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Glitch/inconsistency detection in navigation data
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Rune Allnor
science forum beginner


Joined: 03 May 2005
Posts: 18

PostPosted: Thu Jun 29, 2006 3:26 am    Post subject: Re: Glitch/inconsistency detection in navigation data Reply with quote

Steve Underwood wrote:
Quote:
Jerry Avins wrote:
JF Mezei wrote:

...

So one must really understand the "event" as well as how the data was
recorded for that event before starting to process such data and
eliminate points judged to be "bad".

One must always understand data in order to analyze and interpret it
meaningfully. Believing otherwise is like believing that someone can
manage a business without understanding its nature.

Tell that to an MBA. :-)

You are quite right. When I asked what kind of nav system this is, there
was no response. All the discussion has been about hypothetical
something or others, rather than real world improvement of specific
problems in the data.

Steve

My apologies for that, Steve. These *are* real-world data from
real-world
systems, but as I don't know exactly where the limits go for corporate
"hush-hush" I'll rather play it safe for now. And, of course, I can
guess
but I don't necessarily know the important details.

No offence to you or any other. I saw a way of doing things and I
wanted
to know what the alternatives, preferably commercially available, are.
As
there seems to be no canned solution available, I'll probably have to
program some Kalman filters myself and play with them until I get a
sense for how these things work and how to incorporate the various
ideas.

Rune
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Steve Underwood
science forum beginner


Joined: 26 May 2005
Posts: 7

PostPosted: Thu Jun 29, 2006 1:44 am    Post subject: Re: Glitch/inconsistency detection in navigation data Reply with quote

Jerry Avins wrote:
Quote:
JF Mezei wrote:

...

So one must really understand the "event" as well as how the data was
recorded for that event before starting to process such data and
eliminate points judged to be "bad".

One must always understand data in order to analyze and interpret it
meaningfully. Believing otherwise is like believing that someone can
manage a business without understanding its nature.

Tell that to an MBA. :-)

You are quite right. When I asked what kind of nav system this is, there
was no response. All the discussion has been about hypothetical
something or others, rather than real world improvement of specific
problems in the data.

Steve
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JF Mezei
science forum beginner


Joined: 26 Jun 2006
Posts: 6

PostPosted: Wed Jun 28, 2006 7:13 pm    Post subject: Re: Glitch/inconsistency detection in navigation data Reply with quote

Mogens Beltoft wrote:
Quote:
If the new sampled track point n is outside the "road" defined by track
points n-1 and n-2 plus a margin to each side of the line n-2 to n-1, or
the unit has not recorded a track point for "this long", then record
track point n.


Change in speed also causes a track point to be recorded on sime Garmin
units. And I think that change in heading also does. I don't think
Garmin ever documented the algorythm.
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Mogens Beltoft
science forum beginner


Joined: 28 Jun 2006
Posts: 1

PostPosted: Wed Jun 28, 2006 3:05 pm    Post subject: Re: Glitch/inconsistency detection in navigation data Reply with quote

Ulrich Bangert wrote:
Quote:
Hello JF Mezei,

Ok. fair enough. But that still leaves the requirement that the user
know about the type of data that he has to process, the types of
irregularities which must be retained, and those that can be removed
because this will be needed to decide on the window size. And one also
need to know how the data was collected.

Agreed!

of stray points. With "auto" track recording, chances are very good that
the GPS would record a point at the turnoff, one point at the stop for
water, and again a point once the car gets back to main road and turns
back into the normal direction.

I am not sure if i interprete the term "auto track recording" in the right
way. Perhaps it is even a "standard" term in navigation that i am not aware
of (I have seen the question for outlier detection purely from a
mathematical point of view). But if it is some kind of "event driven" track
recording you are of course right that the proposed algorithm can not handle
data acquired in this way because some frontend entity has already made the
decision what an event is and what not and has missed to acquire the
"surrounding data" that are necessary for the algorithm.

I read somewhere, that some GPS units use a boundary and time check when
recording track points in auto mode.

It went something like this:

If the new sampled track point n is outside the "road" defined by track
points n-1 and n-2 plus a margin to each side of the line n-2 to n-1, or
the unit has not recorded a track point for "this long", then record
track point n.

/Mogens
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Jerry Avins
science forum Guru


Joined: 03 May 2005
Posts: 534

PostPosted: Wed Jun 28, 2006 2:58 pm    Post subject: Re: Glitch/inconsistency detection in navigation data Reply with quote

JF Mezei wrote:

...

Quote:
So one must really understand the "event" as well as how the data was
recorded for that event before starting to process such data and
eliminate points judged to be "bad".

One must always understand data in order to analyze and interpret it
meaningfully. Believing otherwise is like believing that someone can
manage a business without understanding its nature.

