Strava Elevation Question

[nickyboy]

interesting points, however, the speed sensor that came with my Edge 1000 does not use wheel circumference, and does not use a magnet on the spokes...

Another point, (from what I have read eleswhere), is that if you have weak signal on your iphone (such as when riding beneath tall trees, through a tunnel etc), Strava tends to give a faster speed in the areas where it looses signal when you record with your phone

Regarding losing GPS signal beneath trees etc...my Garmin shows an unusually slow speed while this is happening. It all sorts itself out when you emerge from the trees but the GPS trace will show slow in the trees, then a spike in speed when you reemerge and the device recalculates with a strong signal

 

[Tanis8472]

interesting points, however, the speed sensor that came with my Edge 1000 does not use wheel circumference, and does not use a magnet on the spokes...

Another point, (from what I have read eleswhere), is that if you have weak signal on your iphone (such as when riding beneath tall trees, through a tunnel etc), Strava tends to give a faster speed in the areas where it looses signal when you record with your phone

The speed sensor still uses wheel circumference to calculate speed. It needs to know it or it can't work out the speed

 

[jefmcg]

On elevations, if the google camera/recording car has not been on that road, the elevations are incorrect

1. The Google car has been pretty well everywhere in the UK, I think.
2. But this is irrelevant to strava as they stopped using Google maps about a year ago..

instead of just being tracked by a sattelite in a straight line .... thats the way I understand it, but may be wrong

Yeah, you are wrong. GPS uses 4 or more satellites to trilaterate your position in 3d space. Then x seconds later it records your position again, and uses a formula to estimate your route between those two points. Thus your wheel magnet would be more accurate as it records along your actual path, and not an estimate of it.

Of course it's not as simple as that. Altitude is also calculated with a barometer on some devices, and when you upload I think they correct altitude with map data. But again distance is calculated using 3 coordinates, lat, long and (possibly corrected) elevation.

 

[Dogtrousers]

But again distance is calculated using 3 coordinates, lat, long and (possibly corrected) elevation.

Can I just say that when I write code that deals with GPX files, which I do occasionally, I only use lat and long to calculate distances. Because life's too blimmin short to be worrying about elevation - and the data in there is a bit rubbishy sometimes.

So distance is calculated using 3 co-ordinates only if the person doing the calculation can be arsed so to do

 

[Ajax Bay]

The new Garmin speed sensor unit uses an internal accelerometer to measure revolutions, no magnets required. I'm sure it is as accurate as a speed sensor working with a spoke mounted magnet - at measuring revolutions per time unit, but no more accurate. But your Edge 1000 still needs to know the circumference of your wheel (the one that the sensor is attached to the hub of). Cross post with @Tanis8472 . @dim - what circumference is your wheel - to the nearest 2mm, please? Do you use a different wheel on the turbo? That will have a different circumference.

GPS uses 4 or more satellites to trilaterate your position in 3d space. Then x seconds later it records your position again, and uses a formula to estimate your route between those two points.

And what sort of error percentage might that typically result in, I wonder? Depends on the sampling rate (ie period between position records), I guess.

Altitude determination by GPS is a bit iffy, most of the time. Barometric readings can do a good job, provided they are regularly re-calibrated on a known height point as you pass: easy on foot, less so manually when riding, but you can input 'known point' heights.

 

[dim]

The new Garmin speed sensor unit uses an internal accelerometer to measure revolutions, no magnets required. I'm sure it is as accurate as a speed sensor working with a spoke mounted magnet - at measuring revolutions per time unit, but no more accurate. But your Edge 1000 still needs to know the circumference of your wheel (the one that the sensor is attached to the hub of). Cross post with @Tanis8472 . @dim - what circumference is your wheel - to the nearest 2mm, please? Do you use a different wheel on the turbo? That will have a different circumference.

And what sort of error percentage might that typically result in, I wonder? Depends on the sampling rate (ie period between position records), I guess.

