Garmin gps data?

zill
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Garmin gps data?

Postby zill » Sat Nov 22, 2014 8:56 pm

How does the garmin GPS speed reading compare with a sensor speed reading?

lard
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Re: Garmin gps data?

Postby lard » Sat Nov 22, 2014 9:19 pm

It partly depends on the conditions. GPS is accurate with within a few meters. Your wheel speed sensor is as accurate as you've calibrated it, and may vary with tyre changes, tyre pressure and wheel spin. Generally GPS is very accurate, for road riding I'd trust it way more than a wheel sensor. However if you're riding tight twisty single track on the mtb, I'd be more inclined to trust the wheel sensor as in these conditions GPS struggle a bit, mainly due to the slight inaccuracies and the sampling rate.

zill
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Re: Garmin gps data?

Postby zill » Sun Nov 23, 2014 7:01 am

lard wrote:It partly depends on the conditions. GPS is accurate with within a few meters. Your wheel speed sensor is as accurate as you've calibrated it, and may vary with tyre changes, tyre pressure and wheel spin. Generally GPS is very accurate, for road riding I'd trust it way more than a wheel sensor. However if you're riding tight twisty single track on the mtb, I'd be more inclined to trust the wheel sensor as in these conditions GPS struggle a bit, mainly due to the slight inaccuracies and the sampling rate.


Interesting. Will keep that in mind!

mick243
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Re: Garmin gps data?

Postby mick243 » Sun Nov 23, 2014 12:20 pm

A stationary gps is accurate to within a few meters, and will often still show a little movement.

A moving gps can be accurate to within inches (in perfect conditions) and it's 3d velocity is VERY accurate - with the understanding of what it's measuring... It's not measuring current velocity, it's measuring time between the last two valid fixes, so it's readout is the speed you just did, not a problem but if you change speed a lot and often, something you need to consider.

zill
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Re: Garmin gps data?

Postby zill » Sun Nov 23, 2014 5:28 pm

mick243 wrote:A stationary gps is accurate to within a few meters, and will often still show a little movement.

A moving gps can be accurate to within inches (in perfect conditions) and it's 3d velocity is VERY accurate - with the understanding of what it's measuring... It's not measuring current velocity, it's measuring time between the last two valid fixes, so it's readout is the speed you just did, not a problem but if you change speed a lot and often, something you need to consider.


So does the Garmin use a stationary or moving GPS?

Also, if I was changing speeds often, I probably would not bother looking at my Garmin during the accelerations! It's only when I am holding my speed that I will have a glance at the Garmin.

mick243
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Re: Garmin gps data?

Postby mick243 » Sun Nov 23, 2014 6:18 pm

zill wrote:
mick243 wrote:A stationary gps is accurate to within a few meters, and will often still show a little movement.

A moving gps can be accurate to within inches (in perfect conditions) and it's 3d velocity is VERY accurate - with the understanding of what it's measuring... It's not measuring current velocity, it's measuring time between the last two valid fixes, so it's readout is the speed you just did, not a problem but if you change speed a lot and often, something you need to consider.


So does the Garmin use a stationary or moving GPS?

Also, if I was changing speeds often, I probably would not bother looking at my Garmin during the accelerations! It's only when I am holding my speed that I will have a glance at the Garmin.


Ummm, it is stationary if your not moving it, if it's moving it's moving.....


All GPS receivers work the same way, (yes, some are better than others) when it's stationary, various errors in the signal propagation effect it's readings, as it moves, some of these errors are able to be nullified, therefore (within reason) a moving GPS receiver is more accurate than a not moving (stationary) one.

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biker jk
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Re: Garmin gps data?

Postby biker jk » Sun Nov 23, 2014 7:05 pm

Note that if you use the wheel magnet speed sensor with your Garmin it defaults to this to measure your speed rather than GPS. This is more accurate than the GPS based speed measurement.

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simonn
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Re: Garmin gps data?

Postby simonn » Sun Nov 23, 2014 8:26 pm

lard wrote:It partly depends on the conditions. GPS is accurate with within a few meters. Your wheel speed sensor is as accurate as you've calibrated it, and may vary with tyre changes, tyre pressure and wheel spin.


A wheel sensor takes one sample every diameter of the wheel including tyre. It will therefore be accurate to the diameter of the wheel (including tyre, and assuming you have calibrated it correctly and it has not changed during the ride) and a little bit.

In my experience wheel/cadence sensors are more accurate than just a gps for short segments (i.e. speed you are going now).

mick243 wrote:it's readout is the speed you just did


<pedant>
Speed = distance / time

Ergo, your speed is always the speed you just did.
</pedant>

It is certainly the same with a wheel sensor too.

zill
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Re: Garmin gps data?

Postby zill » Sun Nov 23, 2014 11:04 pm

simonn wrote:
In my experience wheel/cadence sensors are more accurate than just a gps for short segments (i.e. speed you are going now).




So the gps is more accurate the faster you go?

I find that hard to believe. Why is that?

mick243
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Re: Garmin gps data?

Postby mick243 » Mon Nov 24, 2014 8:22 am

zill wrote:
simonn wrote:
In my experience wheel/cadence sensors are more accurate than just a gps for short segments (i.e. speed you are going now).




So the gps is more accurate the faster you go?

I find that hard to believe. Why is that?


all to do with introduced errors in the radio signals that the gps reciever is measuring to work out its position fix - things like humidity, temperature inversion layers, multipath signal reflections that the reciever has to process before it can work out what signals are valid and what arent. the military grade gps is more accurate because it uses more frequencies, and therefore has far more data to work with to cancel out the errors.

when its sitting stationary, if you zoom the screen in to the maximum amount, you generally see that it thinks its always moving a tiny bit every which way, if you track this over a few days, it makes for a "ball of wool"on the track plot, usually a couple of meters across (back in the bad old days of "SA" (selective availability, a US govt system to deliberateley degrade GPS fixes for its enemies) the ball of wool would (in australia) be 150-200m across (in other areas it could be several kilometers across, thankfully, SA has been disabled). when you move the reciever at a relativeley constant rate (say, walking, bike ride, drive car) some of the errors get so large so fast that the reciever can figure out what to discard much faster, whilst other errors integrate over time to effectivley become zero. its not quite "the faster you go the better", but at human powered velocities that pretty much holds true. otoh, stick a basic hiking gps in a supersonic jet and the rapid velicity added to potential rapid altitude changes can confuse them very quickly (aviation gps units tend to have bettrer recievers, more processing power and better software loads, however the old garmin "GPS2+" units were fine at general aviation speeds and altitudes (nobody would lend me a fast jet to play with when I was doing my adv dip....))

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