How does a cycling computer know the gradient?

[Deleted member 121159]

I was just thinking, huh, how is my Wahoo recording the gradients of every single bit of road I go on? Does it somehow calculate it itself? or does it communicate with a satellite map where this information is stored? If latter, who went and measured the gradients of all roads?! I guess it can't be that, cos it shows the gradient when you go off road as well. But how does it generate that data??

 

[T.M.H.N.E.T]

Barometric altimeter built in vs GPS data

 

[ColinJ]

I think it is calculated based on the elevation changes and distance between points. That means that the displayed gradient figure always lags behind somewhat. It used to drive me mad to be told that I was going down a 20% descent when I had actually done that 10 seconds ago to cross a bridge and was now climbing at 20% on the other side! I turned the gradient display off.

 

[Deleted member 121159]

Ah of course, it's got a barometer inside. It was kind of stupid of me not to realise that!

 

[ColinJ]

Ah of course, it's got a barometer inside. It was kind of stupid of me not to realise that!

It is possible for GPS devices without barometers to calculate elevation but usually with a fairly wide range of error.

Having said that, my old barometer-free Garmin Etrex typically got it within 20 metres or so, and my Edge 500 which does have a barometer is often out by more than that, despite me setting the elevation at home before setting off.

 

[sleuthey]

It is possible for GPS devices without barometers to calculate elevation but usually with a fairly wide range of error.

Having said that, my old barometer-free Garmin Etrex typically got it within 20 metres or so, and my Edge 500 which does have a barometer is often out by more than that, despite me setting the elevation at home before setting off.

I have the cheapest smart phone you can buy in Argos. It runs Strava perfectly Inc elevation. The phone has a GPS but have always assumed the service pulls info from OS map data somehow to provide the elevation

 

[ColinJ]

I have the cheapest smart phone you can buy in Argos. It runs Strava perfectly Inc elevation. The phone has a GPS but have always assumed the service pulls info from OS map data somehow to provide the elevation

I wouldn't try to rely on a phone signal in many of the more interesting cycling areas up here...!

 

[tyred]

Ah of course, it's got a barometer inside. It was kind of stupid of me not to realise that!

You need to tap the barometer for best results.

 

[sleuthey]

I wouldn't try to rely on a phone signal in many of the more interesting cycling areas up here...!

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The old phone I use in the car for Android Auto has no SIM card, you can download the maps for some but not all aps.

 

[ColinJ]

The old phone I use in the car for Android Auto has no SIM card, you can download the maps for some but not all aps.

In that case it IS using GPS!

 

[ColinJ]

In that case it IS using GPS!

I could use Memory Map on my phone (using GPS) but I don't think the app tells me the elevation directly. I could look at the map to read it off that.

 

[freiston]

From what I understand, both barometric and GPS elevation calculation are problematic and not particularly accurate. Garmin Connect offers an elevation correction that uses survey data instead of device data. When applied, my total ascent/descent always goes down for a ride. My device is a Garmin Edge 530 and I understand that this uses barometric pressure.
I vaguely recall a thread on another forum where one of the members is a bit of an expert on digital mapping/navigation (he is behind a popular online cycle route planner website) and, as I recall it, he explained that online route planners also have discrepancies in ascent/descent stats due to algorithms used to deduce the route elevation from available (survey/map) data.

 

[figbat]

It must be map data as well, since it will show you upcoming gradients as well as what you’ve ridden.

 

[freiston]

It must be map data as well, since it will show you upcoming gradients as well as what you’ve ridden.

I wonder if and how they're "linked" - the upcoming gradient could be from map data but the "ridden" gradient might be from recorded GPS/barometer alone.

 

[Deleted member 121159]

Also silly of me to forget that most online maps show elevation data, so grades should be able to be calculated from that too.

When I make a route on Komoot and ride it, the Wahoo always comes up with quite a bit higher total elevation - it'd go from say 2500ft to 3000ft. I always go by the higher number as that makes me feel better but now I'm not so sure if I should!