# Question regarding strava accuracy



## cavo (Apr 18, 2011)

I have been wandering about this for a while now, but how reliable is when i compare 2 riders times for specific segments or ride with same route. Here is why i wonder: i mostly ride alone but once in a while i ride with a friend. When i compare our strava segments i am generally 0.5mph to 1mph faster according to strava. Both for segments and for overall average speed for same ride. Now when we ride together, i am obviously slower than him, but at the end our average speeds are either same or mine is a bit faster, by 0.2mph or so. This really puzzles me. Is it different phones that could do this? We both use smartphones to track, using viewranger and upload to strava. He claims he rides no faster when he rides with me. Why my stats would show faster speeds and times, while in reality i am slower? 
Thanks


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## *OneSpeed* (Oct 18, 2013)

Cell phone gps may only "ping" once every 2-5 seconds? I'm sure an iPhone is different from a Samsung too. If you want more accurate data get a proper GPS from Garmin or Wahoo or something. 

Phones aren't designed to accurately track distance, speed, elevation, etc. Bike GPS units are. Even a basic bike specific GPS Unit will give you accurate data.


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## Harold (Dec 23, 2003)

*OneSpeed* said:


> Cell phone gps may only "ping" once every 2-5 seconds? I'm sure an iPhone is different from a Samsung too. If you want more accurate data get a proper GPS from Garmin or Wahoo or something.
> 
> Phones aren't designed to accurately track distance, speed, elevation, etc. Bike GPS units are. Even a basic bike specific GPS Unit will give you accurate data.


Ping frequency is a biggie. And, when your recording interval is so low, it matters how closely that ping is to the actual start/end of the segment as to when Strava "matches" your ride to the segment. So there can be additional variation between the amount of time that Strava says you rode the segment, even if you and your riding buddy are the same distance apart the entire stretch of that segment.

What oftentimes seems to happen is that the person whose device has a more frequent recording interval or ping (and thereby most likely the more accurate device) is the one that's consistently marked as the slower rider in these cases. I say that because when a person who uses a phone to track compares to a riding buddy who uses a GPS to track is almost always marked as the faster one, even if they're not actually the faster one. There are a few other threads on this forum going back over the years discussing this same phenomenon with various devices in question.


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## J.B. Weld (Aug 13, 2012)

I've found strava segments to be extremely accurate, If I'm with someone and finish a segment 2 seconds faster than them that's what shows up on the strava feed.


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## Le Duke (Mar 23, 2009)

This picture will help explain the difference between two different GPS units with different reception/processing rates.

Imagine the trail is the black line on the right. And then, imagine that one person has a GPS that is processing incoming data at a lower rate (and calculating a new location solution less often). That person's route will look like the straight line on the left, or at least, straighter than a person whose GPS is processing data and producing a location solution more often.

Thus, the distance covered in a given amount of time, aka SPEED, will be different between the two of them.


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## J.B. Weld (Aug 13, 2012)

Le Duke said:


> This picture will help explain the difference between two different GPS units with different reception/processing rates.
> 
> Imagine the trail is the black line on the right. And then, imagine that one person has a GPS that is processing incoming data at a lower rate (and calculating a new location solution less often). That person's route will look like the straight line on the left, or at least, straighter than a person whose GPS is processing data and producing a location solution more often.
> 
> ...


Yes but strava knows the distance of each segment so the speeds will match.


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## Harold (Dec 23, 2003)

J.B. Weld said:


> Yes but strava knows the distance of each segment so the speeds will match.


But what matters when it comes to segments is the TIME. If the recording interval is very low and strava records you as on the segment 2 seconds AFTER your buddy who is using a much more accurate device which dropped a recorded point closer to the start of the segment, then your buddy will be listed as SLOWER because he spent more "time" on the segment than you did according to the devices you used respectively.


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## J.B. Weld (Aug 13, 2012)

Harold said:


> But what matters when it comes to segments is the TIME. If the recording interval is very low and strava records you as on the segment 2 seconds AFTER your buddy who is using a much more accurate device which dropped a recorded point closer to the start of the segment, then your buddy will be listed as SLOWER because he spent more "time" on the segment than you did according to the devices you used respectively.