Jerry
--
Engineering is the art of making what you want from things you can get.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
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Ulrich Bangert
science forum beginner


Joined: 27 Jun 2006
Posts: 4

PostPosted: Wed Jun 28, 2006 8:07 am    Post subject: Re: Glitch/inconsistency detection in navigation data Reply with quote

Hello JF Mezei,

Quote:
Ok. fair enough. But that still leaves the requirement that the user
know about the type of data that he has to process, the types of
irregularities which must be retained, and those that can be removed
because this will be needed to decide on the window size. And one also
need to know how the data was collected.

Agreed!

Quote:
of stray points. With "auto" track recording, chances are very good that
the GPS would record a point at the turnoff, one point at the stop for
water, and again a point once the car gets back to main road and turns
back into the normal direction.

I am not sure if i interprete the term "auto track recording" in the right
way. Perhaps it is even a "standard" term in navigation that i am not aware
of (I have seen the question for outlier detection purely from a
mathematical point of view). But if it is some kind of "event driven" track
recording you are of course right that the proposed algorithm can not handle
data acquired in this way because some frontend entity has already made the
decision what an event is and what not and has missed to acquire the
"surrounding data" that are necessary for the algorithm.

Regards
Ulrich

"JF Mezei" <jfmezei.spamnot@teksavvy.com> schrieb im Newsbeitrag
news:44A2289A.BF59B898@teksavvy.com...
Quote:
Ulrich Bangert wrote:
very significant. Note, that the algorithm can fit BOTH kind of views by
adopting the window length. If you make the window length greater than 2
X
the "hill length" then the hill will be completely removed from the
data. If
you find that the hill is significant then make the window length
smaller
than 2X the "hill length", in this case the hill will not be filtered
out.


Ok. fair enough. But that still leaves the requirement that the user
know about the type of data that he has to process, the types of
irregularities which must be retained, and those that can be removed
because this will be needed to decide on the window size. And one also
need to know how the data was collected.

Say on a long straight road, a car turns off and drives 100m to a water
hole/pump. With periodic trackpoint recording, you could have a couple
of stray points. With "auto" track recording, chances are very good that
the GPS would record a point at the turnoff, one point at the stop for
water, and again a point once the car gets back to main road and turns
back into the normal direction.

Now, both would have a couple of stray points from a purely
"mathematical" point of view. But in the second case, a human could
more clearly see a path away from road and back to the road at the same
intersection to resume course.

So one must really understand the "event" as well as how the data was
recorded for that event before starting to process such data and
eliminate points judged to be "bad".
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Ulrich Bangert
science forum beginner


Joined: 27 Jun 2006
Posts: 4

PostPosted: Wed Jun 28, 2006 7:52 am    Post subject: Re: Glitch/inconsistency detection in navigation data Reply with quote

Rune,

as a dedicated follower of PASCAL i program in Borland DELPHI which produces
native code that i do not suspect to be significantly slower then C/C++
generated code. But over the years I have found that the Matlab help system
gives me information about mathematical topics at exactly the level that
seems to match me, that's why i pointed to it. If Plotter does not read your
files, then (in case they are ASCII) send me a few lines of it. I am very
interested to make my file read routines as universal as possible, so every
no-go is a object of interest.

Regards
Ulrich


"Rune Allnor" <allnor@tele.ntnu.no> schrieb im Newsbeitrag
news:1151475859.983140.83250@d56g2000cwd.googlegroups.com...
Quote:

Ulrich Bangert wrote:
To Rune:

On a typical pc with a window width of 100 I process 600000 data points
in
1-2 minutes, so it is not THAT slow that my first mail may have
indicated. I
use this algorithm for example in a freeware software named "Plotter".
You
can download "Plotter" from my homepage

www.ulrich-bangert.de

If you manage to load your data files with that (chances are..) you can
immediatly test the quality and the speed of the outlier detection.

I'll definately have a look into this. Your first post indicated you
have programmed these things in matlab? If so, there is a speed-up
potential here. I usually get a speed-up on the order of 10-50x when
I port from matlab to C or C++.

Rune
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JF Mezei
science forum beginner


Joined: 26 Jun 2006
Posts: 6

PostPosted: Wed Jun 28, 2006 6:58 am    Post subject: Re: Glitch/inconsistency detection in navigation data Reply with quote

Ulrich Bangert wrote:
Quote:
very significant. Note, that the algorithm can fit BOTH kind of views by
adopting the window length. If you make the window length greater than 2 X
the "hill length" then the hill will be completely removed from the data. If
you find that the hill is significant then make the window length smaller
than 2X the "hill length", in this case the hill will not be filtered out.


Ok. fair enough. But that still leaves the requirement that the user
know about the type of data that he has to process, the types of
irregularities which must be retained, and those that can be removed
because this will be needed to decide on the window size. And one also
need to know how the data was collected.

