Hello,
My main concern is that Etrex 30 is really sensitive to pressure
changes, and where I ride my mountain bike this is hilly and sometimes
very windy, then altimeter records bumps which do not exist in reality.
I do the same ride on a zero wind day, then on a very windy gusty day
and I can see a "big" difference : 1950 meters instead of 1800 m. Then I
have to filter out the excessive 150 m.
Then in general getting less ascent means for me getting a more
realistic value. In general I am happy with 9 points median filter
(applied to 1 Hz tracks). It seems that it removes wind gusts and keeps
true terrain bumps (at least on a mountain bike on my usual tours).
Ascent value is something difficult, and having it modified by
Douglas-Peuker filter is something I would like to avoid. This is the
target of my modification.
zero
Le 10.11.2017 08:48, Oliver Eichler a écrit :
> Hi,
>
> I can see this occasionally for some tracks, too. So far I did not
> investigate in depth but my explanation:
>
> The caveat of that algorithm is always the last intervall at a local extrema.
> The error can be up to the threshold value. With other words: In the worst
> case you miss treshold meters to the summit. If you change the number of
> points you also change how you approach a local extema. If you are lucky you
> get much closer to the summit now and thus the ascend (same for descend) is
> larger. If you have several local "summits" in your track the difference can
> be quite large. "can" not must be. It depends on how errors sum up or
> compensate.
>
> As far as I understand your changes you you want to sum up multiples of the
> threshold value only. By that you always sum up a bit less. Naturally this
> will lead to a lower value. But is it a better value in the sense of being a
> more precise? You will always favor the lower corner of the error interval.
> It's less irritating for sure. But is that wanted?
>
> Oliver
>
> GESENDET: Donnerstag, 09. November 2017 um 21:46 Uhr
> VON: zero <[email protected]>
> AN: "Qmapshack mailing list" <[email protected]>
> BETREFF: [Qlandkartegt-users] Douglas-Peuker filtering and ascent/descent
> calculation
>
> Hello Oliver,
>
> fact 1) Few months ago we had a discussion about the way ascent is calculated
> in QMapShack. At the end the conclusion was that a threshold of 5 meters is
> used, and this is now in Wiki. Using a threshold is like counting contour
> lines on a map. I fully agree and I do not write this email to attempt
> anything against this.
>
> fact 2) I capture my tracks at 1 pt/s rate (to get more precision on timing
> for my Strava segments and because sometimes filters in my Etrex 30x are too
> rough, even with highest rate). First I send the 1 Hz track to Strava, then I
> archive it after having applied 1 meter Douglas-Peuker filter.
>
> fact 1 + fact2) I always have bigger ascent/descent values after
> Douglas-Peuker filtering. Until a few days ago it was a mystery to me : why
> be removing points, ascent/descent could be higher ?
>
> After some thinking and trials I modified code in CGisItemTrk.cpp, line 919 :
>
> It was :
>
> _ if(lastEle != NOINT)_
> _ {_
> _ qint32 delta = trkpt.ele - lastEle;_
>
> _ trkpt.ascent = lastTrkpt->ascent;_
> _ trkpt.descent = lastTrkpt->descent;_
>
> _ if(qAbs(delta) > ASCENT_THRESHOLD)_
> _ {_
> _ if(delta > 0)_
> _ {_
> _ trkpt.ascent += delta;_
> _ }_
> _ else_
> _ {_
> _ trkpt.descent -= delta;_
> _ }_
> _ lastEle = trkpt.ele;_
> _ }_
> _ }_
>
> it is now :
>
> _ if(lastEle != NOINT)_
> _ {_
> _ qint32 delta = trkpt.ele - lastEle;_
>
> _ trkpt.ascent = lastTrkpt->ascent;_
> _ trkpt.descent = lastTrkpt->descent;_
>
> _ if(qAbs(delta) _>=_ ASCENT_THRESHOLD)_
> _ {_
> _ if(delta > 0)_
> _ {_
> _ trkpt.ascent += _(DELTA / ASCENT_THRESHOLD)*
> ASCENT_THRESHOLD;
> _ }_
> _ else_
> _ {_
> _ trkpt.descent -= _(DELTA / ASCENT_THRESHOLD)*
> ASCENT_THRESHOLD;
> _ }_
> _ lastEle +=_ (DELTA / ASCENT_THRESHOLD)* ASCENT_THRESHOLD;
> _ }_
> _ }_
>
> Now ascent/descent is not modified by Douglas-Peuker filtering anymore.
>
> Should I keep this for my branch or are you OK with the modified code (then
> it means a PR from myself) ?
>
> bye,
>
> zero
>
> [1]
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>
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