Analysing GPS Track Data

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General Gps & Speed Waves Foiling Gear Reviews Lost & Found WA NSW QLD SA VIC TAS

Haggar

QLD

1670 posts

Haggar

QLD, 1670 posts

1 Jan 2007 10:47am

I took a long drive on Saturday to Sandy Point, what an awesome place to sail

Have a bit of a quandry with track data from my Foretrex 101 for the day. The max speed on the display was 32.5 kts, but after I've analysed the data in GPSAR and deleted all the obvious spikes above this speed(dozens of these), I seem to have valid speeds up to 38 knts. Can this be correct or should the recorded speeds always below the GPS max speed displayed ?
I also get an error message when opening the track file in GPSAR "This track does not belong to the 32'th UTM zone(it belongs to the 39th) You may need to restart GPSAR to get better accuracy with this track". Any ideas from anyone ? I'd be happy to email anyone the track file to help me analyse

sonic

QLD

756 posts

sonic

QLD, 756 posts

1 Jan 2007 10:52am

If you register with gps speedsailing and use there foram there are many experts who will help also if you search topics within the forum it may already be there....in my experienec the display speed and max should not be more than a knot diffrent.

drjukka

QLD

258 posts

drjukka

QLD, 258 posts

1 Jan 2007 11:00am

Hagger,

Contact Mal Wright at Intellimass ~~http://www.intellimass.com/RealSpeed/~~
and get a copy of RealSpeed. Analyse the tracks with this program and see what happens.

Had a similar problem with a recent session at Victoria Point where I was clocking 36's according to GPSAR- (One run only)GPS MAX of 33 something for the day - when I Analysed with Real Speed I was getting 32's and 33's which is far more realisttic given the conditions on that day.

When I reviewed the raw data I found a 183 knot spike right in the middle of a runs of 31-33's that completely stuffed the track. Removing the spike using the recommended method with GPSAR just spread the speed out over a longer run giving an inflated value.

Regards,

J

kato

VIC

3530 posts

kato

VIC, 3530 posts

1 Jan 2007 12:29pm

haggar, a few of us will be at sp today.Talk to daffy(orange volvo) or myself(Sail V27) for help

Haggar

QLD

1670 posts

Haggar

QLD, 1670 posts

1 Jan 2007 11:53pm

Thanks guys for your advice, I'll be back in SP in around a week wind gods permitting

elmo

8894 posts

elmo

WA, 8894 posts

2 Jan 2007 3:50pm

G'day Hagar

Heres how I sort out spikes

Open file in GPSAR
First thing is to name your file with the date and "RAW" and then resave with Date and "EDIT"

File/Trackpoints Data Table. This will open up another table
On this table click the top of the Right hand collumn marked kmh (or knots), this will bring your highest speed to the top of the list
make a note of the Line number (left hand Collumn) and click onto the top of the left hand collumn to sort back into correct sequence
Find your offending line and click on it, look to either side of it to see whether it stands out as a spike
if it does, with the offending line highlighted, go back to the main GPSAR page, look at your speed graph to confirm your thoughts, if it is bad then Holding down the SHIFT key (Important else it will delete entire file) and hit the delete button on page
Go back to table and repeat process until data is clean

I don't know how you carry your GPS whilst sailing but I used to have mine in an AQUApac bunged down my Wet suit and used to get a truckload of spikes until Chris L set me straight with the arm mounted AQUApac, spikes are now almost non extistant.

Got to be a bit envious of the Sandy point crew, it looks like the Boys have had a Ball tearer Christmas, work will definitely be required to recover from such marathon speed sessions.

Anyway, hopefully this helps.

Good winds' Good speeds, keep it upright.

Alby

Haggar

QLD

1670 posts

Haggar

QLD, 1670 posts

2 Jan 2007 5:30pm

Thanks elmo for the resorting by id # tip, I did'nt think of that. I am wearing the GPS in an aquapac on my wrist, maybe I have a bad GPS, or maybe on my upper arm may not jar quite as much

I did have lots of stacks on Saturday but I still don't think this explains all the spikes, if it continues maybe I should contact Johny Appleseed. I do turn the GPS of and on quite a bit, could this contribute

Thanks Jason for analysing the track for me, I can post my speed now, cheers

vando

QLD

3419 posts

vando

QLD, 3419 posts

2 Jan 2007 8:16pm

quote:
I do turn the GPS of and on quite a bit, could this contribute

yes

elmo

8894 posts

elmo

WA, 8894 posts

2 Jan 2007 8:35pm

I'd just leave the GPS on making sure you have good batteries before you go out, the Geko records 10,000 track points, so @ 2sec readings gives you 5hrs of recording time

I wear my Aquapac up the top of my arm, I also create a slip loop with the cord and have that attached to my arm (saved gps after a prang which ripped the GPS of off arm). By wearing up the top of the arm you are getting a better signal.

I'd only wear the GPS on your Forearm if you are one of those loonies which look at their arm rather than where they are going.

Have fun,

Alby

OSkl

NSW

5 posts

OSkl

NSW, 5 posts

3 Jan 2007 2:41am

I'm still learning to analyse the data too. Spikes are annoying so I have put the 101 in an AquaPak and strapped it to my spreader bar, facing up. I get about 5 spikes per recorded hour.

mathew

QLD

2172 posts

mathew

QLD, 2172 posts

3 Jan 2007 10:46am

The best place for the GPS is on your arm (upper or lower doesn't really matter all that much). As for spikes, you should really only be getting them if you fall in.

As elmo suggested, use the trackpoints data table to show your points. One item not mentioned is that you can delete the trackpoint directly from the table by selecting the offending row then hitting the backspace key (hold shift to select more than one row).

Also, hit the "neighborhood" checkbox on the RHS of the main window. As you select a row from the datatable, it highlights the track; it if is a spike, you will immediately see the error in the track.

elmo

8894 posts

elmo

WA, 8894 posts

3 Jan 2007 10:49am

quote:
Originally posted by mathew

The best place for the GPS is on your arm (upper or lower doesn't really matter all that much). As for spikes, you should really only be getting them if you fall in.

As elmo suggested, use the trackpoints data table to show your points. One item not mentioned is that you can delete the trackpoint directly from the table by selecting the offending row then hitting the backspace key (hold shift to select more than one row).

Also, hit the "neighborhood" checkbox on the RHS of the main window. As you select a row from the datatable, it highlights the track; it if is a spike, you will immediately see the error in the track.

Mathew

Thanks for that, didn't know those ones (got to read the instructions one day)

Alby

drjukka

QLD

258 posts

drjukka

QLD, 258 posts

3 Jan 2007 1:30pm

Haggar, Et al.

Spikes seem to be more prevalent on overcast days as well. Matthew is right that falling in will cause spikes.

To remove spikes in GPSAR - I open the track file and then sort by ortho acceleration, anything over 1.6 ms-2 I delete. I repeat the same process after sorting on the linear accleration. (to delete it is Shift backspace in GPSAR - (something is lost in the translation from french - in the manual)

I think go back and sort again and keep deleting until there are no ortho or linear accelerations greater than 1.6 ms-2.

Alternatlively - RealSpeed ex Mal Wright seems to do this for you automatically.

- Justin.

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