GPS speed vs. speed sensor speed

by truffy   August 18, 2014  

GPSs measure speed, no?

Speed sensors measure speed, naturally.

So, what is the point of adding a speed sensor to a GPS? Is one demonstrably more accurate? Are GPSs so bad at measuring speed that they need help?

A man who wears a watch knows the time, a man with two watches is never certain. Probably post-dates Confucius, that one.

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Whilst it is true that gps will measure speed and indeed, averaged out over a longer distance it is pretty accurate- there are several occasions when signal can be patchy. These small blips are often responsible for stupidly fast speeds on Strava.

Also, gps can struggle on hills as it 'sees' the world as generally flat and it will on occasion record 0mph or an inaccurate speed whilst you grind up a steep slope.

It also isn't as instant as a wheel sensor and has a subtle lag.

One or twice when riding through a heavily wooded route, my Garmin has recorded me as riding parallel to my actual route, in a nearby field so what hope did it have of recording an accurate speed?

I use gps for route plotting and recording and ignore the fact it measures speed- leave that to the wheel sensor.
The wheel sensor also overrides gps on my Garmin so you can't compare the vagrancies anyway.

Crosshair's picture

posted by Crosshair [11 posts]
18th August 2014 - 18:02

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Yeah, but if you get into a valley, tunnel or tree covered area GPS becomes pretty terrible at measuring speed. Where I live we have some decents down canyons and valleys, and if I ride a bike that doesn't have an ANT speed sensor my Garmin will often record speeds 2 to three times higher.

You also need one if you want speed measured on the turbo trainer, for obvious reasons!

posted by giobox [311 posts]
18th August 2014 - 19:09

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Thank you both! Valleys, tunnels and tree-covered areas are all relevant to me, which is why I was thinking of a unit that also supports GLONASS. In the Garmin range that limits me to the Edge 510 (no mapping) or the Edge 1000 (early beta version in the shops now! Rolling Eyes)

But GLONASS only offers better connectivity, I don't think it would address some of the other shortcomings of GPS-measured speed that you mention. So a dedicated speed sensor is in the bag then.

Thanks once again! Smile

posted by truffy [369 posts]
19th August 2014 - 9:18

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(The wheel sensor also overrides gps on my Garmin so you can't compare the vagrancies anyway.)

How do you get that to work - never seen that as an option. I have an edge 510

Rich

richdirector's picture

posted by richdirector [54 posts]
19th August 2014 - 11:06

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Rich- just pair the speed sensor and it starts using it. Switch on with no speed sensor and it defaults to gps. At least that's my understanding of how it works based on not reading the instructions Smile (edge 800)

The other day, my speed sensor broke and it read 0mph until I switched off and on again so I could continue the ride using gps.

Crosshair's picture

posted by Crosshair [11 posts]
19th August 2014 - 17:00

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Crosshair wrote:
Rich- just pair the speed sensor and it starts using it. Switch on with no speed sensor and it defaults to gps. At least that's my understanding of how it works based on not reading the instructions Smile (edge 800)

The other day, my speed sensor broke and it read 0mph until I switched off and on again so I could continue the ride using gps.

I think Crosshair is right, from what I remember the Edge computers do this automatically. Kind of makes sense- little point in using the GPS if the sensor is present.

posted by giobox [311 posts]
19th August 2014 - 20:36

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giobox wrote:
Crosshair wrote:
Rich- just pair the speed sensor and it starts using it. Switch on with no speed sensor and it defaults to gps. At least that's my understanding of how it works based on not reading the instructions Smile (edge 800)

The other day, my speed sensor broke and it read 0mph until I switched off and on again so I could continue the ride using gps.

I think Crosshair is right, from what I remember the Edge computers do this automatically. Kind of makes sense- little point in using the GPS if the sensor is present.

The Bryton i'm using does it as well - I wouldn't be surprised if it's just the way it's normally done on GPS head units in general, as you say it makes sense.

fukawitribe's picture

posted by fukawitribe [480 posts]
19th August 2014 - 21:01

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I seriously considered the Bryton Rider 60, but no GLONASS support Sad

posted by truffy [369 posts]
19th August 2014 - 21:10

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As others have said, GPS is sufficient for speed data on its own but pairing with a speed sensor will smooth data when the device's view of the sky is obscured (as by trees, tall buildings, etc.).

The difference may be minimal but I have noticed it occasionally so pair my Garmin Edge 510 with a wireless Ant+ speed sensor.

DC Rainmaker discusses it in his review of Garmin's new speed sensor.

Quote:
Here’s the order of precedence for Garmin units on the speed sensor:

1. Power meter sending torque and speed (e.g. PowerTap). In this case wheel speed is needed to compute power.
2. Speed sensor
3. GPS

Note that unlike running footpod’s, there is no method to change which sensor (or GPS) to use on a Garmin unit for cycling speed.

So would I recommend the speed sensor from a data standpoint? For most riders, they realistically won’t likely even notice the noise. Ultimately it’s giving you the basically same speed (albeit with a bit of noise that’s barely noticeable), and it’s giving you basically the same distance. But really for most folks would use GPS outside anyway, so it’s really just indoor training where you might be using some sort of app that utilizes speed.

posted by rosscado [9 posts]
19th August 2014 - 22:05

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It's all Глобальная навигационная спутниковая система to me Wink

posted by IHphoto [105 posts]
19th August 2014 - 22:29

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