Why does a gps need a speed sensor?

[NotSoBigBen]

I only use the sensor indoors and even that is only in exceptional circumstances ... it's rained for a month for example. Outdoors especially on MTB those Garmin sensors are crap!

[lerouc]

also... riding an incline your distance you cover is further than the straight flat surface a gps pics up. basic trig. So the speed sensor is a more accurate reading aswell as cadence senser 99% of the time. and you get speed on a IDT because they usually mount on the rear stay and wheel

 

So you saying all those clever okes designing these units has not taken into account the distance you travel on a incline or decline?

[no calves]

So you saying all those clever okes designing these units has not taken into account the distance you travel on a incline or decline?

Im saying a gps looks straight down and calculates speed as if its a flat surface.

[BDF]

Im saying a gps looks straight down and calculates speed as if its a flat surface.

 

Garmin Fenix 3 can calculate 3D distance.

[Paul Ruinaard]

GPS is a very weak signal so the wheel sensors help when in forests or where the signal is lost.

 

That said GPS is very bad at working out vertical position and often uses information like barometric pressure to assist it in doing the calculation.

 

Maps are kind of 2D in a GPS world.

 

To get them 3D requires lots of maths and some assistance.

 

It also depends on how many satellites you have got communicating with your head unit (channels etc) and then where they are in relation to you. Plus the communication protocols etc.

 

Lots of maths happening in that little unit - and very quickly.

 

Hence you get the setting on Strava and other download sites to"correct" your GPS tracks as the head unit makes some assumptions around where you are if it cant work it out.

 

At a very granular level you can see this - one of the sites (I think it was DC Rainmaker) compared a Garmin and a Suunto. When the Suunto didn't know its position it just drew a straight line to the next point it had a fix on and because of this it  often got distance covered wrong.

 

Also depending on your sampling intervals (even at 1 second) you are going to be plotting a series of lines - especially moving at high speed.

[Chris NewbyFraser]

Hmmm! so it seems that for our truck / sorry, tandem, the standard GPS measuring is fine. Tandems don't do too well on rollers and the wife she don't ride if it's misty/cold/miserable/hungover or if I forgot her birthday (when was it again?).

[Patchelicious]

Im saying a gps looks straight down and calculates speed as if its a flat surface.

No it doesn't. It calculates variances in elevation as speed too.