Say on a long straight road, a car turns off and drives 100m to a water
hole/pump. With periodic trackpoint recording, you could have a couple
of stray points. With "auto" track recording, chances are very good that
the GPS would record a point at the turnoff, one point at the stop for
water, and again a point once the car gets back to main road and turns
back into the normal direction.

Now, both would have a couple of stray points from a purely
"mathematical" point of view. But in the second case, a human could
more clearly see a path away from road and back to the road at the same
intersection to resume course.

So one must really understand the "event" as well as how the data was
recorded for that event before starting to process such data and
eliminate points judged to be "bad".
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Rune Allnor
science forum beginner


Joined: 03 May 2005
Posts: 18

PostPosted: Wed Jun 28, 2006 6:24 am    Post subject: Re: Glitch/inconsistency detection in navigation data Reply with quote

Ulrich Bangert wrote:
Quote:
To Rune:

On a typical pc with a window width of 100 I process 600000 data points in
1-2 minutes, so it is not THAT slow that my first mail may have indicated. I
use this algorithm for example in a freeware software named "Plotter". You
can download "Plotter" from my homepage

www.ulrich-bangert.de

If you manage to load your data files with that (chances are..) you can
immediatly test the quality and the speed of the outlier detection.

I'll definately have a look into this. Your first post indicated you
have programmed these things in matlab? If so, there is a speed-up
potential here. I usually get a speed-up on the order of 10-50x when
I port from matlab to C or C++.

Rune
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Ulrich Bangert
science forum beginner


Joined: 27 Jun 2006
Posts: 4

PostPosted: Wed Jun 28, 2006 6:01 am    Post subject: Re: Glitch/inconsistency detection in navigation data Reply with quote

To Rune:

On a typical pc with a window width of 100 I process 600000 data points in
1-2 minutes, so it is not THAT slow that my first mail may have indicated. I
use this algorithm for example in a freeware software named "Plotter". You
can download "Plotter" from my homepage

www.ulrich-bangert.de

If you manage to load your data files with that (chances are..) you can
immediatly test the quality and the speed of the outlier detection.

To JF Mezei:

I you managed to figure out exactly what the algorithm does, you will have
noticed that for detecting outliers everything is significant, that is
INSIDE the window, nothing else. For that reason, if this algorithm is
applied to the scenario you present, the first thing to say is, that it does
not matter at all whether you have been riding for 6, 12, 18 or anything
hours before you meet the hill. The algorithm is completely insensitive to
that!

The window is something like "If you want to detect outliers look only to
values in the neighbourhood and decide what is normal and what is not for
them". Please note also, that your scenario arises the question for a
definition of "oulier". Other people would pehrhaps think that the "hill
scenario" IS indeed a outlier that should be removed while you think it is
very significant. Note, that the algorithm can fit BOTH kind of views by
adopting the window length. If you make the window length greater than 2 X
the "hill length" then the hill will be completely removed from the data. If
you find that the hill is significant then make the window length smaller
than 2X the "hill length", in this case the hill will not be filtered out.
By applying the rule "a event shorter than n/2 may be a outlier" YOU decide
what is an outlier not the algorithm.

I cannot accept your second objection, it is a outlier detection algorithm,
not a biker's rest detection algorithm. But if you want to put forward the
question whether the rest will be detected as an outlier or not, the same
rules apply as above: If the window length is set to value so that the
length of the braking action before stop and the window length "match" then
the stop will be recognized as a "normal" change in data

Regards
Ulrich

"Rune Allnor" <allnor@tele.ntnu.no> schrieb im Newsbeitrag
news:1151393854.224220.97860@p79g2000cwp.googlegroups.com...
Quote:

Ulrich Bangert wrote: lots of interesting stuff.

Thanks. Sounds like something to look into. Processing speed is
(as of yet) insignificant if it can release man-hours for other duties.
Where I am right now, man-hours are expensive. If a computer
needs 12 hours for this sort of job, then so be it, if it can be done
in the human operator's time off watch.

Rune
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Steve Underwood
science forum beginner


Joined: 26 May 2005
Posts: 7

PostPosted: Tue Jun 27, 2006 8:25 am    Post subject: Re: Glitch/inconsistency detection in navigation data Reply with quote

JF Mezei wrote:
Quote:
Another aspect to consider:

When I processed my australian track logs, those points, taken at 1
minute intervals, that were very near to each other were in fact VERY
significant because this indicated a place where I stopped ( in some
cases, I want those to stand out to show places where whater was available).

So from a simplification of the trackpoints to draw a route, those
points can be removed, but you want to mark those points as significant
stops and possibly create a waypoint for them.