Altitude determination by GPS is a bit iffy, most of the time. Barometric readings can do a good job, provided they are regularly re-calibrated on a known height point as you pass: easy on foot, less so manually when riding, but you can input 'known point' heights.

The Garmin edge 1000 speed sensor does not need a wheel size (as far as I am aware) .... you strap it around the hub and off you go .... I have not seen any setting on the Garmin Edge 1000 where one sets the wheel size .... I am not sure what happens on a turbo trainer though (I have not read the complete manual)

Here is a typical example of differences in elevation with Strava vs my Garmin Edge 1000:

I planned a 92.7 km route on RideWithGPS in combination with Strava Heatmaps, and I wanted to include a few hills. On the RideWithGPS, it says that the elevation for that ride is +702m and -703m:
https://ridewithgps.com/routes/17605671

I cycled the course, and when I downloaded the ride to Strava, it said that the elevation was 702 meters (the same as what RideWithGPS calculated).

Beneath the elevation section on Strava was a question mark .... I clicked on the question mark, and it changed the elevation to 526 meters (thats a difference of 177 meters) .... so which is the correct elevation? (Strava or RWGPS and Garmin) .... here's the ride on Strava:
https://www.strava.com/activities/774268861

Before the ride, I used the little man on RideWithGPS to check each road, and the google van with camera went on all those roads

 

[Tanis8472]

Believe it or not, it does still need to know wheel size.
My 500 has a setting somewhere where you can input the circumference or just let it work it out itself.
Don't know how it does it, but it does.

 

[dim]

Believe it or not, it does still need to know wheel size.
My 500 has a setting somewhere where you can input the circumference or just let it work it out itself.
Don't know how it does it, but it does.

on the new speed sensors, it automatically works out the wheel size:
https://forums.garmin.com/archive/index.php/t-303542.html

I also have a Garmin 500 that I use on my winter bike (Surly LHT) .... i have the old speed sensor with magnet, and I had to input the wheel size. But saying that, if you don't have the speed sensor with the garmins, it still works out the speed with satelites

 

[Tanis8472]

on the new speed sensors, it automatically works out the wheel size:
https://forums.garmin.com/archive/index.php/t-303542.html

I also have a Garmin 500 that I use on my winter bike (Surly LHT) .... i have the old speed sensor with magnet, and I had to input the wheel size. But saying that, if you don't have the speed sensor with the garmins, it still works out the speed with satelites

Which is what i was alluding to. Same as the 500 with a GSC10 Speed/cadence sensor. I never put a circumference figure in to it and its correct

 

[dim]

Which is what i was alluding to. Same as the 500 with a GSC10 Speed/cadence sensor. I never put a circumference figure in to it and its correct

to be honest, you don't really need any speed sensors with most Garmins as they work out the speed via satelites (like Strava on your phone) .... if you are using a turbo trainer, you would need the speed sensors.

 

[Ajax Bay]

That chat site said this:
"The wheel size can be set under the sensor details for the BSS (Bike Speed Sensor). The Edge will default to using an automatic wheel size if you do not set a size manually. The automatic wheel size is computed by the Edge using GPS data and a count of the number of wheel revolutions over a given GPS distance. This value is being calculated throughout your ride and adjusted."
If one thinks about this logically, and you do not set a wheel circumference by hand, the circumference is being 'set automatically' for which read 'deduced by assuming the accuracy of the GPS and making the speed sensor reading (rph) x wheel circumference (assumed) match". I hope you grasp the circular nature of this argument, with regard to one being more accurate. The readings from both should match. If you put the bike on a turbo and use the same wheel, the sensor will carry the assumed circumference into the speed calculation (useful). Less so if you use a dedicated turbo rear wheel (many do) and just swap the sensor over. It will assume you're on the road wheel - giving you an inaccurate reading unless the two tyres are exactly matched in circumference. But does it matter?
So to say the speed sensor will be more accurate is true only if you have set the circumference by hand at a value which is accurate to within 2mm.

