How do Garmins/GPSs cater for hills?

[Dave7]

If I can explain my query (excuse any incorrect terminology).
With a standard mechanical speedo it will lterally meaure the road distance.
With a GPS device (if I am correct) it measures/tracks your location.
Therefore if, as an extreme example I am riding down a straight 45 degree road for 7.5 miles I will only have moved approx 5 miles laterally.
Or to put my query another way......if I measure A to B on a map and it is eg 5 miles but it is actually up a 45 degree hill, have I not actually travel 7.5 miles ?
If I am misunderstanding this then the above could be a load of rubbish.

 

[JtB]

If your GPS reciever can see 4 satellites it can work out your position in 3D thereby taking account of hills.

 

[Drago]

A standard speedo measures your velocity along a constantly changing surface.

GPS measures your speed between 2 constantly recalculated locations in 3D space - the distance travelled through 3D space between 2 points may not be the same as the distance traversed along a surface. Even if a GPS can see 8 or 10 birds it only uses 2 at a time for the data required to feed the algorithm, albeit it does so bloody quickly so it'll make dozens of calculations between different satellites each second.

On a flattish, fairly representative road surface GPS is liable to be more accurate, measuring as it does your velocity between 2 points on a fairly steady plane, which will also correspond to the roads surface. Moving at speed in extremely hilly terrain, a wheel driven device is might theoretically be more accurate as it measures your progress along a surface, while the GPS would be measuring your speed through 3D space, which may differ to the length of actual surface being traversed.

At the sort of velocities a cyclist is liable to generate the GPS will be the best option. A hyper car, or a Kwak H2R or suchlike, and precision wheel driven device might be the more accurate on very hilly terrain.

 

[midlife]

I wonder if you were riding your bike up a vertical face if the GPS tracking across the ground will be zeo? I have no idea just curious .

 

[Drago]

Itll still track your location as you move vertically, although there is an altitude limit above which becomes inaccurate. That's what GPS does - locates items in 3D space. The speed calculation is a device based function off the back of the location data, and isn't universally accurate enough to be trusted for that purpose.

 

[Tommy2]

Depending what Garmin you have it may have an internal barometric altimeter to calculate altitude.
And don’t some others cross reference map data after uploading to correct elevation?

 

[Dave7]

Thanks all.
I confess that some of it is beyond my tiny mind but that bit about 3D is fascinating.
And at least I know my question was not "a load of rubbish".
By the way @Tommy2 my Garmin is a few years old and a basic model so probably lacks the barometric gizmo.

 

[steverob]

Thanks all.
I confess that some of it is beyond my tiny mind but that bit about 3D is fascinating.
And at least I know my question was not "a load of rubbish".
By the way @Tommy2 my Garmin is a few years old and a basic model so probably lacks the barometric gizmo.

I understand that any Garmin Edge from the 500 model upwards has a barometric altimeter, and that particular model came out in about 2010.

 

[Sharky]

I wonder if you were riding your bike up a vertical face if the GPS tracking across the ground will be zeo? I have no idea just curious .

I've ridden up a steepish hill at a very slow pace and my GPS went into "Auto Pause"!

 

[Hugh Manatee]

@Dave7 I know a bit about geodesy and it melts my brain sometimes. With all GNSS, the vertical element is the weaker dimension. At the high end, (a £20k Leica for example) this is because all the satellites are at more or less the same altitude which gives us good open triangles for the horizontal element but flat triangles for the vertical. When you're getting your height from the satellites as well, your vertical will be just about 1.5 times less good than your horizontal accuracy.

I think our bike based devices get the vertical from pressure and as someone else said, referring to the loaded map. I know to the nearest 10mm how high a nail is in my driveway. Sometimes my G armin can be only a metre or two off this. Other times 100m. Once you start your ride, it fixes this plane and takes your 3D speed and height gain from it. Even though the elevation is 100m out, the height gain at the end of the ride is usually pretty close.

 

[ColinJ]

If I can explain my query (excuse any incorrect terminology).
With a standard mechanical speedo it will lterally meaure the road distance.
With a GPS device (if I am correct) it measures/tracks your location.

Therefore if, as an extreme example I am riding down a straight 45 degree road for 7.5 miles I will only have moved approx 5 miles laterally.
Or to put my query another way......if I measure A to B on a map and it is eg 5 miles but it is actually up a 45 degree hill, have I not actually travel 7.5 miles ?

If I am misunderstanding this then the above could be a load of rubbish.

Don't worry about it! I plot long hilly routes on maps all the time and measure the rides with a GPS device and the differences are negligible.

Your imaginary 45 degree road should remain just that - imaginary. Nobody could build one and nobody would be able to use it. (Maybe you should worry about trains using steep funicular railways though ...? )

Here is a real world example, from just down the road to me ...

Assume that the 5.5 miles is along the road and assume that the gradient is constant (it isn't quite but it is pretty steady for most of the climb).

5.5 miles = 29,040 ft. By Pythagoras' theorem, the actual horizontal distance travelled would be 29,016 ft - by any reasonable standards that 24 ft difference isn't worth worrying about.

If you really want to worry about the numbers then you would probably have to take the curvature of the Earth into account too!

 

[Hugh Manatee]

Curvature of the Earth is definitely going to be included. All GNSS equipment however humble works off a geoid. Our bike gizmos will more than likely use WGS1984 or perhaps OSBG36(15) which is a transformation of ETRS(89).

 

[MontyVeda]

Depending what Garmin you have it may have an internal barometric altimeter to calculate altitude.
And don’t some others cross reference map data after uploading to correct elevation?

I posted a thread a few years back after Map My Ride recorded an 8 mile ride along a perfectly flat towpath as something really quite hilly... i recall the proposed reason was that it wasn't pinpointing my position very accurately, eg. it thought i was say twenty yards east of my actual position, thus giving me a hilly graph from map data. It was an old phone so probably no internal barometer.

 

[Dave7]

Don't worry about it! I plot long hilly routes on maps all the time and measure the rides with a GPS device and the differences are negligible.

Your imaginary 45 degree road should remain just that - imaginary. Nobody could build one and nobody would be able to use it.

Thanks.
Yes I realise that but was using that to illustrate my question.
I was surprised by your example of just how little difference it makes in real life though.

 

[steveindenmark]

Take what any gps records regarding altitude with a pinch of salt. The same type of device riding the same route can come up with different numbers.