So one must really be familiar with the activity and how the data was
collected before selecting some logic to simplify a series of points.

Another aspect: many GPS have an "auto" feature to write track points
into the log. Generallly, track points are written when there is a
change in speed/direction and possibly at some regular intervals. But if
you do not know the exact logic used by the unit to decide if a
trackpoint is to be written or not, you cannot really decide how to
remove trackpoints statistically.

Related fun things happen in radar tracking. There you usually have
reasonable x and y estimates (based on the r and theta you measure), and
a weak z measurement (based on a very limit accuracy angular
measurement). You can track a target and predict based on what is a
reasonable side to side turn rate. However, a fighter can roll over then
do a hard downward half loop and go back in the direction it came. Your
nicely filtered track basically stops abruptly and goes backwards, and
your weak height information doesn't always help that much in tracking
in 3D. Airliners are easy to track, but the interesting targets are a
real pain.

This kind of filtering is like the tale I heard of the world's most
accurate weather forecasts. I was told a DJ in the Caribbean spent years
saying the weather would be whatever it was yesterday. Because the
weather there has long period of similar conditions punctuated by abrupt
changes, his accuracy was very high, but his usefulness was zero. :-)

Regards,
Steve
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Rune Allnor
science forum beginner


Joined: 03 May 2005
Posts: 18

PostPosted: Tue Jun 27, 2006 7:37 am    Post subject: Re: Glitch/inconsistency detection in navigation data Reply with quote

Ulrich Bangert wrote: lots of interesting stuff.

Thanks. Sounds like something to look into. Processing speed is
(as of yet) insignificant if it can release man-hours for other duties.
Where I am right now, man-hours are expensive. If a computer
needs 12 hours for this sort of job, then so be it, if it can be done
in the human operator's time off watch.

Rune
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JF Mezei
science forum beginner


Joined: 26 Jun 2006
Posts: 6

PostPosted: Tue Jun 27, 2006 7:29 am    Post subject: Re: Glitch/inconsistency detection in navigation data Reply with quote

Another aspect to consider:

When I processed my australian track logs, those points, taken at 1
minute intervals, that were very near to each other were in fact VERY
significant because this indicated a place where I stopped ( in some
cases, I want those to stand out to show places where whater was available).

So from a simplification of the trackpoints to draw a route, those
points can be removed, but you want to mark those points as significant
stops and possibly create a waypoint for them.

So one must really be familiar with the activity and how the data was
collected before selecting some logic to simplify a series of points.

Another aspect: many GPS have an "auto" feature to write track points
into the log. Generallly, track points are written when there is a
change in speed/direction and possibly at some regular intervals. But if
you do not know the exact logic used by the unit to decide if a
trackpoint is to be written or not, you cannot really decide how to
remove trackpoints statistically.
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JF Mezei
science forum beginner


Joined: 26 Jun 2006
Posts: 6

PostPosted: Tue Jun 27, 2006 7:21 am    Post subject: Re: Glitch/inconsistency detection in navigation data Reply with quote

Ulrich Bangert wrote:
Quote:
this problem is the classical application for so called "robust statistical
methods".

How would your method handle a situation where one is riding a bike at
30km on a very flat road for 6 hours in a steady speed, and for one
minute, you have this kamakaze downhill where you reach 80km/ h and then
ride up a hill at 10kmh for 7 minutes. Would it "filter out" those
readings of going down and then up the hill ?
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JF Mezei
science forum beginner


Joined: 26 Jun 2006
Posts: 6

PostPosted: Tue Jun 27, 2006 6:22 am    Post subject: Re: Glitch/inconsistency detection in navigation data Reply with quote

Rune Allnor wrote:
Quote:
If you have 3 points, A B and C, B can be considred an exception if the
discance between B and the axis A-C is greater than some amount.


I think this approach might be useful. Do you have any references
(books, articles) to how these things are done in aviation?

No. The ideas were given to me here (sci.geo.satellite-nav) years ago
while I was post processing some tracks data I had accumulated during a
bike trip through australia. I built logic to simplify the number of
points (remove those that do not define a change in direction, and
remove those that are "stray".).


Another thing to consider: if you are a large cargo ship overating in
the st-lawrence seaway, a stray point may be really easy to spot because
a ship simply cannot move on a dime. But if you are a canoe on a rapidly
flowing river, you may catch an eddy currnt that quickly pushes you off
course.

But for an aircraft, you need to really define the requirements and
handling os any glitch. Are you allowed to filter data out which will
not get logged onto the flight recorder ? The advantage of aviation is
that you can coorolate GPS with the primary instruments. (UINS and
barometric altimetre). So if you see a glitch on the GPS but no glitch
on the other systems, then the GOPS probably experienced a glitch. But
if it is visible on all 3 systems, then it is probably turbulence etc etc.
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