So, your Garmin is most accurate for Strava hey?

[Stretch]

 

Source?

 

Except for topographical mapping there's no way the satellite can know your height asl

 

You must remember that a satellite can determine how far it is from your device and the fact they you need at least three satellites to determine your x y and z..... finally you need to read up about 3D trilateration to understand exactly how this works

[Pulse]

That's some subtle stuff there hey

 

Jip! Don't know how much difference it will make.

[fandacious]

 

 

You must remember that a satellite can determine how far it is from your device and the fact they you need at least three satellites to determine your x y and z..... finally you need to read up about 3D trilateration to understand exactly how this works

 

There's no way a satellite can determine how far it is from your device.

 

The satellite doesn't even know about your device.

[Slowbee]

maybe a satellite can know your heigh asl, but my garmin sure has heck doesnt know it.

 

I just read the the 705 is barometric, so no amount of satellites going to help me

[FlandersZA]

Jip! Don't know how much difference it will make.

For the ride pictured in the OP, it is reading 1.5km short for that 40km route. If an entire route was filled with these windy curved bits it would be miles out. On a straight line road ride or similar it's pretty much spot on. Hopefully by upping the recording interval it will solve the problem (although increase file size).

[Stretch]

There's no way a satellite can determine how far it is from your device.

 

The satellite doesn't even know about your device.

 

A GPS receiver's job is to locate four or more of these satellites, figure out the distanc­e to each, and use this information to deduce its own location. This operation is based on a simple mathematical principle called trilateration.

 

f you know you are 10 miles from satellite A in the sky, you could be anywhere on the surface of a huge, imaginary sphere with a 10-mile radius. If you also know you are 15 miles from satellite B, you can overlap the first sphere with another, larger sphere. The spheres intersect in a perfect circle. If you know the distance to a third satellite, you get a third sphere, which intersects with this circle at two points.

The Earth itself can act as a fourth sphere -- only one of the two possible points will actually be on the surface of the planet, so you can eliminate the one in space. Receivers generally look to four or more satellites, however, to improve accuracy and provide precise altitude information.

In order to make this simple calculation, then, the GPS receiver has to know two things:

• The location of at least three satellites above you
  
• The distance between you and each of those satellites
  

The GPS receiver figures both of these things out by analyzing high-frequency, low-power radio signalsfrom the GPS satellites. Better units have multiple receivers, so they can pick up signals from several satellites simultaneously.

Radio waves are electromagnetic energy, which means they travel at the speed of light (about 186,000 miles per second, 300,000 km per second in a vacuum). The receiver can figure out how far the signal has traveled by timing how long it took the signal to arrive. In the next section, we'll see how the receiver and satellite work together to make this measurement.

And then it gets more complicated....

At a particular time (let's say midnight), the satellite begins transmitting a long, digital pattern called a pseudo-random code. The receiver begins running the same digital pattern also exactly at midnight. When the satellite's signal reaches the receiver, its transmission of the pattern will lag a bit behind the receiver's playing of the pattern.

The length of the delay is equal to the signal's travel time. The receiver multiplies this time by the speed of light to determine how far the signal traveled. Assuming the signal traveled in a straight line, this is the distance from receiver to satellite.

In order to make this measurement, the receiver and satellite both need clocks that can be synchronized down to the nanosecond. To make a satellite positioning system using only synchronized clocks, you would need to have atomic clocks not only on all the satellites, but also in the receiver itself. But atomic clocks cost somewhere between $50,000 and $100,000, which makes them a just a bit too expensive for everyday consumer use.

The Global Positioning System has a clever, effective solution to this problem. Every satellite contains an expensive atomic clock, but the receiver itself uses an ordinary quartz clock, which it constantly resets. In a nutshell, the receiver looks at incoming signals from four or more satellites and gauges its own inaccuracy. In other words, there is only one value for the "current time" that the receiver can use. The correct time value will cause all of the signals that the receiver is receiving to align at a single point in space. That time value is the time value held by the atomic clocks in all of the satellites. So the receiver sets its clock to that time value, and it then has the same time value that all the atomic clocks in all of the satellites have. The GPS receiver gets atomic clock accuracy "for free."

When you measure the distance to four located satellites, you can draw four spheres that all intersect at one point. Three spheres will intersect even if your numbers are way off, but four spheres will not intersect at one point if you've measured incorrectly. Since the receiver makes all its distance measurements using its own built-in clock, the distances will all be proportionally incorrect.

The receiver can easily calculate the necessary adjustment that will cause the four spheres to intersect at one point. Based on this, it resets its clock to be in sync with the satellite's atomic clock. The receiver does this constantly whenever it's on, which means it is nearly as accurate as the expensive atomic clocks in the satellites.

