Different segment times on strava

davoj

Hi Guys,

I have noticed on a few bike rides with my mate that we have different times on segments even though we did the exact same pace up the hill from start to finish and it can be a difference of between 8sc to 50sc on each segment. I have a Bryton rider 20 and he has a Garmin 500 and i will always get a faster time with my Bryton (He is not happy).

I have also noticed that the Garmin always record that he has climbed a lot more than i did on the same ride even though i will have done more KM as i have to cycle to this house. Maybe this has something to do with it.

So i am wondering how accurate is Strava and has anyone else came across this?

Cheers

slowbike

It's not Strava that's accurate or not - it's the GPS device and how it's recording.

The device periodically creates a record of the current data:
Examples from a recent ride of mine (lon & lat changed to protect the innocent!):

    <trkseg>
      <trkpt lon="-0.797537687420845" lat="56.97728719934821">
        <ele>7.800000190734863</ele>
        <time>2013-08-13T16:06:34.000Z</time>
        <extensions>
          <gpxtpx:TrackPointExtension>
            <gpxtpx:atemp>20.0</gpxtpx:atemp>
            <gpxtpx:hr>99</gpxtpx:hr>
            <gpxtpx:cad>111</gpxtpx:cad>
          </gpxtpx:TrackPointExtension>
        </extensions>
      </trkpt>
      <trkpt lon="-0.7975745677947998" lat="56.977306477725506">
        <ele>7.800000190734863</ele>
        <time>2013-08-13T16:06:35.000Z</time>
        <extensions>
          <gpxtpx:TrackPointExtension>
            <gpxtpx:atemp>20.0</gpxtpx:atemp>
            <gpxtpx:hr>101</gpxtpx:hr>
            <gpxtpx:cad>103</gpxtpx:cad>
          </gpxtpx:TrackPointExtension>
        </extensions>
      </trkpt>

Each record is held in a <trkpt></trkpt> segment - I assume trkpt means Track Point - but it doesn't matter.
This recorded from an Edge 800 with HR monitor & Speed/cadence sensor - so it has a few extensions to the data set - specifically, temperature, heart rate, cadence. It will calculate the speed from the change in Lat/Lon over time.

You can see that my unit is creating a record once per second - which is ok, but has it's limitations.
At 20mph you will travel ~30 feet/10 meters each second. This impacts on when you started/finished a segment as Strava will have to extrapolate between points - on short segments this can be a significant proportion of the distance travelled and explain some differences in calculations between units.

sloppyschleckonds

Slowbike wrote:

It's not Strava that's accurate or not - it's the GPS device and how it's recording.
.

Or how Strava algorithms interpret the data.

adr82

SloppySchleckonds wrote:

Slowbike wrote:

It's not Strava that's accurate or not - it's the GPS device and how it's recording.
.

Or how Strava algorithms interpret the data.

"and" not "or", both your GPS and Strava contribute to this problem!

• Different GPS devices perform better than others. In my experience dedicated devices like Garmins tend to give you more accurate results than smartphones.
• It also depends on which satellites your GPS receiver can see at any given time, if it's only able to receive data from the minimum number and one keeps getting blocked by trees or buildings, accuracy is going to suffer. If you've ever seen a ride where the GPS track seems to wander dozens of metres off the road you were actually on, this is probably why.
• Most receivers are limited to updating their position once per second at most, so getting more fine-grained tracking is usually not possible.

As for Strava, it does some pretty braindead things with the data once you upload it, particularly on short, fast segments. Since it's extremely unlikely that a trackpoint from your ride will fall on the exact start and end points of a segment, they apparently use the closest trackpoints to the start and end of the segment instead. The obvious flaw with this is that the resulting distance for the whole segment will be a) different from the defined segment length and b) different for every person who's ridden it. So you end up riding the segment over a longer or shorter distance than it was actually created to cover. On short and fast segments this leads to ridiculous average speeds being posted. Strava in their wisdom apparently implement their average speed calculations as <time between your GPS start/end points> / <official segment length>, ignoring the fact that the distance between the start and end points from your GPS will almost never be the same as the segment length.

davoj wrote:

I have also noticed that the Garmin always record that he has climbed a lot more than i did on the same ride even though i will have done more KM as i have to cycle to this house. Maybe this has something to do with it.

Wouldn't rely on that elevation data too much either. Try using the "Elevation correction" feature in Strava - under the elevation reading for your ride, just above the map view, there should be some grey text saying "Elevation - ?". Click on that and then click the "Correct elevation" button. Not guaranteed to be entirely accurate either but probably better than the Garmin does a lot of the time.

NITR8s

Ive noticed that on ride with my friends who have garmins, my andriod version of strava will more than not put me about 1 sec behind their time. Even though we were cycling in a train. When this is over a distance of 5 mile + segments, I dont really worry about the slight inaccuracy.

davoj

adr82 wrote:

SloppySchleckonds wrote:

Slowbike wrote:

It's not Strava that's accurate or not - it's the GPS device and how it's recording.
.

Or how Strava algorithms interpret the data.

"and" not "or", both your GPS and Strava contribute to this problem!

• Different GPS devices perform better than others. In my experience dedicated devices like Garmins tend to give you more accurate results than smartphones.
• It also depends on which satellites your GPS receiver can see at any given time, if it's only able to receive data from the minimum number and one keeps getting blocked by trees or buildings, accuracy is going to suffer. If you've ever seen a ride where the GPS track seems to wander dozens of metres off the road you were actually on, this is probably why.
• Most receivers are limited to updating their position once per second at most, so getting more fine-grained tracking is usually not possible.

As for Strava, it does some pretty braindead things with the data once you upload it, particularly on short, fast segments. Since it's extremely unlikely that a trackpoint from your ride will fall on the exact start and end points of a segment, they apparently use the closest trackpoints to the start and end of the segment instead. The obvious flaw with this is that the resulting distance for the whole segment will be a) different from the defined segment length and b) different for every person who's ridden it. So you end up riding the segment over a longer or shorter distance than it was actually created to cover. On short and fast segments this leads to ridiculous average speeds being posted. Strava in their wisdom apparently implement their average speed calculations as <time between your GPS start/end points> / <official segment length>, ignoring the fact that the distance between the start and end points from your GPS will almost never be the same as the segment length.

davoj wrote:

I have also noticed that the Garmin always record that he has climbed a lot more than i did on the same ride even though i will have done more KM as i have to cycle to this house. Maybe this has something to do with it.

Wouldn't rely on that elevation data too much either. Try using the "Elevation correction" feature in Strava - under the elevation reading for your ride, just above the map view, there should be some grey text saying "Elevation - ?". Click on that and then click the "Correct elevation" button. Not guaranteed to be entirely accurate either but probably better than the Garmin does a lot of the time.

Cheers that helps a lot.