True but that hasn't seemed to be much of a factor IME, in all the group rides I've done I've never seen results reversed on strava segments even when many of us had started and finished close together and were using various devices.


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## cavo (Apr 18, 2011)

is this reception rate what you speak of, the application setting of how often the location is pinged? we are both set at 1 second interval.


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## Harold (Dec 23, 2003)

J.B. Weld said:


> True but that hasn't seemed to be much of a factor IME, in all the group rides I've done I've never seen results reversed on strava segments even when many of us had started and finished close together and were using various devices.


It depends whether you care about a couple seconds or not. Some people nitpick over a couple seconds on strava segments and whether they "win" their group ride or whatever. I don't care and only compare segment times with my own, so I handle it roughly similarly as you. As indicated in this thread, though, some people care about those tiny details. We're pointing out that you can't care about those details unless you AND your buddies are using the most accurate recording options you can get.

And on a popular segment with the top 50 or so riders differing by only a couple seconds, device variations can mean the difference between a KOM or not. I've known people who KOM hunt and intentionally use lower accuracy devices so they can try to gain an advantage on timing with the lower frequency sampling. They don't care about actually being faster. They care about stealing the KOM.


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## Harold (Dec 23, 2003)

cavo said:


> is this reception rate what you speak of, the application setting of how often the location is pinged? we are both set at 1 second interval.


Yes, partly. But it can also depend on exactly when you each press the start button, and with how fast you're going when you cross the "start" and "end" of the segments. The two devices won't be recording points at exactly the same time, and the faster you're going, the farther those points will be from each other at any given time interval.


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## J.B. Weld (Aug 13, 2012)

Harold said:


> It depends whether you care about a couple seconds or not. Some people nitpick over a couple seconds on strava segments and whether they "win" their group ride or whatever.


Like I said, it's always seemed to come out right IME. Never lost a segment when I finished ahead of my riding partners by a second or 2 and vice-versa.


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## cavo (Apr 18, 2011)

J.B. Weld said:


> Like I said, it's always seemed to come out right IME. Never lost a segment when I finished ahead of my riding partners by a second or 2 and vice-versa.


i am not just referring to segments, also entire ride average speed for example, he is constantly dropping me by a good margin, yet my average speed for the ride is higher? makes no sense


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## J.B. Weld (Aug 13, 2012)

cavo said:


> i am not just referring to segments, also entire ride average speed for example, he is constantly dropping me by a good margin, yet my average speed for the ride is higher? makes no sense


Don't know, maybe your friend is just soft pedaling around while you catch up? That never happens to me on group rides, average speed always seems about right for everyone.


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## Harold (Dec 23, 2003)

cavo said:


> i am not just referring to segments, also entire ride average speed for example, he is constantly dropping me by a good margin, yet my average speed for the ride is higher? makes no sense


THAT will depend on the distance measured by each device, which is related to device accuracy, and Le Duke's post above.

Especially in a case where the GPS is NOT accurate and is adding distance by wandering all over. For a device that's calculating speed based purely on GPS data, if it's recording MORE distance than you're actually riding (in the same amount of time as your buddy), then it will say you did the ride FASTER.

This is because an ACCURATE GPS will always measure the distance you rode as shorter than what you actually rode (showing a slower avg speed). It will shortcut corners. A GPS that is INaccurate will wander, adding in extra distance. If the device is a little bit inaccurate, you may not notice it unless you closely analyze the track of your data on a map, because the total distance might be about what you actually rode, and will just make up for what gets lost by an accurate device.

I use a wheel sensor on my Garmin because I want to know the actual distance I rode, regardless of what my track says. Sometimes I'll ride through tunnels on the road. The GPS track can do some wacky $hit in a tunnel before it drops out completely, and while the wheel sensor won't fix the GPS data, at least it fixes the distance.


