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elevation data weirdness

PostPosted: Fri May 13, 2011 6:13 am
by rjl6789
Hi

I'm new to Sports Tracks - liked it and bought it. I'm now evaluating a few plugins. I really like the courses and waypoint plug-in. However, I'd like to understand the elevation correction process a bit more - I spotted that there were often significant differences if I use the add/fix elevation function from your plugin or the elevation correction plugin. I checked that the same elevation data was being used (SRTM3), and also ran the activity through http://www.gpsvisualizer.com for a third comparison. It seems that the elevation becomes a lot more jagged for the courses and waypoint plugin. Which then results in an increased climb value in the summary. Just thought I'd point it out incase it's a bug, or find out if I'm using it in error? I've attached an image of the spreadsheet I put together for the comparison. I understand that the actual climb calculation is based on smoothing the elevation data taking moving averages, the degree of smoothing being set in the settings. The data presented in the graphs is from the gpx export so I think should be unfiltered.

Thanks for any insights

Rob

Re: elevation data weirdness

PostPosted: Fri May 13, 2011 7:21 am
by old_man_biking
Hi Rob,

thanks for the thorough analysis.

Did you check which elevation data sources are used by the two plugins?
Both allow to configure this on their settings pages.

If you stil can't account for differences, could you please add a zip file to your post with:
1. an exported FITLOG file of the activity BEFORE the correction with W&C plugin
2. one AFTER the correction
3. a screenshot of the W&C settings page that shows the elevation data source

Cheers,
OMB

Re: elevation data weirdness

PostPosted: Fri May 13, 2011 12:03 pm
by rjl6789
Hi OMB

Yep, the elevation data is from SRTM 3" and all consistent - I'm assuming the "elevation correction" plugin is using this as it matches the elevations from GPS visualizer almost precisely (having selcted to only use this data source in GPS visualizer).

I've attached the fitlogs to this post, and the images to the next ones (the activity was fairly long and I needed to use 7z and do each file individually to sneak in under the 256kB limit)

Thank you very much OMB

Rob

Re: elevation data weirdness

PostPosted: Fri May 13, 2011 12:05 pm
by rjl6789
settings images attached

Re: elevation data weirdness

PostPosted: Fri May 13, 2011 3:28 pm
by old_man_biking
The screen shots tell me that Elevation Correction plugin uses SRTM 3" data and W&C plugin uses SRTM 1" data (which has a 3 times finer grid).
--> W&C plugin will for sure create more changes in elevation.

You can try to change elevation model sequence in W&C to match EC plugin and retry.

Cheers,
OMB

Re: elevation data weirdness

PostPosted: Fri May 13, 2011 4:30 pm
by rjl6789
ok, no worries then. I just assumed because there were 0 tiles and 0 MB of data cached for SRTM 1" that it wasn't in use. Whereas SRTM 3" has 3 tiles and 8MB. (i.e. the activity is in the uk not the us) + is quoted in the notes of the activity after you do the add/fix

p.s. changing the settings to the attached has the same result as before. Sorry for being a pain!

Re: elevation data weirdness

PostPosted: Sat May 14, 2011 1:59 am
by old_man_biking
Yep, if you live in the UK, SRTM 1" isn't available - only for the US and Canada, afaik.
So I'll have a deeper look at your attached data.

Cheers,
OMB