How accurate is a GPS?

[JA-Q001]

So, while riding today and going uphill with the girlfriend, she wasnt happy with the slow speed we were going. This got me thinking, does a gps calculate speed only on a horrisontal scale or does it recon in the hight gained? If using a 45deg triangle to eyplain, horisontal distance would only be 1, while vertical is also 1, but you are actually traveling the vector, thats the squareroot of 2. When looking at a gps map, its totally flat, so does it then just measure the horisontal speed?

[Tankman]

you should spend more time looking at your girlfriend

[AndreZA]

It calculates the time it takes you between two points on the ground. The gradient has nothing to do with it. I doubt it can calculate speed if you travel 90° up.

[Racingsnail]

This is how I understand it...GPS uses 30 satellites in fixed orbits. In order to ascertain position you need to fix between 3 of these (triangualtion) The signal off a 4th satellite is used to solve for altitude.

 

The priciple is fairly simple: the individual satelite transmits a timing signal. When this is picked up by the receiver, the time it took to get there is used to calculate distance from that satellite. Do this same calculation off another 3 satellites and the intersecting arcs of distance give you a reasonably precise position in 3 dimentions.

 

JA-Q001 I had a similar question...

[Guest agteros]

Does a GPS point (not referring latitude/longitude) not also include height? Reason I'm asking is that if your GPS does not have a barometric altimeter, and altitude is calculated from the GPS data, when you stop the altitude does become quite erratic. I've seen this on quite a few gradient graphs, where the gradient spikes at that point to +20% and -20%

[JA-Q001]

So, it shows you the correct speed and distance only on a flat road. On uphills an downhills you actually go faster than the gps is saying?

[chris_w_65]

If you have a Garmin then best you ask them about the model you are using. I have confirmed with them that the 310XT uses the speed/cadence sensor to calculate speed if the sensor is present.

[DirtBag]

Hay Jaq, you are quite right. There is a slight.... very slight discrepancy in the speed reading, dependant on the steepness of the gradient you are riding.

The GPS calculates your distance as if looking straight from above, looking down at a map, so it does not take into account mr-Pythagoras's calculation.

But, as I say, the discrepancy is VERY SMALL!!

 

To give you an idea: if you do a 100km route, and your total ascent is 1400m, you can actually add the 1400m to your total, to give you a more accurate distance = 101.4km.

[Bloupanda]

This is how I understand it...GPS uses 30 satellites in fixed orbits. In order to ascertain position you need to fix between 3 of these (triangualtion) The signal off a 4th satellite is used to solve for altitude.

 

The priciple is fairly simple: the individual satelite transmits a timing signal. When this is picked up by the receiver, the time it took to get there is used to calculate distance from that satellite. Do this same calculation off another 3 satellites and the intersecting arcs of distance give you a reasonably precise position in 3 dimentions.

 

JA-Q001 I had a similar question...

Great explanation and right on the money

[Guest agteros]

Don't agree. Pythagoras statement l^2=x^2+y^2. to get l (actual distance) = SQRT(100,000m^2+1400m^2 ) = 100009.8m ... 100.0098km

 

Flame away

[chris_w_65]

If you have a Garmin then best you ask them about the model you are using. I have confirmed with them that the 310XT uses the speed/cadence sensor to calculate speed if the sensor is present.

 

Some Googling tells me the Edge 500 does the same

[chris_w_65]

Hay Jaq, you are quite right. There is a slight.... very slight discrepancy in the speed reading, dependant on the steepness of the gradient you are riding.

The GPS calculates your distance as if looking straight from above, looking down at a map, so it does not take into account mr-Pythagoras's calculation.

But, as I say, the discrepancy is VERY SMALL!!

 

To give you an idea: if you do a 100km route, and your total ascent is 1400m, you can actually add the 1400m to your total, to give you a more accurate distance = 101.4km.

 

I think your math is dodgy

[Guest agteros]

I think your math is dodgy

 

mi two Simplistic example above where you do not end at the same altitude/point as where you started shows for 1400m ascent (no descent) over 100km the error to be 0.0098% ?

[LeTurbo]

You guys are all a bit extreme. Should "anal retentive" have a hyphen?

[chris_w_65]

Mr.Bump

What GPS are you using? Is it one of the Garmins with a speed/cadence sensor fitted? (I think it is called a GSC10)

[Guest agteros]

You guys are all a bit extreme. Should "anal retentive" have a hyphen?

 

dunno, does it feel like it should have a hyphen?