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## unerlaubt (Aug 2, 2019)

cavo said:


> I have been wandering about this for a while now, but how reliable is when i compare 2 riders times for specific segments or ride with same route. Here is why i wonder: i mostly ride alone but once in a while i ride with a friend. When i compare our strava segments i am generally 0.5mph to 1mph faster according to strava. Both for segments and for overall average speed for same ride. Now when we ride together, i am obviously slower than him, but at the end our average speeds are either same or mine is a bit faster, by 0.2mph or so. This really puzzles me. Is it different phones that could do this? We both use smartphones to track, using viewranger and upload to strava. He claims he rides no faster when he rides with me. Why my stats would show faster speeds and times, while in reality i am slower?
> Thanks


I've recorded a single ride with 3 devices simultaneously (fenix 5x watch, Edge 530, and Iphone6). You might be surprised to find out that the results differed between all of them! It has to do with the sampling rate and resulting track recorded as others have said. Further, Strava is not accurate in the way it interprets GPS data. In many cases it's just a dumpster fire when it come to KOMs and why they can't always be taken seriously.

Here's a segment I ride across frequently








Here is the KOM








Here is the QOM








These are extreme examples on a short segment, but many longer segments in my area have the same kind of errors, crediting rides that are not on route. Consumer GPS alone is not a high accuracy tool, apps processing data from a sloppy source just results in further error, and thus what you have experienced.


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## Harold (Dec 23, 2003)

holy hell, both of those are flag-worthy, but especially that QOM.


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## J.B. Weld (Aug 13, 2012)

As far as gps drift how does that affect segment times? Aren't they just timed?


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## Harold (Dec 23, 2003)

J.B. Weld said:


> As far as gps drift how does that affect segment times? Aren't they just timed?


I dunno the extent of it, but it DOES play a role in whether you are included in a segment or not. I have a feeling that poor GPS performance can affect the times you are considered "on" the segment as well as "off" of it.

I have a KOM on a trail that doesn't exist anymore (crappy trail) because of a GPS timing error that said I completed the segment in 0sec. It was a steep AF segment that the trail designer treated like a cyclocross course and wanted to force riders off their bikes. I could have reported myself, but the trail was so bad that I figured I'd leave it up for laughs and see if anyone else wanted the KOM bad enough to flag it for me. Nope.

If you are looking at trails that mostly just locals ride and you know most of the people on Strava who ride it, you can get a feel for the times that are legit and those that are due to pretty subtle errors. I've flagged a few over the years that were obvious errors. I've seen some that were included on the segment that were actually people riding an adjacent trail, or an old trail that was removed when the entire trail system was redone. I've flagged a few gravel road climbing segments where a shuttle rider obviously left their GPS running during the shuttle up, too. But when there are literally thousands of different people who have logged a time on a given segment, and you don't know anyone in the top 50 or 100 or whatever, it's hard to know what's an erroneous time is unless there's an obvious spatial problem, like the QOM posted above, or someone AVERAGES 30mph on a gravel climb that averages about 8% over 6mi and peaks around 15% as in the obvious shuttles that I've flagged.

I doubt any one of those that I flagged was a KOM maliciously taken. Most of them, Strava included a rider on a segment when they really weren't. Strava's fault for making the cutoff for being included on a segment a bit too loose. But of course they did that because people with crap hardware complained that they did a segment and weren't included on it.

On the shuttle rider one, they just most likely didn't care enough to think to pause the timer when they loaded up after their first descent.


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## unerlaubt (Aug 2, 2019)

J.B. Weld said:


> As far as gps drift how does that affect segment times? Aren't they just timed?


I don't think it comes down to just timing, there must be a geography component (subject to GPS drift) to it in order for the system to know you are on segment. A few weeks ago a guy set a KOM on a fast downhill segment. He crashed about 100' shy of the finish. He never crossed the finish point and headed back the other direction, presumably to nurse his wounds. Strangely, the Strava system still credited him with the KOM. I think the former KOM must have flagged him because it is gone now, I was looking for it to post as another example  Anyhow, between the gps errors and e-bike poachers I find the notion of KOMs to be silly.

Strava isn't like race timing at an organized event, but people sure treat it that way in my neck of the woods. I started noticing these discrepancies as me and my buddies were using it to compare efforts amongst ourselves when not riding together. I have a trail system at my doorstep and ride the same training loop many times per week for fitness. Between GPS drift and alternate lines, Strava can't account for fine details and a few seconds of difference. I find that it is good for timing longer segments and courses for comparing my results against myself. Very handy for practicing for XC races with defined start/finish points.