In order for the distance information to be of any use, the receiver also has to know where the satellites actually are. This isn't particularly difficult because the satellites travel in very high and predictable orbits. The GPS receiver simply stores an almanac that tells it where every satellite should be at any given time. Things like the pull of the moon and the sun do change the satellites' orbits very slightly, but the Department of Defense constantly monitors their exact positions and transmits any adjustments to all GPS receivers as part of the satellites' signals.

[JannievanZyl]

There's no way a satellite can determine how far it is from your device.

 

The satellite doesn't even know about your device.

No, but your device knows how far it is from every satellite it can see. How else can it triangulate your position? Hint: It's all about timing.

 

I see Strech gave an explanation above. I was just going to suggest you type in "how does a GPS calculate elevation" into Google.

[fandacious]

The deviations of the EGM96 geoid from the WGS 84 reference ellipsoid range from about −105 m to about +85 m

 

 

and

 

The GPS receiver does need to know the shape of the Earth too, since it's not exactly spherical, but there is a standard geoid model used for that. It's not exact though which is why altitude information isn't considered as accurate for GPS as position

 

and

 

http://en.wikipedia.org/wiki/Altimeter

Global Positioning System (GPS) receivers can also determine altitude by trilateration with four or more satellites. In aircraft, altitude determined using autonomous GPS is not precise or accurate enough to supersede the pressure altimeter without using some method of augmentation. In hiking and climbing, it is not uncommon to find that the altitude measured by GPS is off by as much as 400 feet depending on satellite orientation

Edited July 23, 2014 by fandacious

[NotSoBigBen]

I'm just happy it tells me more or less how far I've gone or have to go in an 'event', can help me follow a route or my way back home/to the start AND I don't need those damn temperamental sensors with magnets and stuff ....

Edited July 23, 2014 by NotSoBigBen

[Wez-O]

I rode with a rider using Strava on an Android device while I was using Strava on a Garmin and our segments matched pretty much to the second.

[Mileage]

Having just had a quick glance over the posts so far I don't see a mention of the type of antenna that the various models have.

Looking at the pictures above and the descriptions of the routes cycled it seems that the signal is degraded under tree cover or in urban areas which is common with patch antennas.

The cycle and running oriented models have a "patch" antenna but the fancier and bigger models used for hiking navigation have a "quad helix" which is believed to be more accurate under cover. Also as cycling and running GPS’ need to be quite small they normally have the smallest possible antenna required to function adequately.

I have a Garmin GPSMAP 62s and it is extremely accurate for the rides Ive used it on. Messing around with the waypoint averaging ive got spot accuracies of about 30cm.

[Long Wheel Base]

I rode with a rider using Strava on an Android device while I was using Strava on a Garmin and our segments matched pretty much to the second.

Must have been a flipping good cyclist because usually the guys are a few seconds behind you!!

[FlandersZA]

I rode with a rider using Strava on an Android device while I was using Strava on a Garmin and our segments matched pretty much to the second.

What I've found is that the timing on segments matches up across platforms - ie. they all agree that x seconds to get from point a to point b is the same thing. Difference comes in on the summary of a ride with regard to average speed, elevation gain and, to a lesser degree, distance.

[FlandersZA]

http://img.photobucket.com/albums/v648/flannahs/nf_garmin_eTrex_highRate_zps897f872b.jpg

 

This is Saturday's ride result. I set the garmin to record at its highest rate by distance - every 0.01km. A bit of an improvement but still a bit disappointing. The ride was 80km and GPX file size ended up being 1.3mb with under-read of +-1km. The only other way to get an improvement would be to set it to record by time interval but I think this could cause file size to become unworkable as it would also record at non-moving points.

[Tankman]

Accurate info for what it was intended for.

 

I cant remember, what was the yellow brick intended for again?

 

I use it for cycling, hiking and sea kayaking. When I use it for cycling I have it in my back pack and use it to upload rides to strava. For hiking I've used it as a handheld device and it tells me direction and distance to waypoints. Similar story with the fishing ski but on the sea it's in compass mode and tells me distance to waypoints too. Handy little device that.

 

What you can use if for and what it was intended for is not the same.

 

You can use a Mini to pull a double-wheeler Jurgers caravan ... but that was not what it was intended for.

 

A unit intended for cycling, with 1,3,5sec or smart recording would be the Garmin Edge 500, 510, 800 or 810.

 

Use the right tool for the job and you will get your pretty pictures more accurate.

[FlandersZA]

 

 

 

What you can use if for and what it was intended for is not the same.

 

You can use a Mini to pull a double-wheeler Jurgers caravan ... but that was not what it was intended for.

 

A unit intended for cycling, with 1,3,5sec or smart recording would be the Garmin Edge 500, 510, 800 or 810.

 

Use the right tool for the job and you will get your pretty pictures more accurate.

 

The eTrex can record at one second intervals. It can also record at distance intervals. I just haven't tried it on time-based intervals yet. I have another device to read out my speed etc. but ya, thanks for that.