies and me take our GPSr's with us when we ski each weekend and we have reached 60 mph (we hope we don't fall).

 

Last year (4/2/04) I hit 64 at Sugarloaf (iced) then next run went a lot faster but the GPS shut off due to low batteries!

 

So on skis: 64 recorded

Plane: 600 give or take

Boat: 40ish

Car: 147

Swingset: 12.5

 

Beat those numbers! Haha..

 

Randy

 

PS: And more amazingly, that car can get 30 mpg too...

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Most all commercial jets fly around 500 to 575 MPH. Now in some cases there may be a tail wind of up to 100 MPH. Safe crusing speed usually does not exceed 635 MPH. Now some fighter jets fly from 700 MPH to 1100 MPH. So I'm not sure what kind of planes some of you were on.. Maybe the SR71 Blackbird....

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Another Geocacher, Two Tracks, showed me where his MapVI showed a top MPH of 283. He noticed it right after having to slam on the brakes in his truck on the highway to avoid a collision. He wasn't traveling that fast, but his GPS was, toward the windshield!
I think Two Tracks was pulling your leg, the GPS could only go as fast as the car was traveling.
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Didja know that if you jump up at the last second when an elevator is free-falling, you can save yourself?

 

Slamming on the brakes doesn't make the contents of the car speed up. Now if his GPS hit the windshield and was damaged in some way...

Edited by Markwell
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I sometimes come home at the end of a day's hike and upload my tracks to mapping software. There's nearly always a mystery waypoint, where the GPS flipped out for a second and hopped me hundreds of feet away. It's usually proportional to what I'm doing, though: if I'm driving, the bad point could be miles from where I am, but if I'm walking, it's no more than hundreds of feet. That implies some degree of error checking.

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If you are in a car traveling 70 mph and quickly move your GPS forward towards the windshield, the GPS is technically going faster than 70 mph for that one fraction of a second. Are these units sensitive enough to pick up this small burst of speed? I know that mine has read a max speed of 80-some miles per hour when I was sure I had never gone faster than about 70 or 75 mph.

 

…Also, what effect does going around a curve have on the speed of your GPS? When there is a change in direction, there is an increase in velocity (I think I’m remembering that correctly from Physics class), but from the perspective of your tires (and therefore your car’s speedometer) the speed remains constant. Could this account for the discrepancy?

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If you are in a car traveling 70 mph and quickly move your GPS forward towards the windshield, the GPS is technically going faster than 70 mph for that one fraction of a second. Are these units sensitive enough to pick up this small burst of speed?

 

…Also, what effect does going around a curve have on the speed of your GPS? When there is a change in direction, there is an increase in velocity (I think I’m remembering that correctly from Physics class), but from the perspective of your tires (and therefore your car’s speedometer) the speed remains constant. Could this account for the discrepancy?

You would have to throw it very, very hard for it to make any difference.

 

When you change direction, you change the velocity (therefore there is acceleration, which you feel as you slide around in the seat) but you can't speak of increasing the velocity unless you increase the speed component.

 

Velocity has two components: speed, which has magnitude, and direction, which doesn't.

When you go round a corner, unless you brake, the speed component doesn't change.

 

In any case, the way your GPS measures speed is basically by seeing where you were X milliseconds ago and where you are now, then dividing the distance by the time. This has a lot of possibilities for error on each measurement, which greatly exceed the possible slight change in velocity over the period (because maybe halfway through the X milliseconds you turned the steering wheel). So it's a bit like ordering a 16-ounce Coke and worrying whether you got the last drip off the tap, while not specifying how much ice you wanted.

 

FWIW, my record max speed on my GPS is 224 km/h (about 140mph), on my way back home from having a turbo fitted to my car. The speedometer read 234 km/h (145mph) at that point. Previously I hadn't been able to get it over 185 km/h. This was all in Germany, where most major highways have no speed limits...

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Actually, I believe GPS speed is more a measure of doppler effect of the radio signal. Going around a corner won't bother it.

 

OTOH, if you throw your GPS forward while driving, it will go faster than the car. This is how you splatter folks in the cab of a pickup (open windows) from the bed--by throwing a watermelon forward against a telephone pole.

 

If you slam on the brakes (or run into something) and your GPS is unrestrained, it will be slowing from the initial speed--hence seat belts for people.

 

An irrationally high reading is more likely the result of losing/gaining a sat's signal IMO, with the resulting positional change.

 

(If you've ever watched your sat screen while standing still, if a new sat is locked on or one drops, the coord readout usually will change a few digits.)

 

Hence the reason to average coords over multiple days when hiding caches.

 

Lastly on speed vis-a-vis doppler effect, angle of incidence matters. If the current constellation is of sats is directly overhead (with commensurate poor accuracy) the speed readout will be less accurate. However on roads that's almost never the case and since we need triangulation for a fix, moving in any direction will provide good angle.

 

(Same phenomena that requires radar or lidar to be positioned in line with traffic for accurate speed readings.)

 

Enjoy,

 

Randy

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…Also, what effect does going around a curve have on the speed of your GPS? When there is a change in direction, there is an increase in velocity (I think I’m remembering that correctly from Physics class), but from the perspective of your tires (and therefore your car’s speedometer) the speed remains constant. Could this account for the discrepancy?