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## Harold (Dec 23, 2003)

Yeah, the variations inherent with using a GPS as your measuring device (and especially a phone with a poor GPS) become increasingly problematic the shorter the segment is. Strava even mentions this when you create a segment. And I know a few segments that are obvious plays on this, where they were created to be as short as Strava would allow on a straight, flat road segment just to mess with people.

Longer segments do tend to weed out a bunch of the chaff, but it's also more likely to fail to match someone to that segment.


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## Mudguard (Apr 14, 2009)

I think Strava also does some trimming itself. Like Harold I have a wheel sensor for Garmin, at the end of the ride, it syncs with my phone, sends it to Garmin and on to Strava. On longer rides Strava rides show up only about 100m shorter than the ride in Garmin connect. Which I figure is Strava trimming the ride a little based on whoever set the segments. 
I compared a 10km ride I did with a friend who used his phone, and me with my Garmin. The total difference variation was 100m, which is pretty good really. The GPS discrepancy is probably worse for shorter rides. I know of a 1m downhill that has an insane KOM, Strava has cut off the last 100m of the ride, as the trail has a hairpin about 100m before the finish but you also ride quite close to the finish point on the way down. So GPS wander has just put the rider straight into the finish instead of the last 200m of the trail.


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## J.B. Weld (Aug 13, 2012)

It seems like the segments around here are pretty legit, the fastest riders have the kom's and other people slot in the lineups about where they should be. There are some segments I've done over 100 times and I can usually guess by my efforts within a few seconds of what my time is before I see it on strava (I don't look at the times while riding) The few anomalies I've seen are obvious errors and are gone pretty quick.

So in my area anyway I trust the results on the leaderboards.


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## thasingletrackmastah (Nov 15, 2005)

J.B. Weld said:


> As far as gps drift how does that affect segment times? Aren't they just timed?


If your GPS does not know exactly where you are, the segment can't be started and stopped at the exact time.
It's like a race with a variable start and finish line.
We now know your gps does not know exactly where you are, and it only samples this data with a one second interval.
Example.
GPS drift is a dynamic thing. One second the GPS records a position a little to your left, next second a little to your right.
The worst your reception is, the bigger the inaccuracy.
Let's say you enter a segment, while your GPS is a little behind you.
In this situation you already started the segment, but your GPS and Strave don't know, the last sampeled position was 20m/60ft behind you.
This gives you a head start that is unheard of in a normal race.
At the end of the segment, things might as well have changed.
The gps accuracy/error/drift now places your position way ahead of you.
This makes Strave think youv'e already finished the segment, seconds before you actually do.
Strava is a hoax. A toy at best.


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## J.B. Weld (Aug 13, 2012)

thasingletrackmastah said:


> If your GPS does not know exactly where you are, the segment can't be started and stopped at the exact time.
> It's like a race with a variable start and finish line.
> We now know your gps does not know exactly where you are, and it only samples this data with a one second interval.
> Example.
> ...


As mentioned it seems pretty dang accurate for me, if you snag a kom around here you earned it.


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## chazpat (Sep 23, 2006)

Looks accurate to me.


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## *OneSpeed* (Oct 18, 2013)

chazpat said:


> View attachment 1325083
> 
> 
> Looks accurate to me.


I hear the Awesome Strap makes you faster, maybe try that?


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## Harold (Dec 23, 2003)

chazpat said:


> View attachment 1325083
> 
> 
> Looks accurate to me.


Yeah, that's a perfect example of one of those segments that's the minimum segment length, which happens to exaggerate errors with the GPS.


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## bholio2 (Oct 21, 2013)

Phone GPS and phone apps are not accurate at all. I use Strava and Wahoo to record my rides simultaneously. Strava so I get the safety beacon, and wahoo to get HRM. The final ride distance is different between the 2 apps by 10%+ quite often. 

When I ride with my son, he records with Strava on his phone. Many instances of 10% difference in overall distance for the same ride.

Its worst on windy singletrack, as explained above by the wavy line picture.

