Is it possible to use Google Earth/Maps to know how big a track is???

malmo wrote:

Google Earth, GPS devices, etc, are all fun devices that will give you measurements "good enough for government work". They are NOT accurate.

Very interesting catchphrase to use in this context. Very much indicative of your ignorance on this topic, too.

The very CONCEPT of standardized weights and measures emerged from government work (usually military and nautical), as did the metric system, by far the most standardized set of measures we have. Government does poorly in many departments, but measures are not among them.

How about "good enough for Firestone's work"?

malmo wrote:

Interesting. I wonder if Google Earth can come within 2-3 meters of a measured distance of 400 meters if you didn't know beforehand what the distance was?

Couldn't agree with you more. That's why I asked people to participate in a little blind test a few years ago on the measurement forum. I only got two responses, but both were within 0.5% of the correct answer, which is the same as 2 meters on a 400-meter course.

http://measure.infopop.cc/eve/forums/a/tpc/f/69510622/m/965102185?r=7541021591#7541021591

(at the end of the thread)

With a track things are much less ambiguous than a road course, if you can see the lane lines(which isn't always the case). You can download the Google Earth path I posted earlier in this thread and move the my points around if you want. Any reasonable movement of the points that keeps them on the inside lane line won't change the overall distance of the path by more than a meter.

I agree with you though, that when someone comes on here and says they measured a certified 10k course with google earth and got exactly 10000 meters, it means nothing. If you're going to provide evidence that a measurement device is accurate it has to be some evidence that is well documented and/or repeatable. Otherwise it is of little value. But the same is true for people who come on here and say their GPS device gave 7 miles for their 6-mile course.

i got .2479 for my local track on gmap-pedometer

398.87 meters

good enough for training runs

dukerdog wrote:

I agree with you though, that when someone comes on here and says they measured a certified 10k course with google earth and got exactly 10000 meters, it means nothing.

Considering a perfectly measured certified 10K would be 10010 meters, that's right. ;-)

Well, my Christmas present this year was a Garmin nuvi 1300 so I'll have more data points to work with. I was out cruising the Garmin forums today both trying to figure out why Garmin's registration system seems to be broken ad to get more data about the basic operational characteristics. Garmin (and Magellan) seem to keep them pretty close to the vest. I see all sorts of information about how the triangulation works, etc, but not much about how often measurements are taken, what happens when the number of satellites available changes and other things that would impact the measurement.

One theme I saw running though both Google Earth and Garmin discussions was errors in correlation in satellite images and land coordinates. That may be an explanation for the periodically wacky results when drawing a map, and when running/riding a course and looking at the results on a map. In an area like ours that is criss-crossed with raveens the point A to B measures could change significantly if they are attempting to take into account elevation to measure distance.

The track around the corner from me would be a good example. There is a berm and creek running within feet of one side of the track. If the correlation of satellite images to elevation data is wrong, then that easily explains the fact that track shows an elevation change of almost 5 meters. (Hanes Park Track, Winston-Salem). If you look at the images you can clearly see the lime arrows on the service road we put down for the Ultimate Runner XC course the week before.

Keith Stone wrote:

So as far as that "am I making any sense to you", sure you are. I'd sure as s**t like to find a sure fire explanation. Maybe I'm too anal but when I go over the same route multiple times with the same tool and get different results it bothers me. As far as what goes in my log well who gives a crap. That's why I say if you're measuring a training run I don't give a rat's ass if you're measuring it with a 1968 VW Microbus. The gal in the typing pool you're trying to boink doesn't know the difference anyway.

Merry Christmas. If Santa brought you a Garmien Nuvo, I presumed that's the gold standard?

Aghast posted a link that adds more color to the algorithms behind GPS measurements. Check out figure 2 on this link:

While it's specifically dealing with tidal cycles it's clear that a statistical leveling is used smooth out the extraneous data spikes.

Now this link that expalins the general concept of triangulation (actually in 3D trilaterization)

The author of puts GPS accuracy at ."15 meter area" to "Garmin GPS systems featuring Wide Area Augmentation Systems can be accurate to less than three meters." and adds "Differential GPS, which corrects the GPS signals, can be accurate to within three to five meters."

The generally accepted value is 3-5 meters. Which I still find amazing. Think about it.

You need at least three satellites moving from different altitudes and at different speeds in different directions to make those instantaneous point fixes. Then you need to know the positions of the satellites -- so you need at least three more ground receivers to fix the position of those satellites. Only then can you trilaterate the ground position of your GPS device relative to the satellite.

Then you've got to do this all over again another 6 seconds down the road, where you have moved 3 meters, but the earth has moved you westward 45 meters, and each of the satellites have moved another 360 meters - all in different directions! The power of microchips.

malmo wrote:

Then you've got to do this all over again another 6 seconds down the road, where you have moved 3 meters, but the earth has moved you westward 45 meters, and each of the satellites have moved another 360 meters - all in different directions! The power of microchips.

I thought those numbers looked wrong. Change them to 450 meters and 3600 meters.

There was a tsunami detection system deployed a few years back that instead of GPS used a sensor that sat on the ocean floor and measured pressure. It could detect a rise on the surface of as little as 3 cm. Last I heard George Bush was going to put them all over the Pacific. Don't know if that ever happened.