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Elevation data... Which is the most accurate?

skatey-mark's picture

Last weekend, RoadSkater & I were talking about elevation data while on the scenic 51-mile "Hannah" route...  I unfortunately lost my data from that skate due to a hard drive crash, so I can't compare against his.  But it did get me thinking...

 

Which elevation data is most accurate?

1) Polar's barometric sensor

2) Garmin's GPS sensor

3) NASA's 30m elevation data (i.e., on gpsvisualizer.com)

 

My initial thought regarding the NASA data was that it didn't sound very "granular".  However, I did some rough calculations the other day, and just now put some numbers into a calculator and came up with this...

If your watch/gps is set to record data every 5 seconds, then if you're travelling at 6 m/s then you're capturing elevation data at the same granularity of the NASA data.  6 m/s = 13.4 mph.  So if you're going slower than 13.4 mph (and sampling every 5 seconds) then you are (in theory) capturing data at a higher resolution than NASA.

You can increase the resolution by taking more samples.  My Polar HRM can record every 1s, 2s, 5s, 15s or 30s.  So in my case, I set it to sample every 2 seconds on my last skate.  That means I can achieve a higher resolution than the NASA data if I'm skating less than 15 m/s, or 33 mph.  

That assumes, of course, that the whatever sensor you're using (barometric or gps) is as accurate at measuring elevation as whatever method NASA used.

 

With all that in mind, I graphed my elevation data in Polar's software (which uses the barometric sensor) and gpsvisualizer.com.  Polar doesn't save the elevation data in the gpx file, so the only way to draw an elevation profile on gpsvisualizer.com is to use the NASA data.  The results were mildly surprising...

 

Polar (barometric)  gpsvisualizer (NASA) 
   

 

Looking at the charts, the NASA data appears to be more granular than the data I captured, despite the fact that I was capturing many more data points.  My average speed was 13.9 mph and I took samples every 2 seconds.  So that, in theory, works out to sampling every 12 meters - more than double that of the 30m NASA data.

So either the NASA data is artificially "noisy", or the Polar data is "smoothed".  If I had to guess, I'd say the barometric sensor just isn't all that accurate, so it can't pick up the slight changes in elevation when you're sampling more frequently.  But the elevation profile looks roughly the same as the one generated on gpsvisualizer.com, so it's probably "good enough".

I wish I had data to compare from a Garmin unit.  Even a comparison of the captured elevation data versus the NASA data would be useful.  I also wonder what options the Garmin units have for sampling rates...

 

- SM - 

Comments

eebee's picture

Missed it!

I missed this yesterday! Great details. Are those elevation charts from the Hanna route?
skatey-mark's picture

graphs are from a route in Apex

Unfortunately, my new solid-state drive for my laptop decided to die on me the other day, taking the Hannah data with it. Not much else was lost, since I hadn't erased the old hard drive yet. So I'm back up and running with that. Still, I hate knowing there's 51 miles of data missing from my records... The data is actually from "leg 1" of our Apex skate, which is the hillier section. I had intended to do more skating that day, but I started getting some nasty blisters and had to quit. - SM -
timv's picture

Elevation and pressure changes

Nice test! Thanks for posting. It's easy to read 10' off of paper topo maps and I'd expect NASA's elevation data to be at least that good, even if spatial resolution isn't so great. Maybe some crunchiness from snapping your lat/long to the nearest elevation data point... OTOH, it looks like 100' elevation change gives an air pressure change of only about .05 psi. I can see how this might be getting close to the sensitivity limit of a small consumer-grade pressure sensor. So yeah, I agree with you about the baro data probably being less precise, but either probably being good enough compared to the kinds of elevation changes we care about.
skatey-mark's picture

more elevation profiles, including Garmin 301

Okay, here's another try... I skated 65 miles yesterday with someone that had a Garmin 301 HRM/GPS. I got his elevation profile, and then created elevation profiles from my data. I tried to make all 3 roughly the same size so it would be easier to compare them..

 

Some quick observations...

  • The elevation profile from the Polar's barometric sensor appears to be more lik the shape of the NASA/gpsvisualizer profile.
  • The symmetry of the Garmin's profile isn't what I'd expect.  (More comments below...)
  • While the shape of the Polar profile matched pretty well against the NASA data, the actual elevation wasn't even close - this was because I have it set to assume that the starting point is always at an elevation of 0 feet.

 

Here are some points on how the profiles should look symmetric in some places: 

  • The first 20 miles were an "out-and-back" skate,  so the first 10 miles should be a mirror-image of the second 10 miles.
  • Same goes for the next 18 miles...  Miles 20-29 should be a mirror-image of miles 29-38.
  • The last 10 miles should have the exact profile as miles 10-20. 
  • We cross the same bridge on the lake 4 times, so those elevations should be exactly the same...  Looking at the NASA/gpsvisualizer profile, they appear to be around 7 miles, 12 miles, 52 miles, 58 miles... 

 

 

The NASA data produces the most symmetrical profile by far, as you'd expect.  I was very surprised at how un-symmetric the Garmin profile was.  The Polar profile wasn't bad -- but you can definitely see where the barometric sensor was drifting a bit at the end.  The weather was changing that last 10 miles, so while the shape was the same as miles 10-20, it thought we were at a significantly higher elevation.

 

Anyway, I thought the data was interesting enough to follow up.  I'd love to get some Garmin 305 (or even better, Garmin 405) data to compare...

 

- SM - 

 

Garmin:

Garmin 301 elevation profile 

Polar:

Polar RS800CX elevation profile 

NASA:  (gpsvisualizer.com)

NASA/gpsvisualizer elevation profile 

 

 

skatey-mark's picture

one other thought on the Garmin...

I don't know what the sampling rate on the Garmin is... That could explain some of the difference if, say, it's only recording data every 30 seconds. My Polar was set to 2-second sampling for this skate... Oh - and it doesn't appear my Polar is set to assume 0 feet elevation at the start, but I do know there is some calibration that you can do to it. I tell it to calibrate to 0 feet, whatever that means. Maybe if I told it to calibrate to 200 feet, the elevation values might match the NASA data better. But then it's still going to be weather-dependent anyway.. - SM -
skatey-mark's picture

Garmin sampling rate

Perhaps someone might have a more definitive answer, but after some searching, it seems that Garmin does "smart sampling" when recording gps data. That is, the frequency varies depending on how good the gps reception is. Under ideal conditions, it seems to be around 3-7 seconds. Some of the products I think allow you to override the smart sampling and set your own sample rate. I actually looked at the data file for the Garmin profile and the sampling rate did indeed vary. I saw samples anywhere from 1 second apart to 16 seconds apart. In the small section I looked at, however, I'd say the average tended to be around 6 seconds apart, which is pretty good. I have to think Garmin's accuracy could be improved if they separated the GPS sensor from the watch, which would allow for more optimal placement. (Like on top of your helmet.) I'm sure the sensor in the 405 is more accurate than the 301/305. But why not make a change that will make the watch less bulky and more comfortable, and improve the gps reception at the same time? I'm sure it would cost more to do that, so perhaps that's a big enough reason not to do it. The 405 is certainly much cheaper than the Polar RS800 + the gps sensor. - SM -
roadskater's picture

This is great info and thanks

Thanks for doing this, Mark. More more more. I've been too busy to go geek on this one for now, but your observations are interesting and helpful. I think the smart recording on the Garmin is pretty good. I had the idea it kept from taking too many unimportant data points when moving very slowly too, but I'm not sure that's it. Thanks again!

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