Being in the woods probably doesn't help. 

Just do the same loop 2 or 3 times and look at the resulting picture. If the loops are near identical, you've got a great GPS unit. If not, what kind of results can you reasonably expect?


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## offroadcmpr (Apr 21, 2012)

A relative of mine discovered an extreme example of this. She went hiking in the Narrows at Zion National Park and had Strava running. When she got back she was complaining how inaccurate it was, it showed that she had something like 8000 feet in climbing elevation.
When I looked at the elevation map you could see that it would occasionally jump between the top of the cliffs and the bottom, thus making it look like she climbed 1000 feet in seconds. This repeated a few times.


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## Harold (Dec 23, 2003)

offroadcmpr said:


> A relative of mine discovered an extreme example of this. She went hiking in the Narrows at Zion National Park and had Strava running. When she got back she was complaining how inaccurate it was, it showed that she had something like 8000 feet in climbing elevation.
> When I looked at the elevation map you could see that it would occasionally jump between the top of the cliffs and the bottom, thus making it look like she climbed 1000 feet in seconds. This repeated a few times.


That one is classic, and an example of a couple things.

First, it's an example of what happens when you don't have a barometric altimeter and are relying on Strava's elevation "corrections" which are dependent on the accuracy of your GPS. If your GPS is wrong, your spot elevation is, too. With the Narrows being a slot canyon, the margin for error is SUPER tight. Also being a slot canyon, GPS accuracy will be degraded because of the narrow view of the sky. This means that your GPS will be locating you on the rims on either side (as well as occasionally down in the bottom of the canyon) and doing a good bit more wandering than usual. Because Strava's elevation corrections are matching each determined position with a database of elevations (calculated from satellite imagery mostly, though some high grade elevation datasets now are done with LiDAR from airplanes), you'll be getting elevations up on the rim, and every time the GPS wanders to the other side, you'll get a corresponding sudden drop and then rise again in your elevation profile. That's going to add a MASSIVE amount of elevation that you never did since you were walking around in the bottom of the slot canyon the whole time.

It also hints at the problems of trying to use those high grade LiDAR datasets to determine elevations of poor quality GPS data. On the stuff Strava probably uses now, you're talking about a 30m^2 area that's given the same elevation. That'll tune out false elevation changes from a GPS that's wandering some. Some of that high grade LiDAR elevation data is providing 1m^2 to 6 inch resolution data, so in effect, it's more accurate than a consumer grade GPS receiver. The tiniest amount of wander in your GPS (even just sitting stationary on a picnic table) is going to add a whole bunch of elevation if the terrain around you is anything other than flat.


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## acedeuce802 (Jun 30, 2017)

To piggyback off the discussion 2 weeks ago, Strava segments are pretty accurate if the start and finish of the segment are relatively straight. Phone's generally update at 1Hz (whether Strava reads at 1Hz, I don't know), and then the app will likely interpolate in between those GPS points that are 1 second apart. If the lead into the segment isn't a huge change in speed or direction, the estimated location will likely be dead on (without regarding positional accuracy). All that really matters is the time you cross the starting gate and ending gate. GPS errors over tight switchbacks in the middle of the segment mean nothing, unless your GPS is bad enough that Strava believes you aren't on the trail (I believe they reject any segment with more than 10% trail deviation). 

Then phones are generally 5m accuracy, which isn't great. However 5m/s=11mph, which is fairly common for an average speed. So assuming worst case accuracy, you're likely only +/- 1 second. But keep in mind that GPS accuracy isn't like a 5m radius where the position bounces around randomly. GPS inaccuracies are generally a constant shift, which the shift will only change as the earth and satellites move, or the connection to the satellites changes drastically. Which again, if you lead into and out of the segment with speed, will have a minimal impact. You may start the segment 1s early, but then you'll finish it 1s early as well. 

The ride metrics that show up on the main activity screen are poor if calculated purely by GPS, as shown in the diagram earlier in the thread. Segments also get more and more accurate the longer they are. From what I've seen, segments below 30s are useless. Segments around 1 minute are okay but you have to take a couple seconds with a grain of salt, then anything a few minutes or longer is very accurate.


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