I never put a circumference figure in to it and its correct

What do you mean by 'correct'? Do you mean 'is the same as the GPS'? Which system is more accurate?

On my Garmin, I also have the speed sensor. The speed sensor is more accurate, both for speed aswell as distance, as it takes into account the downhills and uphills instead of just being tracked by a sattelite in a straight line .... thats the way I understand it, but may be wrong . . . .
you don't really need any speed sensors with most Garmins

You were the one who said they were more accurate, implying they added value out of doors.
Regarding elevation, as I've already said, GPS (well the type/standard used for recreational purposes) is poor at determining your elevation at any time so the climb record cannot be expected to be accurate. I think the RwGPS planning tool gives good results and I have run a few checks comparing what it says for a long stretch of road with visual/manual counting of the contours on an OS 1:50,000 map.

 

[dim]

That chat site said this:
"The wheel size can be set under the sensor details for the BSS (Bike Speed Sensor). The Edge will default to using an automatic wheel size if you do not set a size manually. The automatic wheel size is computed by the Edge using GPS data and a count of the number of wheel revolutions over a given GPS distance. This value is being calculated throughout your ride and adjusted."
If one thinks about this logically, and you do not set a wheel circumference by hand, the circumference is being 'set automatically' for which read 'deduced by assuming the accuracy of the GPS and making the speed sensor reading (rph) x wheel circumference (assumed) match". I hope you grasp the circular nature of this argument, with regard to one being more accurate. The readings from both should match. If you put the bike on a turbo and use the same wheel, the sensor will carry the assumed circumference into the speed calculation (useful). Less so if you use a dedicated turbo rear wheel (many do) and just swap the sensor over. It will assume you're on the road wheel - giving you an inaccurate reading unless the two tyres are exactly matched in circumference. But does it matter?
So to say the speed sensor will be more accurate is true only if you have set the circumference by hand at a value which is accurate to within 2mm.

What do you mean by 'correct'? Do you mean 'is the same as the GPS'? Which system is more accurate?

You were the one who said they were more accurate, implying they added value out of doors.
Regarding elevation, as I've already said, GPS (well the type/standard used for recreational purposes) is poor at determining your elevation at any time so the climb record cannot be expected to be accurate. I think the RwGPS planning tool gives good results and I have run a few checks comparing what it says for a long stretch of road with visual/manual counting of the contours on an OS 1:50,000 map.

yes, they are more accurate, so I suppose that if you do time trials or triathlons, one would want the most accurate info. The new garmins use 2 different satelite systems to track you .... some switch off the Glosnass feature as the Glosnass uses lots of battery life. Apparantly the Glosnass is the most accurate system (I've only had my new Garmin for a few weeks and am still learning, so I may be wrong, but this is what I've read so far)

My question in difference of height still puzzles me though ....which is more accurate (Garmin in combo with Ride With GPS, or Strava)?

 

[Dogtrousers]

so which is the correct elevation? (Strava or RWGPS and Garmin) ....

This is a frequently asked question about elevations, "moving average" speeds, calories burnt and the like. The answer is that each is as correct as each other. Just choose one method and stick with it. Do not try to mix and match values produced by different algorithms and reference data sets. Choose the one that you like the most.

 

[nickyboy]

As an aside there is something rather odd going on with the way Strava calculates the average watts. Now I know that this number is at best a reasonable guess but I like it and it seems surprisingly consistent and ties in well with my perceived effort.

Say the average power is 150W on a ride. If I ask Strava to recalculate the elevation (there is a button to do this for when you have some GPS problem) then the elevation doesn't change but the average power always increases by about 10%. No idea what's going on

 

[ufkacbln]

As always... It is down to the algorithms and the way that the site / system applies these to the raw data

All you need to work out is how you fiddle that data to get the results you want