After thinking about it for a day, I'm not sure why I even posted this! It certainly wasn't my most brilliant moment. Thanks, everyone for not jumping on me for it. :D Are these forums getting friendlier??

 

I’m convinced that inaccuracies in max speed are because the unit can’t pinpoint your exact location. If your speed is determined by comparing where you were a millisecond ago to where you are now, unless it uses exact locations to calculate this, it will not be entirely precise.

 

I did a quick search of the web and couldn’t find a good explanation of how max speed works using GPS. Oh, well.

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On a flight from Florida to the Dominican Republic the pilot let me use my Garmin Legend.  It was fascinating watching the islands pass underneath and on the GPS itself.  The maximum speed, which I have kept on the GPSr was 926 MPH.  Now that's what I call HAULIN!!

I don't think that the GPS was reading accurately - unless you were on a concorde.

 

Most (non concorde/Tupolev Tu-144) commercial air liners cruise at 550+/-.

 

Speed of sound at cruising altitude is about 660 MPH/573 knots.

 

Of course, you may/probably know this, but just want to clarify.

I'm not trying to start anything, but the ONLY supersonic commercial jet was the Concorde, which only flew from London to New York. I think it also flew to/from Paris.

 

But more on topic, I was flying from Oakland to Atlanta and my yellow eTrex read 551 mph.

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You know I hadn;t really put any thought into the impossibility of the jet going over 900 miles per hour. There may have been issues with using it inside the plane, but the map feature was working and showed my progress both across Florida, the Ocean, and eventually landing in the Dominican Repulic.

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Another Geocacher, Two Tracks, showed me where his MapVI showed a top MPH of 283. He noticed it right after having to slam on the brakes in his truck on the highway to avoid a collision. He wasn't traveling that fast, but his GPS was, toward the windshield!
I think Two Tracks was pulling your leg, the GPS could only go as fast as the car was traveling.

Then what is that whole mass x velocity-thing? I mean, if you are in an auto accident, the mass of a human body increases exponentially during an abrupt stop, or did I misunderstand that science class completely? I'm not the sharpest pencil in the drawer, but I have read a few articles on why people need to wear their seatbelts. Why would this rule not apply to a GPS sitting on the dashboard?

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Another Geocacher, Two Tracks, showed me where his MapVI showed a top MPH of 283. He noticed it right after having to slam on the brakes in his truck on the highway to avoid a collision. He wasn't traveling that fast, but his GPS was, toward the windshield!
I think Two Tracks was pulling your leg, the GPS could only go as fast as the car was traveling.

Then what is that whole mass x velocity-thing? I mean, if you are in an auto accident, the mass of a human body increases exponentially during an abrupt stop, or did I misunderstand that science class completely? I'm not the sharpest pencil in the drawer, but I have read a few articles on why people need to wear their seatbelts. Why would this rule not apply to a GPS sitting on the dashboard?

Well there is the case of inertial mass vs. rest mass, but I don' t think that is what you're talking about (have to be going MUCH faster to see a measurable difference). When you're in a car, you are traveling at the same speed as the car (assuming you aren't running in the bed of a truck or some other problem).

Now if the car slows down slowly enough, the friction between your butt and the seat, or your feet and the floor or what ever is enough to keep your speed essentially the same as the car you are traveling in. Now if you slam on the brakes, or hit a tree, the car is slowing down much faster than friction can slow you down, so your butt leaves the seat, and you fly forwards through the windshield. At no time though, do you move any faster than your initial speed.

 

Now as far as the speed measurement in the GPS, my understanding is (based on the Garmin manual, and an e-mail off to Garmin), that they are VERY precise, and accurate, like in the region of +/- 0.2 m/s. Reason being is that the errors contributed by atmospheric interference are essentially constant over the time between the two readings, and cancel each other out. That isn't to say they can't hiccup though, when I first got my 60CS, it wasn't uncommon for me to see the my max speed at 500+km/hr. I've since updated the firmware, and that doesn't happen any more.

 

Ibycus (B.Sc Physics)

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For some reason it only just occured to me that you might be talking about weight. Your 'weight' is really just a measure of your accerlation. In Earth's gravity, generally you are pulled down by about 9.8m/s/s (or 32 ft/s/s) or 1 g. Now in a sudden stop the deceleration curve may in fact be exponential (well under anything but extremely controlled circumstances it will only be VERY approximately exponential), and hence your weight may increase exponentially.

Oh, BTW, bathroom scales don't actually measure your mass, they measure your weight, and then because we all (or at least most of us) live on the same planet, they divide by a constant conversion factor, and calibrate the scale in mass. If you don't believe me, take your bathroom scale in to the elevator, and see how it reads.

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Car:  147

I decided to use my GPS to find out the top speed of my Suburban when I happened to be on a deserted straight stretch of highway a few months back. I was kinda bent over the steering wheel so I could see the road as well as the GPSr which was laying on the dashboard.

 

The displayed speed was climbing in 7 to 8 mile per hour jumps.

 

About ate the window with the GPSr showing 99.7 MPH.

 

Turns out the ignition is programmed to cut out at 100. At that speed that's like putting the brakes on. The seat belt sure hurts when you don't expect the slowdown.

 

Oh and it wouldn't dream of getting 30MPG going downhill with a tailwind. I have a different car for that.

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