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Old 08-05-2012, 03:00   #1
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Erratic timestamps

I have been looking at many examples of Track Properies that I have produced. All show timestamps with irregular intervals, very different from the Trackpoint Intervals that I selected. These have included 1, 10 and 60 seconds.

I am running the machine in my window at home.

The same thing occurs with a mouse GPS and a Garmin GPS78 both of which work well with OpenCPN.
Can you help?
Cheers
Chris
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Old 08-05-2012, 04:09   #2
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Re: Erratic timestamps

Chris...
Can you produce something we can actually examine? It means exact settings used and a track recorded with these settings, exported as GPX.
If your GPS antenna does not actually change it's position, which is usually the case when it resides on a window in a house, I can imagine more circumstances when this behavior would be normal than otherwise.

Pavel
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Old 08-05-2012, 20:56   #3
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Re: Erratic timestamps

Pavel
I will get on to it as you suggest.
Chris
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Old 09-05-2012, 03:20   #4
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Re: Erratic timestamps

data from GPSMap 78 via USB
OpenCPN 2.5.0 Settings:
GPS
NMEA data source Garmin
Filter period 10 secs.
Etc.
Place trackpoints at time interval 10 secs

I hope to attach a GPX file.
Cheers
Chris
Attached Files
File Type: gpx Timestamp test.gpx (8.5 KB, 49 views)
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Old 09-05-2012, 04:12   #5
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Re: Erratic timestamps

Chris...
This looks OK to me. The reason looks like to be you walking pretty slowly and/or not far enough during those 10 second intervals...
To record a trackpoint in time interval mode, the conditions at https://github.com/nohal/OpenCPN/blo...util.cpp#L2483 have to be met
Translated to English:
The mode has to be turned on AND at least the time set in the configuration must have passed since the last recording (this one should be OK, unless your GPS is sending the data out less than once a second - you can see if it's the case in the NMEA stream window) AND the position has to be at least m_minTrackpoint_delta nautical miles from the last recorded trackpoint. This value comes from the empiric experience and is set in the code to 0.001 nautical mile (approx. 1.9 meters) and it's there to filter out the always present changes in the position reported by the GPS even if it does not move a single millimeter.
This common error in GPS reported positions is also a response to "But I walked more than two meters in those 10 seconds and still nothing was recorded"
As track recording is intended to actually record a way traveled by a moving target, this logic poses no problem in the real usage scenarios IMHO.

Pavel
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Old 12-05-2012, 21:40   #6
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Re: Erratic timestamps

Pavel
Thanks
0.001m per 10sec is about a third of a knot.
I can see that is the speed at which the timestamps become erratic.
It is not uncommon in yacht races to sail at these very low speed, even down to zero knots. Can I adjust the .001m myself to a lower number or 0? (I can't see where to make the change in the code).
Must try it sailing soon. Winter season starts next week.
Cheers
Chris
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Old 13-05-2012, 01:35   #7
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Re: Erratic timestamps

Chris...
If you are able to build the source tree, you can of course change this value in navutil.cpp
On line 2379 (in current git tree) change:
Code:
m_minTrackpoint_delta = .001;
to something like
Code:
m_minTrackpoint_delta = .000001;
I have to say I still don't understand what a track recorded at this interval would be good for to me to analyze my performance in a regatta - for that I simply use the recorded NMEA stream, which includes the environmental data like wind, which I find pretty important on a sailboat

Pavel
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Old 13-05-2012, 16:28   #8
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Re: Erratic timestamps

Pavel
There is a question I have been afraid to ask.
I would love to record the NMEA data to a program, maybe OpenCPN.
Can you guide me to the recipe?
I will have to extract the NMEA data from the back of Raymarine E120W display.
Thanks
Chris
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Old 14-05-2012, 01:42   #9
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Re: Erratic timestamps

Chris...
VDR plugin, cheap Chinese RS-422 to USB converter from ebay, 2 wires connecting to that huge cable leaving your E120W in the middle on it's back side - see page 37 of the installation instructions for wiring and page 106 of the same for what sentences you will receive.
If you don't like VDR, there's plenty of other tools able to record the stream.

Pavel
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Old 22-05-2012, 00:51   #10
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Re: Erratic timestamps

Pavel
Can OpenCPN open a GPX file with all the data columns available from the NMEA stream including wind strength and direction?

Is there another program specifically designed to analyse the full data stream for yacht racing?
Cheers
Chris
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Old 22-05-2012, 01:42   #11
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Re: Erratic timestamps

Chris...
Yes, OpenCPN can open it, but it will not load this extra data as there is no standard defining how they should be stored in the GPX file and thus every vendor stores them differently if at all.
In OpenCPN you can record and replay the whole stream with the VDR plugin. The same is possible with a lot of other tools. As for analysis, there also are tons, The Complete Yacht Race Player and Analyser for one that's free for basic use.
I personally am happy with a track recorded with one minute intervals, (if really in the analyst mode, which I usually am not) the VDR plugin, archived weather forecasts, backlog of meteo data from stations near the course and common sense. But my races sure are usually longer than a windward/leeward in a dinghy.

Pavel
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Old 22-05-2012, 02:02   #12
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Re: Erratic timestamps

Pavel
Thanks for the info.
Our races are in 44 footer, inshore and offshore, all around the coast of SA.
How about you?
Chris
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